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  1. Reminds me a little of the RC article titled something like “CO2 is not the only greenhouse gas and greenhouse effects are not the only CO2 problem” Here it would be “extraterrestrial rock impacts are not the only cause for a sudden cooling, and not all et impacts necessarily lead to a cooling” It seems that you’d have to have multiple, definite, rock layer evidence associated with definite, repeated cooling events for their hypothesis/conclusion to have much weight behind it.

    Comment by Jim Bouldin — 7 Jan 2009 @ 8:37 PM

  2. Evidence of perhaps 40 glacial Lake Missoulas and the Missoula flood events was first seen in the dry rock formations in Idaho, Montana and Washington States.

    It seems plausible that many such humongous glacial seas breached into other areas. Evidence long since eroded may be harder to detect. Although Wikipedia has a list

    Diamond dust may have co-incidentally cast about at that time, but wouldn’t heavy dust impart a serious cooling effect?

    Comment by Richard Pauli — 7 Jan 2009 @ 8:51 PM

  3. (( feel free to ignore my misinformed previous comment ))

    Comment by Richard Pauli — 7 Jan 2009 @ 9:06 PM

  4. Hmmm, I think we could have a drastic climate change without it being grounds for someone declaring that every single drastic climate shift was caused by some extreme outside force and not natural, earth-bound processes.

    I think you’re missing the reason this is getting buzz outside of the climate science community, namely the extinctions and/or the impact of such an event on the Paleoindian population of North America.

    Comment by Brian Cockrell — 7 Jan 2009 @ 9:58 PM

  5. I seems to me that evidence of impact would need to lead to assessment of the energy released. This might give the basis for any estimate of the effect on climate along with the air-borne particles, etc.

    However, if there is sufficiently widespread indication of impact, this should not be discounted as a potential trigger.

    The article implies the Younger Dryas is a Northern Hemisphere event only. Is this the case, as it would effect the potential for impact to cause it? I would have thought that an event large enough to cause a change in climate lasting 1000 years would have to be global in effect.

    Comment by Ricki (Australia) — 7 Jan 2009 @ 10:27 PM

  6. Eric,
    of course you could be right, but I for one am not so skeptical of this theory and it seems very plausible. A barrage of comets could most certainly change climate trends and during a different time period like the last ice age could certainly have a different degree of climate sensitivity. To clarify myself before my peers misunderstand me; I am not blaming all climate change or global warming on a bunch of comets or other phenomena alone and that GHG are not the main current culprit.

    Looking at the evidence and thought process over the possibility of these comets altering seems likely, but far more research is needed as you noted. No, extra terrestrial events alone are not affecting climate, but these and other similar events cannot be overlooked or ruled out at this time.

    Then again your mention of the statistical problems has strong merit; I suggest that the data will lead to somewhere in the middle of these polar extremes you mention, but you already allude to the possibility despite your skepticism of there being an influence of comets whether they were the actual trigger or not. I certainly do not think that comet showers are more common than we think or that the climate is so stable that only in the face of such a drastic change could affect sudden climate change, but we certainly cannot rule out a period of sensitivity to a barrage of comets during one or two time periods. Good post and I understand your skepticism and certainly I will be looking at more data and calling a friend of mine who is more qualified than I to speak about nano diamonds, but if more looking into this confirms these nano diamonds, and this could happen quite quickly, the validity of the hypothesis will increase and from there of course the burden of necessary evidence is still great as you mentioned. Then again such was the case for many other scientific discoveries. I would not make the claim of not holding your breathe, however, on this one, because the plausible factors are far higher than some of the other dubious claims you guys do not hold your breathe for, and for which are not published in reputable journals. Climate is more complex and dynamic than any of us can measure or realize even in light of the trends, other trends we cannot explain or really measure are still very elusive or enigmatic. Still I enjoyed your post and read all your references and I see your points. Time will tell on this one, no one can really say this has no merit or os wrong on scientific grounds as of yet.

    Comment by jcbmack — 7 Jan 2009 @ 10:46 PM

  7. Eric,
    of course you could be right, but I for one am not so skeptical of this theory and it seems very plausible. A barrage of comets could most certainly change climate trends and during a different time period like the last ice age could certainly have a different degree of climate sensitivity. To clarify myself before my peers misunderstand me; I am not blaming all climate change or global warming on a bunch of comets or other phenomena alone and that GHG are not the main current culprit.

    Looking at the evidence and thought process over the possibility of these comets altering seems likely, but far more research is needed as you noted. No, extra terrestrial events alone are not affecting climate, but these and other similar events cannot be overlooked or ruled out at this time.

    Then again your mention of the statistical problems has strong merit; I suggest that the data will lead to somewhere in the middle of these polar extremes you mention, but you already allude to the possibility despite your skepticism of there being an influence of comets whether they were the actual trigger or not. I certainly do not think that comet showers are more common than we think or that the climate is so stable that only in the face of such a drastic change could affect sudden climate change, but we certainly cannot rule out a period of sensitivity to a barrage of comets during one or two time periods. Good post and I understand your skepticism and certainly I will be looking at more data and calling a friend of mine who is more qualified than I to speak about nano diamonds, but if more looking into this confirms these nano diamonds, and this could happen quite quickly, the validity of the hypothesis will increase and from there of course the burden of necessary evidence is still great as you mentioned. Then again such was the case for many other scientific discoveries. I would not make the claim of not holding your breathe, however, on this one, because the plausible factors are far higher than some of the other dubious claims you guys do not hold your breathe for, and for which are not published in reputable journals. Climate is more complex and dynamic than any of us can measure or realize even in light of the trends, other trends we cannot explain or really measure are still very elusive or enigmatic. Still I read all your references and I enjoyed your post. I see your points. Still it is not in the realm of science as of yet to say it is wrong.

    [Response: Thanks for the positive comments on the post. I agree with you -- I do not mean to dismiss this entirely out of hand. I just wanted to put a more sober counterpoint to the breathless excitement in the popular press that the Younger Dryas 'problem' had been 'solved'. It is mostly a non-problem since much of the scientific community (though not necessarily me -- see here) think we understand it already--eric]]

    Comment by jcbmack — 7 Jan 2009 @ 10:48 PM

  8. Just how similar is the YD to D-O events? D-O events seem to be rapid warmings followed by more gradual cooling while the YD was a rapid cooling event. Also, the duration of the YD is nearly as long as the possible period of D-O events (if indeed they occur with any periodicity, which I understand is in doubt). Just curious.

    [Response: See my response to Stefan below. The YD event is not, in my view, unique. Many of the DO events look just like it (indeed, much of the oceanography literature that seeks to explain these events relies on that similarity. There are claims of periodicity (every 1500 years) but updated data from the N-GRIP ice core draws that into question. But the average spacing is about 1500 years.--eric]

    Comment by tamino — 7 Jan 2009 @ 11:45 PM

  9. There is no reason to suppose that all climate change events have the same cause or even similar causes. Dansgaard-Oeschger events don’t have to have the same cause as the Younger Dryas. The earth, the solar system and this galaxy are a very complicated and chaotic system. There may be several or even many groups of causes for climate change events. The end-Cretaceous extinction event was caused by an impact. Impacts come in all sizes, but the larger the object, the lower the probability per unit time. Impactors also vary in structure and composition. Smaller and less solid ones don’t have to spread and iridium layer world wide. Bolides don’t have to contain iridium. The end-Permian extinction event was caused by a super-volcano. The closing of the “Panama straight” [I mean where Panama is now] had something to do with the “ice age” we have been in for 3 million years. Continents move around. The earth changes over billions of years. The sun heats up over billions of years. Planets perturb each others’ orbits. Evolving life changes the earth. Comets and asteroids pass by and sometimes collide. The sun orbits the galaxy and oscillates up and down through the galactic plane. Different stars have very different orbits around the galaxy. In 33000 years, Proxima Centauri will enter our Oort cloud, causing a period of heavy bombardment. Other stars occasionally and at irregular times enter our Oort cloud, causing heavy bombardments of new long period comets. In 25000 years, we will enter a giant molecular cloud, which is an object that astronomers have difficulty looking into. The earth is an active planet in a violent and poorly known universe. Check with your local paleontology, geology and astronomy departments. There is no reason to believe that we know all of the possible causes of climate changes yet.
    At this point, I neither believe nor disbelieve any of the several theories of why the Younger Dryas happened. Further discussion of the Younger Dryas would be interesting.

    My computer is too old to read the Sciencemag articles or the New York Times or PNAS.

    Comment by Edward Greisch — 8 Jan 2009 @ 12:09 AM

  10. From the perspective of an archaeologist, this is very interesting. Why? Because right at the time of this supposed comet strike, we find that the Clovis culture abruptly disappeared from north and central america. Boom. Gone. At the same time was a mass extinction of mammalian mega fauna such as the mammoth, giant sloth, saber tooth, etc. 35 species abruptly went extinct. Why? The archaeological record indicates that something big happened right then. Interesting.

    Comment by Jim O'Donnell — 8 Jan 2009 @ 12:14 AM

  11. I’m with Jim O’Donnell. The Comet Impact Theory would nicely explain the demise of BOTH the megafauna and the Clovis peoples from the paleontological record.

    From a Climate Change perspective, it seems much less interesting. Sure, an impact, in the right place would likely have hastened the demise of an ice dammed lake that in theory would have reduced the ice sheets faster and flooded the Atlantic with fresh water that might have helped produce the Younger Dryas. But the lake would have collapsed eventually anyway – and so maybe the supposed impact simply affected the timing of the Younger Dryas? Which is a much different supposition than being the ’cause’ of the Younger Dryas.

    And the way I read both papers, the impact theory was sought to explain the paleontology not climate change. In a way, it actually refutes the theory that the Younger Dryas (ie climate change) drove the megafauna to extinction. Similar megafauna survived in Asia until human populations eliminated them much later. And so, at the end of the day, it would explain the ‘timing’ of the Younger Dryas AND the extinctions of the Clovis people and the megafauna.

    [Response: This is a very good point. My concern here is the climate impact of the purported comets. I am less skeptical about the significance of these results from an archaeological point of view.--eric]

    Comment by Robin Johnson — 8 Jan 2009 @ 1:14 AM

  12. So the extinction of the dinosaurs and the extinction of the megafauna both were caused by celestial objects crashing into or very near the USA? That is a cosmic coincidence of some magnitude.

    Comment by Anders L. — 8 Jan 2009 @ 3:02 AM

  13. I love ‘crazy’ ideas like this. Thanks for highlighting the issues.

    I agree that far more research is needed, but it is worth considering this hypothesis. So we better start thinking :-).

    Comment by Aslak Grinsted — 8 Jan 2009 @ 3:25 AM

  14. “One would have to either accept the conventional ideas for the causes of these events, or, alternatively, one would have to propose that there was an impact not only before the Younger Dryas, but before each of the earlier events.”

    Eric, I am also very skeptical about this hypothesis. But concerning your above argument: DO events were abrupt warming events, while the YD was a cold event. Although it ended by something similar to a DO warming, the YD is somewhat special and could have a unique explanation different from the regular DO events. However, I think it is more likely that this explanation has to do with the deglaciation going on at the time and the associated ice melt and freshwater fluxes, rather than with comets. – Stefan

    [Response: Stefan. Yes, of course, as I have often pointed out, the defining character of the D-O events is rapid warming, not rapid cooling. But as you well know, many of the DO events end in rapid cooling, rather like the YD. I think the case for saying the YD is special is extremely weak.--eric]

    Comment by Stefan Rahmstorf — 8 Jan 2009 @ 6:10 AM

  15. We have mountains of data on the retreat of the Laurentide ice sheet and the vegetational development as the ice sheet retreated from pollen records. Much of the glacial geologic data is stratigraphic in nature. When one completes such a stratigraphy the key is finding some organic matter including burned material for dating. Thus, it seems odd that this recently noted layer has not been evident. We have lots of evidence that does not support the unrealistic notion of a an impact shattering an ice sheet substantially or melting it. This does not suggest the impact did not happen, just puts into perspective the potential impact. Consider carefully existing data in evaluating the current impact idea. The authors have made some observations of a horizon that suggests an impact at multiple locations at the start of the YD. That is all. They offer no evidence to tie it into ice sheet behavior. Take a look at a couple of examples of the level of detail just from a couple of studies of the Laurentide Ice Sheet. In this case the former Lake Warren, (ancient Lake Erie) and its relative surface level. Affected by ice advances and retreats. or for further chronology of the basin and for lake Agassiz This latter abstract indicates a release at 12.9, but it of the same magnitude of several other smaller releases and much smaller than the largest release. The point is does the hypothesis offered on the affect of the impact test out against the existing data. From what I know, and the links offered here are less than the tip of the iceberg of what is available, the hypothesis with respect to ice sheet impact does not.

    Comment by mauri pelto — 8 Jan 2009 @ 8:10 AM

  16. As John Mashey pointed out in the Open Mind discussion, it’s nice to see a real scientific controversy unfolding. Laymen should note that the debate is not about whether the evidence is “real,” but about which evidence is most important and should be emphasized. This is in contrast to pseudo-debate about climate change where the presence of all the evidence on one side of the debate leaves the denialist side with no strategy but to…, well, deny the evidence.

    [Response: Well said. And I do wish to emphasize that I do not mean to imply with my title about fool's gold that the authors of this work are fools. They are not. This is good work they are doing and I hope they keep doing it. We by no means fully understand the Younger Dryas, and all such ideas ought to be entertained.--eric]

    Comment by Ray Ladbury — 8 Jan 2009 @ 8:19 AM

  17. One other thing to point out about the Younger Dryas/Impact theory… They call upon the rapid break up of a portion of the Laurentide Ice Sheet culminating with the original freshwater routing hypothesis to explain the 1.3 kyr reduction in Atlantic overturning circulation. However, such a break up of the Laurentide would raise sea level and sea level rise slowed down at 12.9 kyr BP and during the Younger Dryas.

    [Response: I'm not sure where the "sea level rise slowed" from. Reference? But in any case this isn't a problem for the comet-impact hypothesis per se. If it is a problem, then it is a problem for the meltwater-caused-the YD hypothesis.--eric]

    Comment by Anders Carlson — 8 Jan 2009 @ 10:03 AM

  18. I find this interesting, but, even as a skeptic looking for justification, I fail to see the relevant effect on the current climate debate. Or is there none implied? If the skeptics are saying (are they?) we don’t have to worry about AGW (if true) because (again even if true) a comet shower will come along and mitigate the problem, that seems to be putting a heavy bet on a horse that might or might not even ever show up for the race.

    [Response: It isn't relevant! Not everything we write about on RealClimate has direct relevance to the "debate", except insofar as research on paleoclimate increases our understanding of the system in general.--eric]

    Comment by Rod B — 8 Jan 2009 @ 10:27 AM

  19. Anders, don’t forget that the “coincidental” events in question were separated by ca. 65 million years, which may render the coincidence less (wait for the pun) striking. For more on hypothesized impacts and alternatives, see this summary:

    (I’m not sure how up-to-date or well-accepted these ideas are, but they are certainly interesting.)

    Comment by Kevin McKinney — 8 Jan 2009 @ 10:42 AM

  20. Rod, I’d say this is pretty much a *paleo*-climate story–as you imply.

    Comment by Kevin McKinney — 8 Jan 2009 @ 10:45 AM

  21. Robin said:

    I’m with Jim O’Donnell. The Comet Impact Theory would nicely explain the demise of BOTH the megafauna and the Clovis peoples from the paleontological record.

    I don’t think it’s massively more successful than any other other “natural” explanation for megafaunal extinction. If this impact did cause extinction, it had remarkable powers of selectivity.

    IMO, the overkill hypothesis is by far the most parsimonious. John Alroy published an excellent paper a few years back in Science which demonstrates quite clearly the plausibility of overkill. You can read it at his website:

    “A Multispecies Overkill Simulation of the End-Pleistocene Megafaunal Mass Extinction”

    This isn’t to say that a comet wouldn’t have been an influence, but I doubt it was the primary mechanism.



    Comment by SteveF — 8 Jan 2009 @ 11:20 AM

  22. “….. If it turned out that rapid climate change events are caused by comets, it would imply the climate system is far more stable than we thought, that abrupt climate change events are not part of the inherent variability of climate during glacial periods. That would perhaps allay fears that we could be pushing the system towards an abrupt climate change in the future. On the other hand, it would also suggest that cometary impacts are far far more common than we thought. …”

    As a retired invert paleontologist, I’ve seen this controversy over comet/asteroid impact induced climate disruption several times before. My first reaction in 1979 to the K-T asteroid impact event was the same as that of most paleontologists–disbelief and denial that such a thing could ever happen so late in Earth history. Students were asked to counter the hypothesis. It was quite an easy task to accomplish, but our ‘evidence’ was never disproof–merely ‘objections’.* Especially when some of that evidence was disbelief that such events could be more common than we’d thoguht-a denial that turned out to be false if only on statistical grounds. Another was the suggestion that if the K-T extinction were impact induced then wasn’t it likely that all other extinctions may also have had an extraterrestrial forcing factor? 25 yr later we can say that some were impact-induced, others not and some others are still undecided.
    Both of these arguments are easy to make, just as you have done. But neither argument follows logically as any sort of dis-proof. All you’re really saying is that if due to an extraterrestrial impact, the YD is (feared to be) taken out of your sphere of study and influence. This isn’t a well-founded fear as both paleontologists and geologists have come to realize.
    As you said…you’re not experts on nano-diamonds, so let’s wait until the verdict is in on that one. Just as the debate on shocked quartz and the K-T went on for needless unproductive years, this one could too. However, our instrumentation and methods are much improved over those of 25 yr ago, so this matter should soon be decide.
    And please, let’s not complain if we don’t soon (or ever) find a crater. Remember ice cover was much greater at the time and there was still nearly 75% of the surface covered by oceans. No, lack of evidence isn’t evidence on the pro-comet/asteroid side either…However, a nanon-diamond-bearing layer in the right place is rather good evidence itself unless one can come up w/a way for gradual, common sedimentological process to concentrate nano-diamonds in certain layers by chance immediately at an extinction & abrupt climate-change horizon.
    It’s sad to see that you’re leaning toward the ‘denialist’ side just because the Younger Dryas event might be snatched (partially) from your realm of study by an extraterrestrial event. We’re all too familiar w/GW denialist claims and their reasoning already. Don’t fall into the trap. Let the experts do their work, and we’ll go from there.

    *To my knowledge the single most vexing aspect of the K-T extinction has never been clearly explicated. Why the large animals and all dinosaurs, many birds, many shallow water marine shelled organisms among others, to the exclusion of insects, small mammals, lagoonal, swamp and estruarine animals and plants such as alligators, frogs, turtles, etc? Perhaps the following 1993 hypothesis (perhaps made by others, but personally propoed to & rejected by “The New Scientist”) may find some following. So please indulge me for a pararaph…
    I find the following realization–based on damage to ground-nesting birds’ eggs by acid rain–quite convincing. Dinosaurs were ground-nesters and could utilize either ‘upland’s', deserts and forests or fluvial, lacustrine or onshore swampy environments for nests. Thus indicating no particular adaptation to typically stressful environments where all habitat variables-especially water chemistry vary wiidely and inpredictably (e.g., alligators and croc’s). The same was/is true of most shallow off-shore marine invertabrates. But these stressful habitat conditions could easily be tolerated by small burrowing mammals reptiles and insects. This also explains the selective survival of ground-nesting Cretaceous water-birds over others. Acid rain fall-out from the impact is thus the likely culprit causing the Selective extinction.
    It’s not that acid rain hadn’t been considered an important killing mechanism before this time (& thus the superficial reason for the paper’s rejection). What hadn’t been recognized earlier or by the editors was acid rain’s probable greater/est importance and it’s central role in explaining the Selective Extinctions.

    Comment by David R. Hickey — 8 Jan 2009 @ 11:46 AM

  23. In evaluating how widespread the affect of the potential impact let us not forget the data that has been meticulously put together and represents more than 50 years of paleoclimate research efforts in the area or pollen. Eric Grimm worked hard to create this database, so let us not forget about it. If you go to the following site you can view pollen diagrams from specific sites. The scientists are not only noting pollen counts but also are looking for any burn horizons etc. For Minnesota alone there are 120 records, shows the emphasis at the U of Minnesota. For the US there are 510 and for Canada 335. It is too bad we do not have a similar database for glacial geologic stratigraphy

    Comment by mauri pelto — 8 Jan 2009 @ 12:25 PM

  24. “ocean circulation change (the most well-evidenced proximal cause of rapid climate change)”

    Are there others?

    What if the comet stuck in such a way that most of the water precipitated over Greenland, the Arctic, and Siberia?

    Comment by Jim — 8 Jan 2009 @ 12:49 PM

  25. SteveF Says:
    8 January 2009 at 11:20 AM
    Robin said:

    I’m with Jim O’Donnell. The Comet Impact Theory would nicely explain the demise of BOTH the megafauna and the Clovis peoples from the paleontological record.

    I don’t think it’s massively more successful than any other other “natural” explanation for megafaunal extinction. If this impact did cause extinction, it had remarkable powers of selectivity.

    IMO, the overkill hypothesis is by far the most parsimonious.

    It’s not necessarily the most parsimonious if it is established that a comet did hit in that location at that time and that massive fires/floods occurred. Then any explanation would have to take that into account and such assumptions as ‘overkill’ would not necessarily be the most parsimonious.
    There’s huge difference between saying ‘maybe a comet did it’ i.e. introducing a new ‘entity’, and saying ‘maybe the comet impact that we know happened at that time and place did it’ where no new ‘entity’ is introduced.

    Comment by Phil. Felton — 8 Jan 2009 @ 12:57 PM

  26. It seems to me (although it’s not my field) that the evidence for an impact is very strong. Whether or not it was the “trigger” for the YD is quite another question.

    I’m not persuaded by the argument based on D-O events. It appears to be an open question whether or not the YD is really like D-O events, and even if it is there’s no reason the YD couldn’t be triggered by an impact while D-O events have some other trigger.

    In my post on the subject, I didn’t say the evidence was a “done deal” but I certainly argued its plausibility more than I would today. The main reason I’ve revised my opinion is Mauri Pelto’s reminder that there’s a lot more evidence to consider than just the evidence presented for an impact event. There’s lots more stratigraphy, plenty of pollen data, and since that’s nowhere near my field I have to accept the fact that I don’t just lack thorough knowledge of it, I lack even the passing acquaintance required to combine old data with new and get a reliable perspective.

    But the single comment I agree with most is Ray Ladbury’s paraphrase of John Mashey. It’s good to discuss some actual controversial science! As important as it is to debunk climate junk, this is way more fun.

    Comment by tamino — 8 Jan 2009 @ 1:37 PM

  27. Phil. Felton said:

    It’s not necessarily the most parsimonious if it is established that a comet did hit in that location at that time and that massive fires/floods occurred. Then any explanation would have to take that into account and such assumptions as ‘overkill’ would not necessarily be the most parsimonious.

    Sure. However, it would still have to account for the faunal structure of the event. Which a purely natural mechanism struggles to do, IMO, given the selectivity involved. I think there’s a fundamental issue of parsimony that natural mechanisms will struggle with in this regard.



    Comment by SteveF — 8 Jan 2009 @ 2:12 PM

  28. Does the shape of the temperature curve during YD not rather argue against a single event cause? The graphs I’ve seen show a square-wave shape, with a rapid fall, then about 800 years of sustained low temperatures followed by an equally rapid rise to the trend line. From a single event cause I’d expect a sawtooth shape, rapid fall followed by a steady(ish) climb back up.

    [Response: Not necessarily. The shape is widely believe to reflect hysteresis in the way that the ocean responds -- the circulation gets suppressed, and then it take a while for it to "re-start". See Stefan's various writings on this (starting here]-eric

    Comment by Jerry — 8 Jan 2009 @ 2:16 PM

  29. > more fun
    Amen. And I’d love to see the various scientists who specialize in different bits of this puzzle suggest to one another what sort of thing they might look for in their own areas. If there are enough sediment cores (I recall Gavin’s comment that those folks rarely cross-correlate their work, they tend to go deep in one spot!) maybe a map of the impact(s?) or airburst(s?) could be made, to get some idea if it was one big whack or flash, or a myriad of smaller ones, and whether the result was one continent-sized firestorm or many “smaller” ones.

    Did anyone ever look for example downwind of Tokyo or Dresden to see how far the soot was distributed and how well it could be documented? I suppose that sort of thing was done for the big volcanos.

    Comment by Hank Roberts — 8 Jan 2009 @ 2:23 PM

  30. This is a really interesting topic from the perspective that it, at least temporally, links failure of human populations with rapid climate change and biodiversity loss. However, I am interested in any comments folks on this site may have on a related issue.

    At the same AGU conference, there was a presentation, that has also gotten some headlines and publicity, by researchers who were reporting that the 65 Ma K-T extinction was not caused by ET impact. The evidence, as I understand, was that their finer dating technique showed that the impact and extinctions were offset by something less than a million years, but did not coincide. The massive volcanism of the Deccan Traps, however, occurred during the course of the extinction and drove the global changes. (Which if I recall from my old geology books was a favored explanation before the impact theory.)
    Anyways, I wanted to hear any opinions from the folks here (esp anyone who may have attended the talks) how this ranks up and is being received. Is the timing difference they cite within the error associated with dating things from 65 m.y.a?

    Also, I can see this gaining (or at least staying in the news) in the same way this theory has (without specific debate to merits) because it’s about a pop-culture relevant topic and apparently History Channel is already making a program on “what really killed the dinosaurs” much like they did last year on the Y-D impact to coincide with the “10,000 BC” movie. So the potential for further public misunderstanding of scientific developments is there too.

    Comment by John A — 8 Jan 2009 @ 3:38 PM

  31. Eric (18), thanks; makes sense.

    Comment by Rod B — 8 Jan 2009 @ 5:00 PM

  32. That some extraterrestrial body or swarm of bodies struck much of North America about 12,900 years ago seems, to me at least, well established. Naturally, I’d like to see more evidence of fires or the lack thereof at more locations. I’d especially like one or more impact craters. But for what I take to be good reasons, neither additional bits of evidence may ever be forthcoming. What we do have is

    (1) The end of Clovis culture without any continuation, AFAIK.

    (2) The extinction of many large mammal species. I attribute this to a combination of (i) climate change stress, (ii) human predation somewhat reducing population sizes, but not that much, (iii) extraterrestial body impact taking the population below viability.

    (3) Variious indicators of extratrestrial impact, but no crater(s).

    So a reasonable hypothesis is that a comet hit the Laurentide Ice Sheet along the southern portion somewhere. This is hypothesized to occur so that an ice damed proglacial lake, going to release sometimes in the next few hundred years anyway, was suddenly released. It carried an unusual quantity of shattered ice with it; this gave rise to the long cool period until all this ice melted.

    Now I found a program here

    which calculates effects of small impacts quite nicely, but after some experimentatiion, does not seem quite so good for comet impacts on ice. Noetheless, assuming a comet similar to Shoemaker–Levy 9 in size and density

    impacting ice of very great thickness, the program rapidly calculates

    Your Inputs:
    Projectile Diameter: 1800.00 m = 5904.00 ft = 1.12 miles
    Projectile Density: 500 kg/m^3
    Impact Velocity: 51.00 km/s = 31.67 miles/s
    Impact Angle: 35 degrees
    Target Density: 1000 kg/m3
    Target Type: Ice

    Energy before atmospheric entry: 1.99 x 1021 Joules = 4.74 x 10^5 MegaTons TNT
    The average interval between impacts of this size somewhere on Earth during the last 4 billion years is 2.6 x 10^6 years

    Crater Dimensions:
    Transient Crater Diameter: 18.6 km = 11.6 miles
    Transient Crater Depth: 6.58 km = 4.09 miles

    Final Crater Diameter: 27.4 km = 17 miles
    Final Crater Depth: 0.802 km = 0.498 miles

    The crater formed is a complex crater.
    The volume of the target melted or vaporized is 9.46 km^3 = 2.27 miles^3
    Roughly half the melt remains in the crater , where its average thickness is 34.8 meters = 114 feet

    I interpret this to mean that all ice in the transient crater would be removed and some of the bedrock below would be shattered. Either such shattered rock lies at the bottom of one of the Great Lakes or else was scraped off by subsequent reformation of the local portion of the ice sheet.

    Whether a crater of this size would be sufficient to promote Younger Dryas release I must leave to others; I am not an expert on the cryosphere. What matters to me is that no surviving crater remains; indeed there is no crater of the right age which matches the rest of the evidence (which might well include many of the meteorites picked up in the Great Plains in the 1930s).

    We certainly do not use extraterrestrial impacts to explain

    nor do extraterrestrial bodies of this magnitude strike often enough to explain both Younger Dryas and the similar termination 3 event.

    We know that proglacial lakes drain through their ice dams; indeed it is not necessary to suppose otherwise for Younger Dryas except for two matters. (i) Younger Dryas is much the most massive during the destruction of the LGM ice sheets, much more so than the 8.2 kya event for example. If the corresponding event during termination 3 is of about the same size, then it would seem that such can occur without extraterrestrial intervention. (ii) Some extraterrestrial body (or bodies) did impact, leaving traces from Alberta to Belgium, but without a crater. Which is the point I made before; while not necessary, such might well have been sufficient.

    Comment by David B. Benson — 8 Jan 2009 @ 6:17 PM

  33. Most of you who object to the ‘comet selectivity’ do not understand the argument that the scientist made. Simply speaking, a swam of comet pieces (many hundreds or thousands entered the atmosphere just over North America. These hyper-velocity fragments would heat the upper atmosphere to a few hundred centigrade. Any animal unable to take cover would be burned alive or badly injured by burns and have trouble lasting. Small animals and animals that tend to be shielded (live in water/mud/underground) will fully escape. Birds can cover huge distances so the lost of nearly all of these creatures in North America would not appear on the geo record since they would be replaced in just a few years. This does not mean the idea is correct, just that its selective nature is easy to explain. Of course, forest MUST be destoryed and leave a heavy soot footprint or else the idea is nonsense. Of course, the breakup of a comet that has vast number of fragments will leave few (if any) and very small (shallow) impacts that may not leave any long term evidence.

    Comment by DBrown — 8 Jan 2009 @ 6:33 PM

  34. Please do not put too much confidence on the History Channel. I just watched its program on “Dispensationalism” (a theological topic, prophecy) and it was pathetic.

    Comment by Ron Taylor — 8 Jan 2009 @ 7:00 PM

  35. re: #16, #26
    It’s not just way more fun, it’s way more instructive for people learning about science:

    1) It’s a paleo problem, requiring ferreting out of partial, difficult data sources. i.e., it’s detective work done millenia after the crime :-)

    2) It’s a highly multi-disciplinary problem, with a variety of data sources, not all of which are necessarily visible to any given investigator, and some of which may be or seem contradictory.

    3) It’s not a problem of measuring/modeling some behavior, say like temperature records, where people generate reconstructions with error bars, and reconstructions may differ, but one can analyze the data and look at the graphs to see if they make any sense together. (I.e., like Figure 6.10 in IPCC WG I, which do. Anyone sensible knows that different lines whose error bars generally overlap are more in agreement or not.

    It’s an *attribution* problem: the major effect is known, but it’s unclear *why*, and specifically, whether or not a particular event happened to trigger the effect, and how. What makes it fun is that it’s a murder mystery where the butler doesn’t have “murderer” written on his forehead.

    4) We’re still in a state where reasonable people can disgree.

    5) Of course, as usual, it’s not a problem you can do in a high school lab… In particular, replicating this experiment is not recommended…

    Comment by John Mashey — 8 Jan 2009 @ 7:11 PM

  36. For a comet impact animation, look at this:

    It is not clear to me that a comet impact directly into the Laurentide ice sheet would leave either an impact crater or evidence of soot and fires. Depending upon the size the comet, it might seemly vaporize a big chunk of ice and eject it high into the atmosphere. From there, much of the water vapor might condense and fall in the direction opposite from the comet. If that direction was towards the north, much might fall as snow or ice or water that rapidly freezes leaving relatively small amounts of fresh water injected into the North Atlantic. Significant but smaller amounts would go higher and possible circle the world. This could cause massive floods in some areas but might also block sunlight sufficiently to trigger major cooling along with the longer term effect of enhanced albedo of the expanded snow/ice area in the Northern Hemisphere.

    This would not be inconsistent with DO events occurring independently from this and either before or after it.

    Comment by Jim Cross — 8 Jan 2009 @ 10:22 PM

  37. I think most modeling should wait until the evidence of impact(s) has been better evaluated, and estimates of the mass of extraterrestrial material are available. One question for the glaciologists, if we postulate that an impact might have triggered the release of a glacially dammed lake, just what sort of time window are we talking about? I.E. how long is such a lake vulnerable to say an impact generated tsunami, versus when it would have failed without any help.

    Comets, and some asteroids may well be extremely fragile objects. At a low impact angle, it is possible that the energy release might be spread over a large area, perhaps without the formation of a large crater. At any high impact angle, once the density per unit area of the impactor exceeds the area density of the atmosphere (roughly 3KG/cm**2) the mass of the impactor should be able to ram through the atmosphere, regardless of its cohesive characteristcs.

    Comment by Thomas — 8 Jan 2009 @ 10:31 PM

  38. Latest wisdom from the BBC
    “Heat may spark world food crisis”

    Comment by PHE — 9 Jan 2009 @ 12:43 AM

  39. The craters exist. There is a band of lakes across North America. The traditional explanation is that the lakes were caused by the ice sheet. Another possible explanation is that the lakes are the results of a Shoemaker-Levy type comet impact. The linear relationship between the Great Slave, the Great Bear and the Great Lakes seems a little improbable. The other lakes seems to form a band. I would think that the lakes formed by the ice sheet would be more randomly scattered.

    Comment by Mark Stewart — 9 Jan 2009 @ 1:01 AM

  40. Eric, I agree with much of what you said but I do not really agree with your line of reasoning. I have not gone through the comments here so I apologize if this is repetitious.

    I am also skeptical of the impact idea, but that’s because (like you) I have not seen any convincing evidence for a mechanism on how it produces a YD-like climate change. The skepticism does not arise because it contradicts the other abrupt changes (which obviously makes no sense).

    For one thing, the YD was more like a seventh Heinrich event (H0) rather than a D-O event (which is marked by relatively rapid warming, not cooling). The YD was not at all like a D-O event and probably had causes much more similar to Heinrich events, of which we have a good story for. This involves the sudden delivery of freshwater influx into the North Atlantic after successive but progressively cooler D-O events (or a Bond Cycle).

    You’re certainly right in that there is no need to throw in another forcing mechanism. There’s not much room for big forcings like cosmic rays in modern times either, and they don’t do any good for predictive ability, but they should not be excluded because we “don’t need them.” The study of the abrupt changes is in its infancy, but I agree the impact idea is unlikely to make significant progress. RC’s own Ray Bradley has proposed that paleocryptic ice from an isolated arctic ocean (thick multi-year sea ice overlain by ice and firn) forced a reduction of the THC and was largely responsible for the YD, with the termination of the paleocryptic ice possibly resulting from warm water inflow from a relatively warm Atlantic and Pacific after sea levels rose, the Bering Strait was flooded over, and the end of arctic isolation.

    Comment by Chris Colose — 9 Jan 2009 @ 1:14 AM

  41. How would icesheet dynamics be effected by a small to medium impact? My guess is that
    A. You’d melt a bit of ice.
    B. You’d shatter the underlying ice below the melt zone.
    C. That would allow the water to drain down to the icesheet-bedrock interface.

    However, The reasoning expressed above makes no sense. Nobody is arguing that impact is the ONLY way to cause kiloyear duration North Atlantic cooling events. So there is no need to re-examine D-O events.

    Even if you think you understood the YD before the impact hypothesis was raised, if you accept an impact, it means that your assumptions about the initial conditions for the YD were incorrect. So you need to demonstrate that the primary and secondary effects of impact won’t alter the pre-existing climactic state before you can discount it as a potential cause.

    Comment by Lab Lemming — 9 Jan 2009 @ 5:47 AM

  42. DBrown

    Most of you who object to the ‘comet selectivity’ do not understand the argument that the scientist made.

    I do understand the argument made. Unfortunately it doesn’t fly IMO. There strikes me as a fair bit of special pleading in your argument. Moreover, the distinction between small and large can’t explain the selectivity of the impact precisely because the small and large distinction does not neatly exist. Not all large megafauna went extinct, only around 3/4.

    It wasn’t simply a neat divide between big beasties and little ones; the faunal structure of the event was much more complex. Therefore a blanket natural event will struggle to explain this. It is much easier to explain the structure with an overkill framework. The natural changes of the time would certainly have added greater stress to animal populations, but humans were required to complete the picture.



    Comment by SteveF — 9 Jan 2009 @ 5:56 AM

  43. Hi folks,

    Slightly off topic, I know, but are there any predictions yet of the expected minimum Arctic ice cover for this year? Last year’s minimum areal extent was a bit higher than the record set in 2007, but I have read that this was probably offset to some degree by the thinning of the ice, so that the volume was a minimum, or near minimum, record. Also, high pressure over much of Europe (and other parts of the near Arctic) seems to have encouraged unusual wind and temps, leading to a reduced freezing (formation of sea ice) over the last couple of weeks. Could we be seeing the setting up of conditions for even greater ice loss this year? Any thoughts, anyone?

    Nick O.

    Comment by Nick O. — 9 Jan 2009 @ 5:58 AM

  44. I still see an irrational tendency to want to invoke widespread catastrophic floods and fires to this potential event. Each of these affects would leave telltale and obvious evidence in the stratigraphy that would not have been missed in the literally thousands of stratigraphy’s completed. This does not mean there was no impact or some fires, as the authors found. To date no evidence of flooding has been cited. People including the scientists got way ahead of their evidence, which was black mats and nanodiamonds. Their evidence says nothing about climate, glaciers, or megafauna directly. We have lots of evidence that does speak directly to these topics, that can be trusted. If a scientific idea does not fit the existing data all that well, then there is something wrong with it. In this case it is the scale of the supposed affects. In looking back through the pollen diagrams and glacial geology in the Lat Quaternary Einvronments of the United States book and in The Last Great Ice Sheets, I am not seeing anything suggestive of a catastrophic change. A glacier dammed lake outburst leaves some pretty tell tale signs as well, particularly in water level drops. There is shoreline data as I noted yesterday available for the lakes. If there is a sudden lake level change it is apparent. Such events do not occur without leaving evidence that survives a few thousand years. If you buy into hugely catastrophic impact, then you have to ignore all the commentary on glacier recession and pollen changes that do not indicate anything that abrupt.

    Comment by mauri pelto — 9 Jan 2009 @ 7:41 AM

  45. Mauri Pelto said:

    In looking back through the pollen diagrams and glacial geology in the Lat Quaternary Einvronments of the United States book and in The Last Great Ice Sheets, I am not seeing anything suggestive of a catastrophic change

    There are potentially problems with cause and effect. The transition into the YD in terms of vegetation was pretty substantial (if not necessarily catastrophic). It’s a sufficiently significant shift and this could be overprinting the very temporally precise signal of the impact in the pollen, if it were the impact that “caused” the YD.

    So, to elaborate somewhat, the scenario could be:

    1) An impact. Impact causes catastrophic vegetation change immediately. A high resolution event.

    2) The impact results in a rapid climate change. The ecosystem responds to this resulting in a cold climate flora. The brief period of time of total vegetation catastrophe has therefore not been recorded. The YD has, effectively, overprinted it.

    Hopefully what I’m trying to say is clear. I’m not familiar with North American YD pollen diagrams, which doesn’t help. That said, your wider point is well taken. There should be good evidence from within the old fashioned, tried and tested late quaternary palaeoarchives for this impact.

    Comment by SteveF — 9 Jan 2009 @ 8:41 AM

  46. SteveF/mauri pelto, how many/little years of transition from previous to “cold climate” flora is needed to “overprint” such an event?

    Comment by Sekerob — 9 Jan 2009 @ 10:25 AM

  47. Mark Stewart, have you read something suggesting those particular lakes are craters, or is this idea your own work? How do you distinguish a crater from a lake besides the fact that they line up?

    Comment by Hank Roberts — 9 Jan 2009 @ 10:55 AM

  48. Ron — I hear ya about the Discovery Channel. They also run UFO stuff. “Discover how to misinterpret evidence so it’s cooler!”

    Interestingly, the CAPTCHA words are “claims Mackay.” Does anyone remember a famous book by an author with that name?

    Comment by Barton Paul Levenson — 9 Jan 2009 @ 11:26 AM

  49. Jim: 30 years is a conclusion about the particular data you’re trying to understand. It’s not arbitrary; it’s done to know what you can understand from the data, so you can collect useful facts.

    For climate:

    You can learn:

    “… to discuss climate trends in global mean temperature, you need to use 20-30 years of data centered on the date of interest.”

    It’s worked out there for students. He will convince you, if you work through the steps shown.

    Comment by Hank Roberts — 9 Jan 2009 @ 11:33 AM

  50. Sekerob,

    SteveF/mauri pelto, how many/little years of transition from previous to “cold climate” flora is needed to “overprint” such an event?

    That was my suggestion and I should say that it isn’t gospel. I was just batting around ideas as to why the impact might not show up in pollen diagrams (if indeed it did occur). Thinking aloud as much as anything.

    That said, we have an idea of roughly how long the transitions took and not just from ice cores. I’m most familiar with European work – there was an excellent multidisciplinary project at a site in Norway called Krakenes (and also Gerzensee in Switzerland). In particular see this paper:

    Birks, H.H et al. (2008) The development of the aquatic ecosystem at Krakenes Lake, western Norway, during the late glacial and early Holocene – a synthesis. Journal of Palaeolimnology, 23, 91-114.

    there’s also a shorter summary here:

    Birks, H.H. and Ammann, B. (2000) Two terrestrial records of rapid climatic change during the glacial-Holocene transition (14,000-9,000 calendar years BP) from Europe. PNAS, 97,1390-1394.

    This showed extremely rapid response from the vegetation to the Younger Dryas event. It’s hard to be entirely precise; the chronologies are never going to be as good as we want them to be. Nonetheless, as the latter paper says:

    Both studies demonstrate the sensitivity of terrestrial and aquatic organisms to rapid temperature changes and their value for quantitative reconstruction of the magnitudes and rates of the climatic changes. The rates in these two terrestrial records are comparable to those in Greenland ice cores, but the actual temperatures inferred apply to the terrestrial environments of the two regions.

    Wether all this is good enough to support the explanation I suggested, well I wouldn’t bet my house on it. It’s something to think about though.

    Comment by SteveF — 9 Jan 2009 @ 11:46 AM

  51. Hank – I am pretty sure Mark Stewart is hypothesizing and not very well at that. Just think, if any single one of the Great lakes, Bear Lake or the Great Slave Lake (let alone all of them and the thousands of smaller lakers in between) were formed by an extraterrestrial impact(s) ~12K years ago, the likelihood that we would be sitting here today is essentially nil. Any one of those features is much, much larger than the Chixculub crater (yucatan) believed to the impact point for the asteroid/comet leading to the K-T event.

    To distinguish between just another lake and an impact crater, one might look for stratigraphic evidence of impact debris at the bottom of the lake or in the area immediately surrounding the lake, evidence of disruption of the local bedrock (for example, the stratigraphic sequence at Barringer Crater in Arizona was folded over on itself in the area immediately around the crater), and for high pressure/low temperature mineral species such as stishovite(a high pressure/low temp polymorph of quartz which is considered diagonistic of impact events).

    For a cool site on some example of impact craters, check here: Geology.Com- Meteor Craters . I particularly like the one in Quebec a little north of the mouth of the St. Lawrence (Manicouagan). It is clearly recognizable from a very high altitude.

    Comment by Bob North — 9 Jan 2009 @ 1:30 PM

  52. If the Younger Dryas was initiated by a comet strike, what ended the Younger Dryas? The termination of the Younger Dryas was the rapid one, not the initiation.

    For example, 2007 Abrupt resumption of the African Monsoon at the Younger Dryas—Holocene climatic transition

    The possible role of volcanism in the creation of evidence similar to that of a meteorite has also been noted, but osmium ratios apparently differ: AGU meeting abstract: Search for Extraterrestrial Osmium at the Allerod – Younger Dryas Boundary

    This is much different from the K-T boundary, which left a gigantic crater under the Yucatan. It would probably have been less significant than the Long Valley Caldera explosion in western U.S.:

    The Glass Mountain eruptions, which were fed by a large, chemically evolving magma chamber in the shallow crust, culminated in the cataclysmic eruption of 600 cubic kilometers of high-silica rhyolite 760,000 years ago. This massive eruption resulted in the widespread deposition of the Bishop Tuff and the simultaneous 2- to 3- km subsidence of the magma chamber roof to form the present 17 by 32 km, oval depression of Long Valley Caldera.

    As far as the mammoths go,

    They lived from the Pliocene epoch from 4.8 million years ago to around 4,500 years ago.

    Why didn’t the Long Valley Caldera also wipe out the mammoths?

    The probable fact is that the rapid growth of human populations at the dawn of the Holocene was responsible for the extinction of much of the North American megafauna from 50,000 to 10,000 years ago. The changing climate was a contributing factor, but it appears likely that without human beings, some 2/3 of those pre-existing large mammalian species would still be around: The causes could involve everything from hunting to habitat encroachment to the introduction of non-native diseases.

    This is an uncomfortable topic for those who wish to deny any human influence on natural systems. It’s easy to see how various sides would use these scientific arguments to push their viewpoints, but that’s not science. The arguments are simplistic and repetitive, like all good propaganda: “Nature is robust, and there is nothing little humans could do to affect it”.

    Comment by ike solem — 9 Jan 2009 @ 1:38 PM

  53. Comets do not form swarms, but do easily breakup. Shoemaker–Levy 9 falling into Jupiter is an example. So a possibility is that the impactor broke up into an ellipse of fragments, all of which impact the ice dam of the proglacial lake. Once again running the program at

    changing only the diameter to be 500 meters, the result of interest is

    Crater Dimensions:

    …Crater shape is normal in spite of atmospheric crushing; fragments are not significantly dispersed.

    …Transient Crater Diameter: 5.34 km = 3.31 miles
    …Transient Crater Depth: 1.89 km = 1.17 miles

    …Final Crater Diameter: 6.67 km = 4.14 miles
    …Final Crater Depth: 0.524 km = 0.325 miles

    …The volume of the target melted or vaporized is 0.065 km^3 = 0.0156 miles^3
    …Roughly half the melt remains in the crater , where its average thickness is 2.91 meters = 9.54 feet

    which suggests damaging the ice sheet down to the bottom. Enough of these, close enough together, cause the ice dam to fail.

    “The prevailing theory holds that the Younger Dryas was caused by a significant reduction or shutdown of the North Atlantic thermohaline circulation in response to a sudden influx of fresh water from Lake Agassiz and deglaciation in North America.”


    and “Climatologists believe that a major outbreak of Lake Agassiz in about 11000 BC drained through the Great Lakes and Saint Lawrence River into the Atlantic Ocean. This may be the cause of the Younger Dryas stadial.”


    which is then in agreement with an ice dam failure in the southern part of the Laurentide Ice Sheet.

    Comment by David B. Benson — 9 Jan 2009 @ 2:18 PM

  54. Another Problem with the Comet Hypothesis

    Another problem with the comet hypothesis is how to explain geographically disburse (multiple burn marks in North America and Europe) by which comet fragments can create burn marks over large areas at multiple locations on the planet but leave no craters.

    There is a second set of peculiar burn marks (The burn marks in question are all elliptical and all have an axis that points in the North-west direction. Some of the burn marks are overlapping.) along the east coast of the US (roughly 100,000 elliptical burn marks.) from New Jersey to North Carolina, that date around 38,000 BP. (Differ event than the YD event of course.)

    Is there another possible mechanism that can create large numbers of burn marks on the planet?

    A possible clue is the anomalously hot inner core temperature (difficult come up with a mechanism to keep the core of the planets hot. Appears an external energy source.) of all planets with exception of Uranus. Uranus is pecular as it magnitude field is at right angles to its rotation and displaced from its axis of rotation.

    Comment by William Astley — 9 Jan 2009 @ 2:42 PM

  55. Hank @49, might you have intended this post to be in response to Jim Cross in the “Environmental reporters ought to be more responsible” thread?

    Comment by Jim Eager — 9 Jan 2009 @ 2:56 PM

  56. The point behind my comment #53 was that aappropriate size fragments would leave no impact craters in the underlying bedrock. So it is possible that there are no impact carters to be found; possibly there are some in various Great Lake bottoms.

    Comment by David B. Benson — 9 Jan 2009 @ 2:58 PM

  57. Actually not all impacts cause surface craters, FYI the Siberian impact early last century. I say this on TLC or one of the science channels where a it was found that if the material can vaperize and comes in at the right impact angle you get massive pressure and thermal bloom but no impact crater. You can get the damage without the impact crater.

    Comment by Vernon — 9 Jan 2009 @ 3:21 PM

  58. First, to record a report of additional evidence: “Exploding Asteroid Theory Strengthened By New Evidence Located In Ohio, Indiana”

    Second, Vernon (57), several months ago Scientific American had a very good article on the Tsungka event. The impact crater is a lake north of the center of the explosion; the remaining portion of the bolide has been magnetically located as below the lake bottom.

    But back to the Ohio-Indiana evidence. This strongly suggests there should be an impact carter somewhere in eastern Canada, whereever the ‘diamond fields’ are.

    Comment by David B. Benson — 9 Jan 2009 @ 4:16 PM

  59. From

    “Although a sinkhole in the limestone terrane is a possibility, an origin related to a meteor crater, that was subsequently glaciated, seems more likely. Aeromagnetic mapping by the Geological Survey of Canada revealed a negative magnetic anomaly over Charity Shoal, which is a characteristic feature of simple impact craters.”

    Comment by David B. Benson — 9 Jan 2009 @ 4:28 PM

  60. David (58), try the Northwest territories, not eastern Canada.

    Comment by Jim Eager — 9 Jan 2009 @ 5:41 PM

  61. Charity Shoal is at the far northeastern end of Lake Ontario.

    Captcha: Arctic signifi-

    Comment by Jim Eager — 9 Jan 2009 @ 5:43 PM

  62. #49, is misplaced, yep, meant to be in the other thread. Thanks.

    Comment by Hank Roberts — 9 Jan 2009 @ 5:54 PM

  63. David #58. The crater has not been seen. There are anomalies and if the crater *were* in the lake, that would explain why they can’t find a crater.

    You’re getting the horse before the cart.

    Comment by Mark — 9 Jan 2009 @ 6:02 PM

  64. William 54. It’s a little premature to say whether the magnetic axis being so different is that important to what is happening to Uranus (snigger, sorry, can’t help it :-)). After all, our magetic axis is quite a long way away from our rotational one and it drifts about all over the place.

    And when it breaks down to reverse polarity, it could be anywhere. It could even turn out to be multiple poles.

    It could take decades of measurements to work out if the magnetic anomaly of Uranus is real or just a sampling error.

    Comment by Mark — 9 Jan 2009 @ 6:06 PM

  65. Jim Eager (60) — The diamond fields in eastern Canada certainly are not in the Horthwest Territories. With evidence from Alberta to Belgium, north of 60 degrees north latitude is not implicated. You are welcome to look there if you wish.

    (61) — Thank you, I thought that clear from the url. I looked there because it is close to a kimberlite field near Kingston. There is, of course, no evidence of just when that meteor crater (if it is one) was formed, other than too many of the numerous glacial advances through Lake Ontario ought to remove the evidence.

    Mark (63) — The lake is the impact crater of the Tsungka bolide.

    Comment by David B. Benson — 9 Jan 2009 @ 6:45 PM

  66. and on 63 I made a boo boo.

    Darnit. It’s been a long week…

    Comment by Mark — 9 Jan 2009 @ 7:27 PM

  67. Re 54:
    “difficult come up with a mechanism to keep the core of the planets hot. Appears an external energy source.”

    Radioactive decay in the Earth’s mantle. See “What Heats The Earth’s Core?” here:

    Given that the mantle is outside the core, it might be considered an external energy source.

    Comment by spilgard — 9 Jan 2009 @ 8:01 PM

  68. Tunguska, not what I wrote before.

    We all make mistakes.

    Comment by David B. Benson — 9 Jan 2009 @ 8:03 PM

  69. Ah yes, I had forgotten about the pipes in northern Ontario. Sorry, David.

    Comment by Jim Eager — 9 Jan 2009 @ 8:21 PM

  70. #56, 49

    Thanks, Jim, for reminding Hank about which thread he is posting!


    Sometimes I find the scroll bar useful (it’s to the right of the window of your browser) for checking which thread you are on.

    Perhaps this Google search may help you:

    Comment by Jim Cross — 9 Jan 2009 @ 8:35 PM

  71. Attached is additional information & questions concerning burn marks that have been found throughout the Northern Hemisphere dated: 12,900 BP and around 32000 BP.

    This is a link to the data and analysis that shows some event created approximately 500,000 elliptical burn marks along the Atlantic coast from New Jersey to Alabama. (See figure 7 in the attached for an aerial picture of the burn marks.)

    I believe other analysis has dated the 500,000 Carolina burn marks to around 32,000 BP rather than 12,900 BP.

    This same link shows other burn marks in the Northern Hemisphere that were dated to 12,900 BP and hence coincide with the Younger Dryas event.

    In addition to the peculiar regular elliptical shape of the Carolina Bay burn marks and the great number of burn marks (i.e. A forest fire would not create elliptical burn marks. What could cause the very sharp edges of the burn marks?), the burn marks show other anomalous patterns.

    There is evidence of overlapping burn marks. There are very small intense burn marks mixed in with large burn marks. The burn marks are separated by areas that are not burned.

    The burn residue is as deep as 15m. There is no evidence at any of the sites of impact craters.

    The authors of this paper hypothesize that all the burn marks where caused by an extraterrestrial impact, however, I suspect that hypothesis is not correct.

    Carolina Bays. The Carolina Bays are a group of »500,000 highly elliptical and often overlapping depressions scattered throughout the Atlantic Coastal Plain from New Jersey to Alabama (see SI Fig. 7). They range from ≈50 m to ≈10 km in length (10) and are up to ≈15 m deep with their parallel long axes oriented predominately to the northwest. The Bays have poorly stratified, sandy, elevated rims (up to 7 m) that often are higher to the southeast. All of the Bay rims examined were found to have, throughout their entire 1.5- to 5-m sandy rims, a typical assemblage of YDB markers (magnetic grains, magnetic microspherules, Ir, charcoal, soot, glass-like carbon, nanodiamonds, carbon spherules, and fullerenes with 3He). …

    Fig. 7. Aerial photo (U.S. Geological Survey) of a cluster of elliptical and often overlapping Carolina Bays with raised rims in Bladen County, North Carolina. …
    …The largest Bays are several kilometers in length, and the overlapping cluster of them in the center is ≈8 km long.

    Comment by William Astley — 9 Jan 2009 @ 11:51 PM

  72. More observational data from the paper & questions.

    This is a link to the data from the paper. The paper present observation evidence that some event created approximately 500,000 elliptical burn marks along the Atlantic coast from New Jersey to Alabama. (See figure 7 in the attached for an aerial picture of the burn marks.) I believe other analysis has dated the 500,000 Carolina burn marks to around 32,000 BP rather than 12,900 BP.

    This same link shows other burn marks in the Northern Hemisphere that were dated to 12,900 BP and hence coincide with the Younger Dryas. As noted below both sets of burn marks 32000 BP and 13000 BP have common characteristics.

    In addition to the peculiar regular elliptical shape of the Carolina Bay burn marks and unusually large number (i.e. A forest fire would not create elliptical burn marks. What could cause the very sharp edges of the burn marks? It seems difficult to image a comet that could break in 500,000 pieces.), the burn marks show other anomalous patterns. There is evidence of overlapping burn marks. There are very small intense burn marks mixed in with large burn marks. The burn marks are separated by areas that are not burned.

    The burn residue is as deep as 15m. There is no evidence at any of the sites of impact craters.

    The authors of this paper hypothesize that these early burn marks where caused by an extraterrestrial impact however I have problems with that explanation.

    Carolina Bays. The Carolina Bays are a group of »500,000 highly elliptical and often overlapping depressions scattered throughout the Atlantic Coastal Plain from New Jersey to Alabama (see SI Fig. 7). They range from ≈50 m to ≈10 km in length (10) and are up to ≈15 m deep with their parallel long axes oriented predominately to the northwest. The Bays have poorly stratified, sandy, elevated rims (up to 7 m) that often are higher to the southeast. All of the Bay rims examined were found to have, throughout their entire 1.5- to 5-m sandy rims, a typical assemblage of YDB markers (magnetic grains, magnetic microspherules, Ir, charcoal, soot, glass-like carbon, nanodiamonds, carbon spherules, and fullerenes with 3He). …

    Fig. 7. Aerial photo (U.S. Geological Survey) of a cluster of elliptical and often overlapping Carolina Bays with raised rims in Bladen County, North Carolina. …
    …The largest Bays are several kilometers in length, and the overlapping cluster of them in the center is ≈8 km long.

    Comment by William Astley — 10 Jan 2009 @ 12:07 AM

  73. To David B. Benson, who said:
    “That some extraterrestrial body or swarm of bodies struck much of North America about 12,900 years ago seems, to me at least, well established. Naturally, I’d like to see more evidence of fires or the lack thereof at more locations. I’d especially like one or more impact craters. But for what I take to be good reasons, neither additional bits of evidence may ever be forthcoming. What we do have is

    (1) The end of Clovis culture without any continuation, AFAIK.

    (2) The extinction of many large mammal species. I attribute this to a combination of (i) climate change stress, (ii) human predation somewhat reducing population sizes, but not that much, (iii) extraterrestial body impact taking the population below viability.”

    What we also have, when you look further than North America, are megafaunal extinctions on other islands and continents that correlate quite well with the time of human arrival on those islands and continents (ranging from ~50 000 years ago in Australia/New Guinea to ~800 years ago in New Zealand). Whether or not a comet impact happened 12 900 years ago, it isn’t necessary to invoke a comet to explain megafaunal extinction.

    Comment by Cathe P — 10 Jan 2009 @ 2:26 AM

  74. William Astley posts:

    Is there another possible mechanism that can create large numbers of burn marks on the planet?

    A possible clue is the anomalously hot inner core temperature (difficult come up with a mechanism to keep the core of the planets hot.

    William, the cores of planets are hot for two reasons: 1) Primordial heat, since planets form by accretion and the kinetic energy of the infalling material has to go somewhere. 2) Radioactive decay. For Earth, the planet we know best, those sources account very neatly for the known thermal structure of the planet, within the limits of present empirical knowledge. No need for exotic mechanisms.

    Comment by Barton Paul Levenson — 10 Jan 2009 @ 5:40 AM

  75. Several people have suggested that a cometary impact could explain the mass extinction and end of the Clovis culture at around the same time. Steve F. has already referenced a good study of the overkill hypothesis. It’s also worth noting that wherever modern humans have arrived for the first time, the event has been followed in fairly short order by a mass extinction (Australia, New Zealand, Hawaii, various Atlantic islands, possibly even Eurasia, although due to the latter’s size and the time taken to occupy it the case is less clear – and there were already pre-modern humans present.

    Comment by Nick Gotts — 10 Jan 2009 @ 6:01 AM

  76. David #65 No, if the crater were the lake, the blast would have had to have been quite a bit bigger.

    There is an *anomaly* that can be found from magnetic surveys that *can* be the result of some types of meteor strike but it’s only in part of the lake.

    IIRC too the distance and angle from the forest flattening centroid to the lake isn’t in too great an agreement with the supposed track.

    The lake could still be an explanation to fit the expectation rather than the facts.

    Unless they were ahead on the scheme to dive into the lake and take more measurements.

    Comment by Mark — 10 Jan 2009 @ 6:49 AM

  77. Other evidence that suggest that the Younger Dryas is related to Dansgaard Oeschger events is the deuterium excess in the Greenland Ice cores (Jouzel et al 2005) as well as the isotopes in several oceanic cores and the Cariaco grey scale (Hughen 2000), which does not support the YD cause.

    Another problem is the coincidence with the Laacher see eruption dated multiple times at 12,900 cal BP. several varve counting in the German maars (Lith, Lucke & Brauer 2004 etc) give very robust evidence that the Younger Dryas started actually some 200 years after the 12,900 eruption

    Comment by Andre — 10 Jan 2009 @ 9:13 AM

  78. Barton, Actually latent heat of solidification from the liquid outer core crystallizing on the solid inner core is also a fairly significant mechanism and accounts for a fair degree of the vigorous circulation of the former.

    Comment by Ray Ladbury — 10 Jan 2009 @ 10:13 AM

  79. I fail to understand how the Younger Dryas delay would invalidate the impact hypothesis involving ice sheet disintegration acceleration, species extinction, and the collapse of a putative Clovis ‘culture’. That is what I would expect, a short delay as the ice sheet collapses.

    The impact hypothesis is now very robust.

    Comment by Thomas Lee Elifritz — 10 Jan 2009 @ 10:37 AM

  80. The problem with relying on an impact or a volcanic explanation to explain the Younger Dryas cold period is the length of the Younger Dryas – over a thousand years.

    The length of the period implies that there was a shift in global ocean and atmospheric circulation patterns that persisted for a long time.

    Then, the question is, what induced such a shift, and what caused it to end? The draining of a large glacial lake into the northern oceans? Would that really alter the global ocean circulation to such an extent?

    This is an important question, as it ties into the much-repeated notion of global warming leading to a European cooling trend due to “shutdown of the conveyor belt that brings warm water northwards”. This idea was all the rage a few years ago, but has now been discredited, mostly – although it is still the leading explanation for the Younger Dryas cooling event (with dissent, though). However, we still live on the same planet, so how can that be?

    The answer could be that the background situation is different – in the modern world, any reduction in poleward heat transport via the oceanic route would likely be compensated for by the atmospheric route. This is a very difficult topic – perhaps the most difficult one – because it is hard to say how atmospheric and oceanic circulation patterns will change in a warming world.

    At the time of the Younger Dryas, there was also still a large remnant ice sheet – the Laurentide – that stretched across Canada. Basically, the idea being promoted is that a comet impacted the ice sheet, led to catastrophic melting, and that initiated a huge meltwater pulse that initiated the Younger Dryas. However, the meltwater pulse would have happened anyway – and it’s still unclear if a meltwater pulse would have led to a thousand-year cooling event.

    It’s also hard to understand how the termination of the Younger Dryas is supposed to go along with the ice-melt pulse hypothesis. Wouldn’t the rapid termination of the Younger Dryas have led to another meltwater pulse and a subsequent cooling? And what caused the rapid termination of the Younger Dryas?

    If there’s anything at all to be said with confidence about the Younger Dryas, it’s that with no Laurentide ice sheet, there would have been no Younger Dryas. In any case, before jumping to the conclusion that a comet initiated the Younger Dryas, please recall the movie “The Day After Tomorrow”, in which Britain is turned into a glacier overnight.

    In paleoclimatology, there are grand hypothesis and there are detailed chronologies and careful reconstructions – and the latter two are what you want to look at, for example this one in Nature Geosciences:

    Tree rings and ice cores reveal 14C calibration uncertainties during the Younger Dryas (2008) Muscheler et al

    What’s most intriguing is the changes in the carbon cycle around the Younger Dryas. During that period, upper-ocean 14C appears to have been unusually high, which has been interpreted as due to a slowdown of the North Atlantic deep water ventilation, since that brings up older CO2 from deep waters with a lower 14C content.

    That’s not the only way to explain changes in 14C content in the atmosphere, however. such as increased methanogenesis in the tropics. There are all kinds of strange methane spikes from that period as well, meaning that the whole system is not completely understood.

    Take a look at the graph – even as CO2 continued to rise during the Younger Dryas, methane plummeted. How is a comet hypothesis supposed to explain a 1000-year drop in methane concentrations, followed by an explosive rise at the end of the period?

    That’s interesting, isn’t it? What’s going on with methane?

    We can make all kinds of suppositions, looking at that graph. For example, what about the role of people and fire? If human populations were suddenly impacted by very cold weather, they would be burning much more wood in the winter – multiply that across the entire Northern Hemisphere, and it could explain the rise in atmospheric CO2… or could it?

    The problem, if you ask me, is that the biological component of the climate system has never really been fully grasped by most of the climatology crowd, who are largely based in non-biological sciences. Paleoclimatologists tend to view biology as a passive tracer of environmental conditions, and forget how biological activity can itself influence the climate (look at us, for example). This is very clearly seen if one looks at the major greenhouse gases – CO2, CH4 and N2O – all of which are intimately involved in biological pathways.

    Comment by ike solem — 10 Jan 2009 @ 12:27 PM

  81. Marvellously informative discussion that seems to sum up as:

    Impact event large enough to leave clear traces: Likely.

    Impact was a sufficient cause of the Younger Dryass: Unlikely.

    With anything further depending on additional data and more penetrating analysis.

    Comment by D iversity — 10 Jan 2009 @ 12:36 PM

  82. Ike Solem writes to remind of the lack of information about
    > the biological component of the climate system

    Yes. The work by Le Quere (including the guest topic here at RC a while ago) and no doubt by many other scientists whose work I haven’t found yet is beginning to illuminate this.

    I wonder if anyone has tried to isolate DNA or RNA fragments from any of the peculiar layers of the stratigraphy to find out what kind of organisms bloomed during rapid climate change, compared to before and after such events. I wonder if it’s even possible to do that (yet).

    Comment by Hank Roberts — 10 Jan 2009 @ 1:02 PM

  83. Here is a very interesting related paper from respected Clovis Archaeologist Vance Haynes. Make sure to see the numerous photos of Black Mats in the supporting info:

    Comment by Ray Percatr — 10 Jan 2009 @ 1:07 PM

  84. Cathe P (73) & Nick Gotts (75) — There is a noticable difference between islands and big continents. Humans lived on the Mammoth Steppe for a long time without (more than a few) large mammal extinctions. I recommend

    R. Dale Guthrie
    The Meaning of Paleolithic Art
    Univ. Chicago Press


    Stephen Oppenheimer
    Out of Eden
    Constable & Robinson, 2003

    as starting points. The large mammal extinctions of North America were different in kind from those in Asia, but more sources than those above are strongly recommended. In any case, a nearby archaeologist, Bill Lipe, was the one to suggest the “three strikes and you’re out” hypothesis that I wrote about. For more see at least the abstract of the paper by C. Vance Hayes linked by Ray Percatr in comment #83.

    Andre (77) — Thank you. My main interest is in finding, if possible, an impact crater. But do note the hypothesis that YD is related to the draining of proglacial Lake Agassiz down the St Lawarence at about the same time as YD initiation; that such might have occurred some 200 years after the extraterrestrial impact says very little about the possiblity of ice dam weakening by such a strike.

    ike solem (80) — How about a lot of the Laurentide Ice Sheet being transported into the North Atlantic? Might take some time for that to melt. When it does the climate of Greenland and Europe suddenly warms.

    But there were so few humans at that pre-agricultural time that anthropogenic effects on climate would be very small. “The First Farmers” is an ok place to start reading about the origins of agriculture.

    Ray Percatr (83) — Thank you for this link! The paper is excellent, as one would expect from C. Vance Hayes.

    Comment by David B. Benson — 10 Jan 2009 @ 2:22 PM

  85. “The high concentration of magnetic grains with terrestrial composition at Gainey suggests that they are local ejecta from a nearby impact site. High water content was found in the magnetic grains at all sites, e.g. 18 at.% H at Gainey, which is consistent with their formation in a steam explosion following an impact into the nearby Laurentian Ice Sheet. Although no craters have been identified with the YDB impact, four deep holes extending to 193-723 feet below sea level in Lakes Superior, Michigan, Huron, and Ontario are candidates. Also in Lake Ontario, a proposed Pleistocene-aged 1-km impact crater has been identified at Charity Shoal where a negative magnetic anomaly also exists.”

    from Firestone, R. B.; West, A.; Revay, Z.; Hagstrum, J. T.; Smith, A.; Que Hee, S. S., “Elemental Analysis of the Sediment, Magnetic Grains and Microspherules from the Younger Dryas Impact Layer”, Fall 2008 AGU meeting abstract:

    Comment by David B. Benson — 10 Jan 2009 @ 3:04 PM

  86. re: #84

    re: early farmers, I agree.
    While I think Ruddiman has some reasonable evidence for his hypothesis of agriculture-related CO2 uptick starting around 8,000 years ago, I haven’t seen anything that makes sense for 13,000 years ago. “Plows, Plagues & Petroleum” Figure 9.2.

    Comment by John Mashey — 10 Jan 2009 @ 4:02 PM

  87. We’d have to find some other comet impact events to explain the Oldest Dryas and the Older Dryas periods as well – in addition to the few thousand other rapid temperature change events in the ice core data.

    Comment by John Lang — 10 Jan 2009 @ 4:42 PM

  88. John Mashey (88) — There were some proto-agricultuists, settled but not growing field crops, just harvesting what grew (together with some plant selection). The Jomon Jin in northern Japan and the Amur River basin

    and the Natufians

    with both cultures starting before YD and continuing through it are examples. I know of no others and would appreciate references (or even hints).

    Comment by David B. Benson — 10 Jan 2009 @ 5:01 PM

  89. #80 Ike

    I agree with most of your observations.

    However, there is still the assumption that a comet would have caused the YD by melting the ice causing it to flow into the North Atlantic.

    What if the comet caused the YD in some other way?

    If a comet struck a massive ice sheet, wouldn’t a large amount of the water go high into the atmosphere. What would be the effect of a large amount of water in the upper atmosphere? And what if it came down in frozen form in the days or weeks following scattered over a much large area in the Northern Hemisphere? Wouldn’t there be a dramatic increase in the planet’s albedo?

    Comment by Jim Cross — 10 Jan 2009 @ 5:39 PM

  90. John Lang (87) — No we won’t. Only Younger Dray is associated with extraterrestrial ejecta, end of Clovis culture, end of very many large mammal species, such a large change in Greenland and European temperatures (maybe North American as well).

    Comment by David B. Benson — 10 Jan 2009 @ 5:40 PM

  91. This is a fascinating discussion, great fun to watch over the shoulder a bit as paleoclimatological events get examined. I understand eric (and others) having a professional reluctance to invoke extraterrestial impacts as a cause for climate change, because it undermines the paradigm that humans are mostly responsible for the current energy imbalance (and that is a paradigm that I subscribe to, BTW).

    But I sense a stronger than usual antipathy to consideration of impact effects in the whole slew of Younger Dryas events.

    As an amateur, what makes the Younger Dryas interesting is both its rapidity of onset and exit, AND that it is concurrent with the retreat of the last great glaciation. As such, it’s likely that more of the details around the event should be discernable (they haven’t been scrubbed away by geological changes or subsequent ice ages).

    My opinion is that here’s enough evidence to support an impact hypothesis about the same time as the onset of Y-D. Fair arguments about what current observation is attributable to what effect. Perhaps not as a trigger, but as an accellerant to already proceeding processes.

    But let’s assume a frozen iceball (or multiple iceballs) kind of impact. Other posters have estimated how big the hole would be if it hit an ice sheet, and whether there would be a crater in the bedrock. But what are the estimated effects of the cubic kms of water vapor which suddenly get introduced into the atmosphere? Would it rain out locally and melt more glacier, or get transported away and regionally reduce ocean salinity?

    I hope someone with access to the right kind of model can take a poke at it. Let the idea rise and fall on its merits.

    Comment by Jerry — 10 Jan 2009 @ 5:58 PM

  92. David @88

    Yes, sorry for the ambiguity.

    When I wrote “I haven’t seen anything that makes sense…” I meant that I hadn’t seen any CO2 data that would hint at noticeable agriculture-induced changes. There might have been hordes running around cutting trees, but if so, the CO2 records don’t show it :-)

    Comment by John Mashey — 10 Jan 2009 @ 6:35 PM

  93. Jim Cross (89) — The assumption is that the impact(s) shattered the ice, enabling proglacial Lake Agassiz to flow to the sea. I had the impact program calculate for a sample impactor and copied the important results into a prior comment. For melt+vaporization, not so much.

    John Mashey (91) — I assure you no hordes, cutting trees or no. The proto-agriculturist societies amounted to so few people that I am sure there is no detecable impact on CO2 measurements from those times.

    Comment by David B. Benson — 10 Jan 2009 @ 7:42 PM

  94. “Three of the largest outbursts correlate closely in time with the start of large {delta}18O excursions in the isotopic records of the Greenland ice cap, suggesting that those freshwaters may have had an impact on thermohaline circulation and, in turn, on climate.”


    James T. Teller and David W. Leverington, “Glacial Lake Agassiz: A 5000 yr history of change and its relationship to the {delta}18O record of Greenland”

    Comment by David B. Benson — 10 Jan 2009 @ 8:03 PM

  95. #92 David

    The calculator you mention doesn’t have ice as a target option but, even so, using water as target and using average comet parameters for a comet the size of Hale Bopp (40 km diameter) gives this:

    The volume of the target melted or vaporized is 108000 km3 = 25900 miles3

    If you reduce this to 10km diameter, you get this:

    The volume of the target melted or vaporized is 103 km3 = 24.8 miles3

    If you use 5km diameter and increase the impact angle, you get this:

    The volume of the target melted or vaporized is 3.14 km3 = 0.752 miles3.

    Assuming a slightly higher density and returning the angle to 45 degrees, gives this for 5mk diameter.

    The volume of the target melted or vaporized is 5.6 km3 = 1.34 miles3.

    You can get a lot of different numbers.

    Comment by Jim Cross — 10 Jan 2009 @ 8:46 PM

  96. So… why didn’t the Long Valley Caldera explosion, (800,000 years ago) which injected 600 cubic kilometers of material into the atmosphere and which left massive deposits all across the western U.S., also lead to mass extinctions? It left a very visible crater, as well.

    Secondly, climate effects from volcanic eruptions tend not to last very long – a few decades or so. Even stratospheric aerosols don’t last very long.

    Thirdly, I’ve always heard of the termination of the Younger Dryas as the rapid event. For example, see the original Dansgaard paper in nature:

    PREVIOUS studies on two deep Greenland ice cores have shown that a long series of climate oscillations characterized the late Weichselian glaciation in the North Atlantic region1, and that the last glacial cold period, the Younger Dryas, ended abruptly 10,700 years ago2. Here we further focus on this epoch-defining event, and present detailed heavy-isotope and dust-concentration profiles which suggest that, in less than 20 years, the climate in the North Atlantic region turned into a milder and less stormy regime, as a consequence of a rapid retreat of the sea-ice cover. A warming of 7 °C in South Greenland was completed in about 50 years.

    All in all, it seems very unlikely that the Younger Dryas was initiated by a comet impact, impact or not. It also seems that the extinction of large animals (from 50,000 to 10,000 years ago) was due to the combination of human pressure and climate change – not due to some pulse event.

    Comment by ike solem — 11 Jan 2009 @ 2:01 AM

  97. The Firestone et al article, which got this impact hypothesis off the ground, was mainly an attempt to account for both the black mats and megafauna extinction. But think about it this way: There are two things we know certainly happened – the megafauna went extinct, and the black mats formed. Haynes 2008 paper also makes it pretty clear that the megafauna extinction immediately preceded the formation of the black mats. The most parsimonious way to begin thinking about what happened is to ask if the megafauna extinction could have caused the black mats to form. Instead, the approach taken has been to invoke something that we are uncertain about (the impact) to explain two things that we are certain about. This is a bit messy.

    Megafauna extinction is readily explained by human arrival. The link between megafauna extinction and black mats could be that the removal of most large herbivores resulted in increased deposition of organic matter in sediments, maybe along with raised water tables, as well as increased fire. There is evidence for these effects from other studies. The YD was probably part of another story, and the previous comments in this thread make me more doubtful than ever that the impact, if it happened, was part of that story.

    Comment by Chris Johnson — 11 Jan 2009 @ 2:10 AM

  98. I have to say, you boys flubbed this one. As a couple said above (#4 said it first), the problem with your critique is that you were critiquing something they didn’t say: One impact affecting the Younger Dryas equals all or many climate events being impact-related. Where did you get that from?

    That you wrote your article at all is a real head-scratcher. First time I can recall such a stumble on your parts.


    PS I’m having connection issues. Hope I haven’t double posted.

    Comment by ccpo — 11 Jan 2009 @ 3:00 AM

  99. Massive volcanism, asteroids, comets, etc… causing massive changes in the atmosphere rapidly, the lines of evidence are compelling, but the more drastic the events and consequences become, the further back in time we have to go. The age of the dinosaurs and their extinction is still hotly debated just as the far more recent last ice age. With so many variables and supplemental factors, this will not be an issue solved overnight, but climate change can be driven to the right quickly with some of the aforementioned events if they are of enough magnitude. I agree with Eric that the issue is not closed and the evidence is neither unequivocal or with a 99.5% level of confidence etc… I like the idea though because it makes so much sense and seems consistent with many findings. What I am wary of, however, is the tactics less reputable persons may use in interpreting such findings to downgrade AGW. That is my chief concern, because whether this really pans out, is largely shown to be of far less magnitude and thus influence, or abandoned in most of the primary literature, the science is doing a great job of looking for the answers… psychological techniques to downplay man’s influence on climate on the other hand is antithetical to this process.

    Comment by jcbmack — 11 Jan 2009 @ 3:57 AM

  100. Jerry suggests: “I understand eric (and others) having a professional reluctance to invoke extraterrestial impacts as a cause for climate change, because it undermines the paradigm that humans are mostly responsible for the current energy imbalance (and that is a paradigm that I subscribe to, BTW).”

    In no way does the impact theory have anything to do with the evidence for the cause of the current warming epoch. I believe Eric’s reluctance is truly skepticism. First impacts of celestial bodies sufficiently large to inflict hemispheric-scale damage are (thankfully) rare. Second, there are still reasons to be skeptical–the lack of a crater, for instance. Third, there could have been an impact, but it may have had nothing to do with the YD.
    I’m sure we all think this stuff is cool and encourage more research.

    Sometimes a scientific controversy is just a scientific controversy, and utterly unrelated to the debate between science and anti-science about climate change.

    Comment by Ray Ladbury — 11 Jan 2009 @ 8:25 AM

  101. I think some climate scientists have a degree of investment in the DO explanation. It is something that has happened multiple times and doesn’t seem random like a comet. It also plays into the “tipping point” narrative – warming reaches a certain point and suddenly bad things happen.

    Keep in mind that the article has to do with the finding of nanodiamonds in the YD sediment layer. Black mats, megafauna extinction, and the YD may be able to be explained in ways other than invoking the comet hypothesis, but I’m not sure the nanodiamonds can be explained as easily, alhough there is some controversy over whether the kind of nanodiamonds found are what would be expected from a comet.

    Comment by Jim Cross — 11 Jan 2009 @ 10:28 AM

  102. Ike excellent post #80. David Benson your doggedness is searching for answers serves us all well. If we had an index for the degree to which a post stayed on topic, this would have to rank at the top, great stuff. I want to return to the idea of Lake Agassiz inducing the YD regardless of what would have released the meltwater. The proponent of this initial idea was Broecker. He is concerned by the recent information not finding an identifiable pathway for this water and given the volume of water needed it would leave evidence. Broecker misses the point that even an under ice release would leave plenty of evidence of its passage. . Pay attention to the last two paragraphs. This more cautionary viewpoint is the result of several detailed studies that are not finding anything convincing about a drainage of Agassiz at the start of the YD

    We are getting ahead of ourselves to think about the shattering impact on the ice sheet of an impact, when we have not been able to document the flood event itself. That is conjecture built on conjecture not a strong foundation. It is also worth noting that an ice dam in this case is not likely some narrow delicate thing. We have an ice sheet advancing up

    Comment by mauri pelto — 11 Jan 2009 @ 10:34 AM

  103. First impacts of celestial bodies sufficiently large to inflict hemispheric-scale damage are (thankfully) rare.

    ‘Rare’ doesn’t tell us anything. The question is rather how often do they occur. Now we know they occur fairly regularly on millennial time scales, since we have two pinned down at 12,900 and 4800 years ago.

    Those are just the two we have hard evidence for. Surely there must be more, but one every 5000 years or so sounds about right, and fits the evidence thus far.

    Comment by Thomas Lee Elifritz — 11 Jan 2009 @ 10:56 AM

  104. > how often … fairly regularly … we have two pinned down …

    Wait — two data points suffice to determine … what?

    Comment by Hank Roberts — 11 Jan 2009 @ 11:51 AM

  105. > the removal of most large herbivores resulted in
    > increased deposition of organic matter …

    Interesting idea. Few people realize how incredibly efficient grazing animals are in wildland numbers — removing the large herbivores would be followed by an overgrowth of the tallgrass prairie, buildup of fuel and then episodes of increased wildfire. Just speculation.

    Comment by Hank Roberts — 11 Jan 2009 @ 11:55 AM

  106. Wait — two data points suffice to determine … what?

    Hemispherically catastrophic impacts on millennial time scales, I thought I was fairly clear on that statement.

    Add that to catastrophic earthquakes on yearly time scales and global warming on decadal time scales, and global economic collapses on centurial time scales.

    Compare that to where we are on mitigating any of this.

    Comment by Thomas Lee Elifritz — 11 Jan 2009 @ 12:01 PM

  107. Thomas, I’m afraid the mechanism for the putative YD trigger seems a bit vague. It seems to me that they’ve had to make some rather tenuous arguments to explain the YD as resulting from impact. What is more, even if we take two events in 13 K years, that puts the mean periodicity at anywhere from 2000 to 15000 years.

    I 100% agree wrt the misplaced emphasis for mitigation efforts.

    Comment by Ray Ladbury — 11 Jan 2009 @ 1:04 PM

  108. It seems to me that they’ve had to make some rather tenuous arguments to explain the YD as resulting from impact.

    I wasn’t making that connection, but clearly there is one that must be more fully refined. The big problem here is that the melting ice sheet washed away much of the evidence, and they are working the peripheries of it.

    Just like the rising seas washed away whatever was happening on the Bahama plateau for 100 thousand years.

    Clearly a catastrophic impact occurred at 12,900 B.P. Clearly also a catastrophic impact occurred at 4800 B.P.

    The evidence for these things is irrefutable, what remains is getting people out there to find more of them, and determine most probable impact scenarios for them.

    Captcha : inevitable value

    Comment by Thomas Lee Elifritz — 11 Jan 2009 @ 2:28 PM

  109. Mauri Pelto

    One of your links has a abstract that says this:

    “During the life of Lake Agassiz, 4 of the 5 largest catastrophic outbursts occurred at 12.9, 11.7, 11.2, and 8.4 ka cal yrs BP, which released 9500, 9300, 5900, and 163,000 km3 of freshwater, respectively (0.30, 0.29, 0.19, and 5.2 Sv if released in one year). Because these freshwater additions to the oceans occurred near the start of the three largest cooling events during this period of deglaciation–the Younger Dryas, Preboreal Oscillation, and the “8.2 ka cal yr cold event”–they may have been the trigger for changes in thermohaline circulation.”

    If I read this right, the amount of fresh water released during the 8.2 ka event was over 16 times what was released prior to the YD. Yet the 8.2 ka event was much less severe and shorter than the YD. Does this pose a problem for trying to explain the YD solely with Lake Agassiz release, whatever may have caused it?

    Comment by Jim Cross — 11 Jan 2009 @ 3:13 PM

  110. I have to agree with ccpo at 98. It reads like a repeat of the wrong end of the “CO2 leading/lagging indicator” argument.

    Comment by Jeffrey Davis — 11 Jan 2009 @ 3:40 PM

  111. ike solem (80) — Broecker’s paper, linked above, states the delta methane is unique to Younger Dryas, not found otherwise in Antarctic ice cores. Quoting from the Firestone et al. PNAS paper, “As evidence for biomass burning, Mayewski et al. (42, 43) reported large ammonium and nitrate spikes in the Greenland GISP2 ice core at the onset of the YD. These GISP2 data are consistent with strong geochemical evidence in the GRIP ice core for massive biomass burning at the YD onset, especially a major ammonium spike, in association with peaks in nitrate, nitrite, formate, oxalate, and acetate (44).” This suggests some chemical removal?

    A couple of years ago Scientific American had a very good article on super-eruptions, mostly about Mt. Toba, 74,000 years ago. There was a bit about Long Valley super-eruption. Other than species with highly constrained ranges, super-eruptions do not cause extinctions. With regard to humans and Mt. Toba it was a near thing however, one creating a definite east-west genetic divide; see the book by Oppenheimer mentioned in an earlier comment.

    All — Do recall that Bison bison survived the large mammal extinction event. So it is distinctly odd that the American lion did not, especially as there is no evidence of humans hunting them. Again quoting from the Firestone et al. PNAS paper, “It is likely that some now-extinct animals survived in protected niches, only later to become extinct because of insufficient food resources, overhunting, climate change, disease, flooding, and other effects, all triggered or amplified by the YD event.”

    mauri pelto (102) — Thanks. One certainly prefers to have good evidence. There is what I take to be a deep submarine canyon connecting Lake Mickigan and Lake Huron on the northern Lake Michigan bathymetric graphic from NOAA’s NGDC. (There is also a couple of features which might be impact craters.) An indicator of massive flooding, if any, ought to be the depth and size of the sediment deposits at the foot of the St. Lawerence rapids just west of Montreal, although subsequent floods or glacial re-advances might well spoil the evidence.

    That only a few thousand years of weathering can remove evidence is shown by stages of

    where driving along the Clark Fork one cannot recognize what occurred there (at least I could not). Again west of Longview, the 13,000+ years of heavy rain together with sea level rise has left no sign of those Bretz floods, to my eye.

    In any case, I’ve now seen enough evidence to find the case of ET air explosions and impacts into the Laurentide Ice Sheet compelling. Whether — and the extent to which — this YDB event promoted proglacial Lake Agassiz rapid drainage (about 150 years, start to finish) remains open. Detractors, however, are going to have to find an alternate hypothesis which takes into account to coincidence of timing between YDB an the onset of Younger Dryas itself.

    [reCAPTCHA entones "is disturbing."]

    Comment by David B. Benson — 11 Jan 2009 @ 6:22 PM

  112. Jim Cross says:

    If I read this right, the amount of fresh water released during the 8.2 ka event was over 16 times what was released prior to the YD. Yet the 8.2 ka event was much less severe and shorter than the YD. Does this pose a problem for trying to explain the YD solely with Lake Agassiz release, whatever may have caused it?

    An additional possibly relevant factor when comparing the 8.2ka event with the YD comes from work from Real Climate’s very own Stefan Rahmstorf:

    “Simulation of rapid glacial climate changes in a coupled climate model”

    The paper looks at the stability of various climate systems.

    Comment by SteveF — 11 Jan 2009 @ 7:36 PM

  113. Re: #111 (David B. Benson)

    Detractors, however, are going to have to find an alternate hypothesis which takes into account to coincidence of timing between YDB an the onset of Younger Dryas itself.

    I won’t opine on whether or not the impact event was a cause (or partial cause) of the YD, but I will mention that one explanation for the (possible) timing coincidence is: coincidence. Statistically, it does happen — in fact, coincidences are inevitable.

    [Response: Yes. This is a key point I made in the post: ".. the point is that if these events can happen as part of the inherent variability of the ocean-atmosphere-ice-sheet system, then there is no need to invoke the impact hypothesis in the first place. And indeed it would be virtually impossible to show it was other than mere chance that comet impacts occurred at the right time ...." --eric]

    Comment by tamino — 11 Jan 2009 @ 8:04 PM

  114. Also interesting to look at what can destabilize a glacier, though all the current observations are on mountain sites, not continental glaciers. I don’t know how to compare a large bolide to an avalanche.

    Rock avalanches that travel onto glaciers and related developments, Karakoram Himalaya, Inner Asia

    “… Glacier movement increased sharply at and below the rock avalanche deposits and, within a few months, the glacier surged. A second surge occurred 2 years later. Major slope failures and debris flows were triggered beyond the ice margins, and ponded melt water led to small outburst floods. By 2005 the landslide material had been transported some 9 km, about one third of this distance in the surges. Eventually it was fully reworked to become less readily distinguishable from other heavy supraglacial debris. A large area of thickened ice persisted where the debris reduced ablation; a positive impact on mass balance equivalent to a 20% increase in annual accumulation. However, it occurred as a moving segment of the ablation zone. In 10 years it had, in effect, replaced the mass transferred in the surges. Data for the mid-1980s indicate the rock avalanches exceeded pre-existing supraglacial debris by roughly five times and, over a 30 year period, will equal almost 500 years of normal supraglacial transport to the glacier margins. Other impacts of the episode suggest a doubling of this contribution to denudation….”
    —-end excerpt—–

    Similar developments were observed at three other glaciers

    EOS also just recently had an article documenting aa change in frequency of mountain avalanches, and included one that destablilized a glacier.

    Whether a large bolide going boom overhead packs anything like the impact of many tons of local avalanche, I dunno.

    Comment by Hank Roberts — 11 Jan 2009 @ 8:12 PM

  115. What is more, even if we take two events in 13 K years, that puts the mean periodicity at anywhere from 2000 to 15000 years.

    I’m sorry I didn’t address this comment earlier, but impacts aren’t periodic, nor are they even random, and they’re not even stochastic. Like global warming, the problem is nearly deterministic to first order. In theory these objects can be identified, cataloged and tracked nearly down to the individual, and lesser fluxes may be observationally verified on relatively short time scales. I think that lunar surface observations are helping here, and should be helpful in the future, as well as closer atmospheric observations and cataloging of fireballs.

    Comment by Thomas Lee Elifritz — 11 Jan 2009 @ 8:37 PM

  116. tamino (113) — Yes, that needed to be stated. What was a coincidence was that it appears that the Laurentide Ice Sheet was ‘surging’ off into the sea, calving rock-bearing iceburgs, rather like a small Heinrich event. Some have gone so far as to term Younger Dryas H0. In any case, this presumably somewhat preceeded YDB.

    But the short term effects of YDB included a massive release of black carbon, i.e., “soot”. Enough of that might well have encouraged accelerated melting of all the continental ice sheets. So possibly the dating is off and H0 was caused by the YDB explosion(s) and impact(s).

    Hank Roberts (114) — Thanks. The YDB events must have been far more destabilizing, except that the Laurentide Ice Sheet was quite flat. Still, your except is indicative of what might have occurred along the southern margins.

    Comment by David B. Benson — 11 Jan 2009 @ 9:02 PM

  117. In reply to Barton Paul Leveson’s comment #74:

    “William, the cores of planets are hot for two reasons: 1) Primordial heat, since planets form by accretion and the kinetic energy of the infalling material has to go somewhere. 2) Radioactive decay. For Earth, the planet we know best, those sources account very neatly for the known thermal structure of the planet, within the limits of present empirical knowledge. No need for exotic mechanisms.”

    I believe there is now consensus that it is not possible to explain the planetary heat balance anomaly with the radioactivity mechanism. At most ¼ of the current internal heat is due to gravitational collapse. The problem with the radioactive mechanism is an unrealistic amount of radioactive material is required which creates too much heat at early stages in the planet. (If the planet is too hot the core would not have solidified.)

    This article in the January, 2009 issue of Scientific American discusses the planets’ anomalously high internal temperatures. The article not one researcher hypotheses that dark matter could be the mechanism supplying the missing heat to the planets’ cores, however, experimentally detailed experiments have failed to directly dark matter by searching specifically for very small heat unbalance.

    Also, I find it hard to believe that an impact of a body would remove the dark matter from Uranus.

    …Jupiter, Saturn, Uranus and Neptune. If these massive worlds have gravitationally captured dark matter, then dark matter particles could smash into them—rare events but enough to heat up the gas giants and account for why the insides of these planets (and even Earth) seem hotter than known mechanisms can explain. It might also account for why Uranus seems anomalously cold—the planet is bizarrely tilted, perhaps because of a colossal impact, and Adler surmises that this collision might have knocked away most of the dark matter cloud that might typically have heated Uranus…

    Comment by William Astley — 11 Jan 2009 @ 9:08 PM

  118. > impacts aren’t periodic, nor are they even random

    Well, starting with the number of near misses reported since people started watching, we ought to be seeing some numbers about likely frequency. Is the assumption that these objects are randomly distributed broadly across the solar system, and we’re just seeing those within limits of our immediate attention? or is there some argument made that the’re more common crossing Earth’s orbit? Or even that they’re more common in the plane of the ecliptic? If they’re perturbed out of the asteroid belt or Oort cloud, wouldn’t the perturbation throw them into any possible orbital plane?

    Well, off topic, pointer welcome if this is summed up elsewhere.

    Comment by Hank Roberts — 11 Jan 2009 @ 9:15 PM

  119. Tamino, Eric, Ray, it does not seem to be just coincidence, I draw the line there. The impact hypothesis needs to be further studied and ongoing data collection and exploration of the alternative hypotheses need to be looked at with greater emphasis, not written off as it could be a coincidence or that it might be statistically insignificant, this is not proper use of mathematics or science. Asteroids and comets can alter climate in and of themselves greatly for the long term, however, many points in the Earth’s history cannot be clearly demarcated to reveal clear lines of cause and effect. Look at 100 years ago at the huge explosion in Siberia; we still have hundreds of ideas and hypotheses, could have been comets or asteroids, both, tectonic plate shifts, or some other combination. Point is chalking this comet hypothesis or early working theory for the last glacial period borders on ludicrous without any solid data to support that assertion.

    I am currently scouring many papers of geologists, paleo-
    biologists,astronomers, Astrophysicists, paleontologists and many expert interviews from History channel,, PBS and PBS,com as well and I have to tell you the data is pretty compelling and the arguments well thought out.When I finish I will cite all my sources.

    Comment by jcbmack — 11 Jan 2009 @ 9:35 PM

  120. To be sure there has to be coincidence at times and it is important to see through insignificant occurrences and those that actually telling data, but this is very premature, such speculation about a very very feasible cause and effect relationship or as a supplemental occurrence of still great magnitude and importance.We are not, however arguing over the bible code.

    Comment by jcbmack — 11 Jan 2009 @ 9:59 PM

  121. Hank wrote:

    “Few people realize how incredibly efficient grazing animals are in wildland numbers — removing the large herbivores would be followed by an overgrowth of the tallgrass prairie, buildup of fuel and then episodes of increased wildfire. Just speculation.”

    And not only the herbivores, apparently–it seems that such effects can ripple down all the way down from the top of the food chain:

    Comment by Kevin McKinney — 12 Jan 2009 @ 9:26 AM

  122. William Astley says: “Also, I find it hard to believe that an impact of a body would remove the dark matter from Uranus.”

    Wow, you’re a comedian, too!

    However, RE: core energy balance, a quick search finds:

    As with the climate, let’s try known physics before traipsing off into the unknown, shall we?

    Comment by Ray Ladbury — 12 Jan 2009 @ 9:36 AM

  123. we ought to be seeing some numbers about likely frequency.

    Actually it’s a fairly new science with observations only for 200 years, and only theoretical fluxes calculated. You have to remember that they are distributed in size as well as orbits, composition, etc, and a lot has to do with angle of entry, altitude of impact, etc. The fluxes of the larger objects is clearly underestimated, so for the last ten years or so we’ve been sitting around waiting for some data points, knowing full well we should be seeing them, wondering where they were, and thinking about oceanic fireballs and impacts, and finally fully realizing that possibly the flux calculations were wrong. Then someone hit on the idea of monitoring lunar impacts by their optical emission. My criteria for a data point was any large tracked fireballs with debris, visible cratering, etc. Finally there was on object that hit a mountainside in Europe somewhere, this Peruvian impact which finally restored our confidence in overall skill, and there was recently a large fireball in Canada with debris. That was a big boost for optical tracking, which is mostly done by amateurs. There is a long oral history of these things as well, so we need to prepare ourselves.

    The dragon will return to the forest, trust me. Rather than just waiting around for the statistics to reveal themselves, it’s much easier to track the larger ones. The statistics is just for the small and insignificant stuff, the regional, local and city wide catastrophes.

    Comment by Thomas Lee Elifritz — 12 Jan 2009 @ 10:19 AM

  124. I’m not sure how well vetted this site is:

    But I found the topic germain to this very interesting discussion.

    Comment by PeterB — 12 Jan 2009 @ 2:30 PM

  125. The site of Montreal underwater at the time of Yunger Dryas: “The sea lasted from about 13,000 years ago to about 10,000 years ago and was continuously shrinking during that time, since the rebounding continent was slowly rising above sea level. At its peak, the sea extended inland as far south as Lake Champlain and somewhat farther west than the site of Ottawa, Ontario.”


    and “After the Port Huron advance, the ice retreated above the Straits of Mackinac and exposed a lower outlet at Kirkfield, Ontario. This outlet lead into the Trent Valley from the Georgian Bay and into Lake Ontario. With this new, lower outlet, the waters of both Lake Michigan and Lake Huron were diverted from the Chicago outlet and into the Atlantic Ocean (Farrand, 1988). During this same interval the earliest lake formed in the Superior basin, Lake Keweenaw. This lake occupied about two-thirds of the basin.”


    so an established flow path from Lake Michigan-Huron to the North Atlantic appears to have existed at the time of Younger Dryas.

    If so, and assuing that a subglacial flow from Lake Agassiz across the current location of Lake Superior to the Straits of Mackinac area, I opine that little evidence for such might still remain.

    Comment by David B. Benson — 12 Jan 2009 @ 2:33 PM

  126. Re#111: “ike solem (80) — Broecker’s paper, linked above, states the delta methane is unique to Younger Dryas, not found otherwise in Antarctic ice cores.”

    Actually, the ice core records are synchronous, according to past research, with some variation in amount but not timing.

    Antarctic Ice Core Hints Abrupt Warming Some 12,500 Years Ago May Have Been Global (1998)

    James White, a paleo-climatologist at the University of Colorado at Boulder, said changes in stable isotope ratios — an indicator of past temperatures in the Taylor Dome ice core from Antarctica — are almost identical to changes seen in cores from Greenland’s GISP 2 core from the same period. “The ice cores from opposite ends of the earth can be accurately cross-dated using the large, rapid climate changes in the methane concentrations from the atmosphere that accompanied the warming,” White said.

    The atmospheric mixing time is very rapid (several hundred days?), and the lifetime of methane in the atmosphere is about one decade (modern conditions), so any increase in methane will be recorded globally, though gradients in amount can happen, which may indicate the difference between tropical and boreal increases or decreases. There is disagreement over the cause of the methane decreases and increases, but terrestrial ecosystems seem more likely.

    See also (pg 1336) for the methane ice core curves.

    Here is a study of the methane issue during four methane events in the ice core records from Greenland and Antarctica:

    …During these events, atmospheric methane concentrations increased by 200-300 ppb over time periods of 100-300 years, significantly more slowly than associated temperature and snow accumulation changes recorded in the ice core record…. We find no evidence for rapid, massive methane emissions that might be associated with large-scale decomposition of methane hydrates in sediments.

    In the case of the warming episodes – the lower tropical latitudes could be the primary drivers behind abrupt polar climate changes. Take a look at this study: (Science Jun 2008)

    “Greenland Ice Core Analysis Shows Drastic Climate Change Near End Of Last Ice Age”

    Both dramatic warming events were preceded by decreasing Greenland dust deposition, indicating higher tropical temperatures and significantly more rain falling on the deserts of Asia at the time, said White. The team believes the ancient tropical warming caused large, rapid atmospheric changes at the equator, the intensification of the Pacific monsoon, sea-ice loss in the north Atlantic Ocean and more atmospheric heat and moisture over Greenland and much of the rest of the Northern Hemisphere.

    “Here we propose a series of events beginning in the lower latitudes and leading to changes in the ocean and atmosphere that reveal for the first time the anatomy of abrupt climate change,” the authors wrote. White likened the abrupt shift in the Northern Hemisphere circulation pattern to shifts in the North American jet stream as it steers storms around the continent.

    That’s similar to the modern case – tropical clouds being a major uncertainty in climate models, for example, and warming at the poles driven mainly by heat transfer from the equator.

    The comet hypothesis doesn’t match any of that, being a cooling scenario. What it does look a lot like, however, is the “nuclear winter” scenario. For that, you’ll want to look to Rutgers & Alan Robock.

    Take a look at the nuclear winter scenarios, the most recent being here:

    Would that correspond to the effects of a rare swarm of comets air-bursting all across North America?

    I am starting to wonder if this is an archaeologically-inspired drama, with all the references to Clovis man and the overkill scenario. See Charles Mann, “1491″, Chapter 5 “Pleistocene Wars”. Basically, it now appears likely that humans were in the Americas as long as twenty or thirty thousand years ago, and in largish numbers, meaning that extinction of megafauna was likely a typically slow process (as elsewhere), not one driven by bands of recently arrived and overeager hunters in the classic Clovis overkill scenario proposed by C. Vince Haynes c. 1964. Note that the comet hypothesis goes along with the Clovis scenario, but avoids the need for rapid overkill.

    The place to look for climate change is things like ice core records and lake sediment cores – pollen types, isotope ratios, and so on. The human angle is obviously of great interest, as are the mammoths, but that’s not going to give you too much information about past climate change.

    Comment by ike solem — 12 Jan 2009 @ 3:22 PM

  127. One thought, I recollect from about water under the Antarctic currently that subglacial water can definitely flow ‘uphill’ in situations there’s higher water/pressure somewhere and a solid confinement between rock and ice through which the water can escape. So I wouldn’t assume the usual “water flows only downhill” rule applies for sure when trying to figure out where drainage occurred.

    Comment by Hank Roberts — 12 Jan 2009 @ 3:36 PM

  128. The main reason for a link to

    Feng Sheng Hu, Herbert E. Wright, Jr., Emi Ito, Kathryn Lease, “Climatic effects of glacial Lake Agassiz in the midwestern United States during the last deglaciation”

    is for the sketch map of proglacial Lake Agassiz II. That was one big lake!

    ike solem (126) — Thank you. I obviously was not sufficiently clear. According to Broecker’s paper no other such cold interval event shows a corresponding decrease in methane concentration. Another unique aspect of Younger Dryas, it seems.

    Comment by David B. Benson — 12 Jan 2009 @ 4:38 PM

  129. “The Lockhart Phase was terminated when the Kaministikwia route to Lake Superior was deglaciated, causing a rapid drop in lake level and the abandonment of the southern outlet (e.g., Fenton et al. 1983; Teller and
    Thorleifson 1983) (Fig. 1, outlet K).”


    David W. Leverington and James T. Teller, “Paleotopographic reconstructions of the eastern outlets of glacial Lake Agassiz”

    which is a web accessible pdf file to which I am not capable of posting a link.

    About Kaministikwia River, cf

    and its headwater is

    but other flow possiblities are suggested by this (long download time) geologic map of the area:

    Comment by David B. Benson — 12 Jan 2009 @ 5:16 PM

  130. L. H. Thorleifson, “REVIEW OF LAKE AGASSIZ HISTORY”

    appers to indicate flow to the Mississippi during the 12.9 ka — 11.6 ka period of interest, as well as the development of some flow to the east. This paper seems to ignore the possibilites of sudden flooding.

    Quite professionally done, as should be expected from a member of the Geological Survey of Canada.

    From just this paper, taken alone, one would find it difficult to support the idea of a sudden draining of Lake Agassiz initiating Younger Dryas. However, the connection between the latter draining out Hudson Bay and the 8.2 Kya seems to be quite well established. From that one can infer a causal connection for the earlier Younger Dryas initiation,

    Comment by David B. Benson — 12 Jan 2009 @ 6:21 PM

  131. “During the subsequent Moorhead Phase (about 11,000 to 10,100 14C yr B.P.), drainage was east to the Great Lakes region, via the Kaministikwia route to Lake Superior [7][8], and the lake became as large as about 190,000 km^2 and 20,000 km^3 [5].”



    This 2 page review paper is recommnded. I was earlier confused between the two dating systems, radiocarbon years and calendar years. Given the researchers and their propensity to write joint papers I can better appreciate the prior papers as well.

    The issue, as I see it, is how the lake water is to cross the continental divide just to the west of Dog Lake. Presumably the lake level was high enough, but there was a considerable amount of ice preventing flow to the east; see the map in this short paper. The ET impact hypothesis is that enough of this ice was seriously shattered, melted, vaporized or otherwise removed to initiate flow. To support this, one would need magnetic surveys of every one of the smallish lakes westerly of Thunder Bay. Then one would have to attempt to see if impact craters could be located and, ideally, dated.

    That no major flow channels seem to be present in this part of western Ontario is problematic. But I’ve been through there once on the Cnadain Transcontinental highway so I can assure you there is essentially no relief; it is flat country. So I am supposing that the flow channel(s) through the ice kept changing position; at that depp time the land would generally be sloping downhill to the north amd east. The assumption is that the flowing water kept eating away and the ice on its left bank and not so much into the bedrock.

    Comment by David B. Benson — 12 Jan 2009 @ 7:05 PM

  132. Just thinking.

    Predator and prey populations tend to achieve a sort of equilibrium. It could be that the equilibrium is near the sustainable harvest point of the prey. (Of course, there will be boom-bust cycles, but those could smooth out over time.) So, let’s imagine that the human and large prey animal populations have achieved this balance point about 13,000 years ago.

    Let’s carry on with the hypothesis that there was a large air-burst collision; something had to form the nanodiamonds. Further, let’s continue with the line that smaller animals would be better able to duck-and-cover than large animals. Under this line of reasoning, you suddenly have an imbalance between the smaller predators (man) and the large prey. It takes a generation or two to make cultural changes; so, the people are going to continue to hunt the large game and quite possibly drive them off into extinction where otherwise, without the people, they may have been able to recover.

    Now, if my understanding is correct, Clovis points are mostly spear tips. You can hunt large targets with spears; it is much harder to hit small targets with spears. So, if the large animals are becoming scarce, the people would have to shift to other technologies to survive.

    I can’t explain the survival of the bison; maybe it had to do with migratory patterns at the time.

    Comment by Chris G — 12 Jan 2009 @ 7:28 PM

  133. Chris G (132) — The Clovis culture peoples had bison for about 35% of their meat. With lots of bison and few people, I suspect that American lions were the primary predator of the bison.

    With regard to the mammoths (3% of meat), I seriously doubt that human predation had very much to do with the untimely extinction. Humans mostly hunted immature male mammoths; probably lions did as well. In any case this would not have changed the population dynamics of the mammoths by much.

    There is no evidence of the use of Clovis points after YDB. It could well be that none of those who knew how to make Clovis points survived YDB. The closest subsequent culture appears to be Fulsom point culture. The Fulom point is smaller, enough for bison but not for the (extinct) mammoths perhaps.

    But in addition, many smaller species of mammals and also birds also became extinct at the time of this event.

    Comment by David B. Benson — 12 Jan 2009 @ 8:06 PM

  134. Re David Benson’s links @125, coincidentally, this past summer I canoed much of one former outlet along parts of the French River, Trout Lake and the Mattawa River.

    Comment by Jim Eager — 12 Jan 2009 @ 8:23 PM

  135. Oops. With regard to perscentages, I seem to have misremembered. Here is a paragraph from the C. Vance Haynes, Jr., paper previously linked:

    “Of the 70 sites in SI Table 2, 56 (80%) have skeletal elements of the Rancholabrean megafauna directly underlying the YD black mat (Table 1). Approximately 38.6% have mammoth remains, 37.1% bison, 8.6% horse, 7.1% camel, 2.9% mastodon, and 8.6% other extinct-species remains, all on the Clovis-age surface, and only bison remains appear in the overlying YD black mat.”

    which actually dosn’t determine food proportions.

    Comment by David B. Benson — 12 Jan 2009 @ 8:32 PM

  136. ike # 126, I like your post, but disagree with your conclusion a bit… The study of human populations at different time periods and correlating them with approximations of the climate along with other animal populations can be very useful. Archaeologists offer an important component of study to all sorts of things including possible climate and triggers/consequences thereof.

    I know what you are saying and can agree for the most part with your line of thinking, however, ice cores and such are not the only ways to get good inferences about climate long before the historical record. There is much dispute actually highlighted regarding the Pleistocene era in this month’s Scientific American (pop evolutionary psychology) and excellent articles on evolution. At any rate I enjoyed reading your post and your references; if you have more I would be happy to read them as well, but I think we are getting ahead of ourselves on this issue of YD, especially what limitations we have in regards to inferring climate changes that wiped out or changed life form bio mass and diversity.

    Comment by jcbmack — 12 Jan 2009 @ 8:51 PM

  137. Of course Ike my own view is influenced by my Biology training and the desire that more biological realities including paleontological be included in climate science in several applications. I am not saying that Biology or Archaeology is completely ignored, but rather that looking at more data from these areas may improve our understanding of paleo climate. There are pretty good dynamic vegetation models and certain other Biological facets being modeled as well. I think fossil data can be revealing even if incomplete and oftentimes the source of further questioning.

    Comment by jcbmack — 12 Jan 2009 @ 9:11 PM

  138. Ike, I’ve been following your articles with much interest. Do you have an explanation for why ammonia (in the ice record) will go up following a major forest burn event, or an impact for that matter?

    Comment by Pt — 12 Jan 2009 @ 11:43 PM

  139. Using
    Spahni, R., et al., 2005.
    EPICA Dome C CH4 Data to 650KYrBP.
    IGBP PAGES/World Data Center for Paleoclimatology
    Data Contribution Series # 2005-078
    NOAA/NGDC Paleoclimatology Program, Boulder CO, USA.

    I checked, by eye, for an negative methane spike remotely similar to that of Younger Dryas, about -180 ppbv. The 8.2 kya event shows up with around -20 ppbv and no earlier excursions appear to be more than +-70 ppbv per sampling interval. Granted the resolution grows coarser the deeper the time, but I didn’t notice any other, roughly similar short negative spikes. In magnitude combined with short duration, Younger Dryas methane change appears to be a unique event in the last 650,000 years.

    Comment by David B. Benson — 13 Jan 2009 @ 5:10 PM

  140. PT, if you look up “forest fire” and ammonia in Scholar, and poke around, you’ll find ammonia comes out into the air as one result of a lot of natural processes; forest fires would be one very rapid process putting a pulse of ammonia into the air. If you want the specific chemistry involved you’ll have to look deeper than I did!

    Comment by Hank Roberts — 13 Jan 2009 @ 7:03 PM

  141. “Our proxies, combined with planktonic δ18O seawater and δ13C, confirm that routing of runoff from western Canada to the St. Lawrence River occurred at the start of the Younger Dryas, with an attendant increase in freshwater flux of 0.06 ± 0.02 Sverdrup (1 Sverdrup = 106 m3·s−1). This base discharge increase is sufficient to have reduced Atlantic meridional overturning circulation and caused the Younger Dryas cold interval.”

    from A.E. Carlson, P.U. Clark, B.A. Haley, G.P. Klinkhammer, K. Simmons, E.J. Brook and K.J. Meissner, Geochemical proxies of North American freshwater routing during the Younger Dryas cold event, Proceedings of the National Academy of Sciences 104 (2007), pp. 6556–6561:

    “… in records of atmospheric methane concentration mainly reflecting precipitation and temperature impacts on boreal and tropical wetlands …”

    from Anders E. Carlson, Why there was not a Younger Dryas-like event during the Penultimate Deglaciation, Quaternary Science Reviews, Volume 27, Issues 9-10, May 2008, Pages 882-887: (link in footnote of Eric’s main post).

    Comment by David B. Benson — 13 Jan 2009 @ 7:27 PM

  142. Regarding the black mats, there is an interesting article:

    “Strata above and below the black mat reflect drier conditions with lowered water tables, and buried features such as wells dug by humans or animals and dry spring conduits indicate fallen water tables and drier conditions. Fallen water tables commonly result from a relatively warm, dry climate, whereas black mats, as used here, may have accumulated under conditions of increased precipitation and/or colder climate when rainfall is more effective in recharging water tables because of reduced evapotranspiration.”

    Many posts here and the main article arguing for the comet itself ascribes the black mats to wildfires, presumably caused by a large comet explosion over N. America.

    Comment by Jim Cross — 13 Jan 2009 @ 7:44 PM

  143. Anyone tested for DNA in the black mats?

    Comment by Hank Roberts — 13 Jan 2009 @ 9:21 PM

  144. Adding some additional thoughts to my last post.

    To believe that the YD was caused by something like a DO event not caused by a comet, one has to accept the following scenario occurring within a few years or decades:

    1- Megafauna and clovis people are alive

    2- Something deposits a layer of magnetic microspherules and other possible ET evidence.

    3- The DO event occurs unrelated to the ET effect.

    4- The climate grows dramatically colder and wetter. (By the way, do any of the other DO events show a black mat possibly caused by a colder and wetter transition?).

    5- Megafauna and clovis people are gone.

    6- We begin 1300 years of relatively cold climate.

    Comment by Jim Cross — 13 Jan 2009 @ 9:22 PM

  145. Regarding the CH4 oddities with the YD.

    Much of the methane contribution to the atmosphere comes from the peat bogs of Siberia.

    There is some evidence that these peat bogs expanded greatly at the transition to the Holocene from the YD.

    Whatever happened may have caused a massive die-off and suppression of CH4 production at the beginning of the YD. The die-off might have caused an expansion of the peat bogs. This would have been followed by jump start at the transition to the Holocene possibly caused by the insolation increase which was ongoing (as long as we are looking for tipping points).

    Comment by Jim Cross — 13 Jan 2009 @ 10:50 PM

  146. NH4 would also be given off from burning plant life and bacteria. Environmental Chemistry should clear up some basic issues; they are available on scholar.

    Comment by jcbmack — 13 Jan 2009 @ 11:57 PM

  147. #141 In the Carlson paper two additional key points are 1. They note several times that there was a progression of increasing values from the start of the yd to 12,500 and a bigger change at this point. This does not argue for a catastrophic release, but the opening of a new outlet that continues to expand its drainage area. Second there Figure 1 map provides ice marginal boundaries that do not fit the best data we have which shows ice advance across Lake Superior overrunning several forests in northern Michigan post 12.5. This map from Dyke (2004), which is based on earlier work as it is just a compendium map allows for the Lake Agassiz drainage but as the other papers that postdate Dyke indicate this drainage way is not evident. The increased flow through the St. Lawrence at the start of the YD could also be related instead to Lake Warren-Lake Whittlesey changes as well.

    Comment by mauri pelto — 14 Jan 2009 @ 7:49 AM

  148. Jim Cross (144) — Whatever Younger Dryas cause, it was not a DO event. DO events are warming events, not cooling. One of the earlier comments includes a link to a paper regarding THC modes which I found quite helpful.

    mauri pelto (147) — Thank you. As best as I can make out Younger Dryas initiation requireed about 150 years to full development. That suggests a more gradual release of proglacial lake Agassiz from Lockahart stage to early Morehead stage.

    But indeed, there is no reason for the fresh water to have all (or any) originated from Lake Agassiz (except that the flow had to go somewhere). Indeed the freshening could have been enhanced by other releases.

    Comment by David B. Benson — 14 Jan 2009 @ 5:04 PM

  149. #148

    Check posts #14 and #8 with Eric’s answers regarding warming and cooling.

    Comment by Jim — 14 Jan 2009 @ 5:33 PM

  150. The introduction linked below gives a general overview of the retreat of LIS from the Great Lakes, but the details may no longer agree with evidence developed later. In any case, here is a quotation: “Figure 9: Lake Algonquin stage was initiated when the Trent valley outlet was dammed by a local readvance. (about 11,000 years ago)” wherein the figure clearly indicates flow to the Strait of St. Lawerence from all the Great Lakes.

    Even if Lake Superior was solid ice, it seems to me that all that is necessary to initiate eatward flow from proglacial Lake Agassiz is for the water to move over the continental divide a few tens kilometers west of Thunder Bay. Subglacial and ordinary flow does the rest.

    Comment by David B. Benson — 14 Jan 2009 @ 5:50 PM

  151. Jim (149) — With all respect, from

    we have “In the Northern Hemisphere, they take the form of rapid warming episodes, typically in a matter of decades, each followed by gradual cooling over a longer period.” and “The course of a D-O event sees a rapid warming of temperature, followed by a cool period lasting a few hundred years (Bond et al. 1999).” These agrees with every text I have studied or even glanced at. Younger Dryas begins with a fairly rapid cooling (of dramatic amplitude in Greenland) followed about 1300 years later by a most dramatic warming.

    For a view of THC hysterisis, etc., to partily explain this, see the link in comment #112 by SteveF.

    Regarding my #150, perhaps somewhere between Figures 8 & 9 of the linked pdf paper is appropriate. Quoting from that paper, “The ice front continued its retreat in the Upper Great Lakes area ultimately moving north of and freeing the Straits of Mackinac and also uncovering a lower outlet at Kirkfield, Ontario, leading into the Trent Valley, across southwestern Ontario from Georgian Bay to Lake Ontario.”

    The timing is a rather delicate matter or else one needs posit a very much larger ET object explosions and impacts. However, for this timing see

    Louis J. Maher, Jr. and David M. Mickelson, “Palynological and Radiocarbon Evidence for Deglaciation Events in the Green Bay Lobe, Wisconsin”, Quaternary Research, Volume 46, Issue 3, November 1996, Pages 251-259:

    with a quotation “… the Straits of Mackinac were free of ice about 13000 yr …”

    Comment by David B. Benson — 14 Jan 2009 @ 6:41 PM

  152. Here is termination 3 methane

    238199 650
    238623 500
    238943 554
    239249 538
    239545 522
    240205 498
    240576 484
    240966 470
    242067 509
    243657 402
    244198 437
    244861 401
    245481 380
    246709 405
    247436 399
    248083 424
    248977 447
    249751 465
    250460 467
    251519 421
    252183 410
    252957 511
    253889 486
    255230 398
    256038 408
    256489 385

    Petit, J.R., et al., 2001,
    Vostok Ice Core Data for 420,000 Years, IGBP PAGES/World Data Center
    for Paleoclimatology Data Contribution Series #2001-076.
    NOAA/NGDC Paleoclimatology Program, Boulder CO, USA.

    in which

    240205 498
    240576 484
    240966 470
    242067 509

    might be considered like Younger Dryas, although only -79 ppbv, but note that is preceeded by

    242067 509
    243657 402
    244198 437

    which is preceeded by

    244198 437
    244861 401
    245481 380
    246709 405

    which might also be thought of as smaller Younger Dryas-like. Using opposite phase, some of these could be considered represetative of DO events, I suppose. By way of comparison, here is termination 1, from LGM to Holocene:

    10189 667
    11013 621
    11143 511
    11329 477
    11719 501
    12626 662
    13412 610
    13457 642
    14241 445
    16417 395
    17695 363

    since we now where to look, Younger Dryas is within

    11013 621
    11143 511
    11329 477
    11719 501
    12626 662

    (providing another reason to doubt the Petit et al. attempt at dating Vostok ice core), where the delta methane agrees moderately well with that obtained from GISP2.

    To sumarize, termination 3 appears, in this record, rather more complex, with many more muted events, that termination 1. This amateur opines that termination 3 did not actually have a true Younger Dryas like event according to just this data.

    Comment by David B. Benson — 14 Jan 2009 @ 9:17 PM

  153. I was trying to determine whether the increased ammonia concentration could have been due to a change in the nitrogen cycle. Ammonia has a low residence time (

    Comment by Pt — 15 Jan 2009 @ 12:06 AM

  154. The hydrogen peroxide data from GRIP during the YD period and beyond looks most interesting. H2O2 rapidly decreases at 11.9kya:
    11965 4.53
    11982 2.5
    11998 2.09
    12013.5 1.77
    12029 0
    12044 0
    12062 0
    … (these must be low-detects because missing data is denoted by 9999)
    12622.5 0
    12638.5 0
    12651.5 0
    12665 1.23
    12677.5 5.48
    12689.5 9.2
    12702.5 16.16
    12713.5 14.97
    12724 6.57
    12735 29.82
    12746 5.38
    12758 7.48

    It stays at a low level, with fluctuations 14.5 kya. The question is whether there was high SOx in the atmosphere to wash off any H2O2 in the atmosphere, or was this due to change in the ozone cycle?

    Comment by Pt — 15 Jan 2009 @ 12:15 AM

  155. #151

    I am not disputing that the DO event is warming but it is followed by cooling. My original post is somewhat confusing because I speak of a DO event as if it were something that happens in a year or two. I really meant that the transition from warming to cooling would have had to take place coincidentally within a few years or decades after some sort of ET event because the black mat, which is caused by cold and precipitation (see my earlier post), is directly on top of the layer containing the ET evidence.

    The same would go for any other explanation – freshwater melt, some cyclic change in the N. Atlantic.

    In other words it would be incredible coincidence that almost immediately after the ET event the climate cools for some other reason unrelated to the event.

    What’s more, as many posts here have pointed out, The YD looks different from other climate episodes of this sort (although Eric disputes this). It seems to have started more abruptly and has the odd methane behavior. There is also the black mat presence which is suggestive of unusually high precipitation along with cold. Can the black mat be found at any of the other cooling/warmings in the Holocene? This suggests that the usual explanations – DO, freshwater in the NA, etc-. – which may be correct for other episodes at least isn’t the complete story here.

    Comment by Jim Cross — 15 Jan 2009 @ 7:35 AM

  156. My #151 post didn’t fully get through. If you examine the ammonium ion concentration, it reaches a max. value of 76 ppbw about 500 years after the proposed 12.9 event. Doubtful that it would take that long for any “ammonia” related event to precipitate out in the ice record.

    Comment by Pt — 15 Jan 2009 @ 9:28 AM

  157. Jim Cross (155) — Younger Drays and, to a lesser extent, the 8.2 kya event both begin by abrupt cooling followed by warming. This is ‘upside down’ from the definition of a DO event. I’ll agree that both are most likely examples of THC transition events.

    Both YD and the 8.2 kya event have methane downturns; that for YD is much larger and indeed much larger than any I can find in methane records. Regarding black mats, it seems these have only been located during archaeological digs (although I am unsure about the ones in Europe).

    The usual explanation of freshwater into the North Atlantic causing the THC transition seems perfectly reasonable since this occurred at times other than YDB. The magnitude of YD appears to be unusual, not to mention the other YDB evidence.

    Pt (156) — Interesting What about the other trace substances mentioned by Firestone et al.?

    Comment by David B. Benson — 15 Jan 2009 @ 2:15 PM

  158. James T. Teller, Matthew Boyd, Zhirong Yang, Phillip S.G. Kor and Amir Mokhtari Fard, “Alternative routing of Lake Agassiz overflow during the Younger Dryas: new dates, paleotopography, and a re-evaluation”, Quaternary Science Reviews, Volume 24, Issues 16-17, September 2005, Pages 1890-1905 states “This research raises questions about whether catastrophic overflow from Lake Agassiz ever spilled into the Superior basin near Thunder Bay, and even whether overflow from Lake Agassiz ever was routed into the Superior basin through the Thunder Bay area. The geomorphic form of channels west of Thunder Bay does not suggest that large overflows from Lake Agassiz sculpted that region, although the topography along the lower, more northern route over granite to Thunder Bay, via Dog Lake and the Kam Valley, is a possibility. As well, the absence of coarse-grained fluvial sediment at the surface in valleys west of Thunder Bay, previously interpreted as Agassiz spillways, indicates that if strong overflow ever did occur, that record must lie buried, perhaps by till or glaciofluvial and glaciolacustrine sediment associated with the Marquette ice advance about 10 14C ka; alternatively, this overflow may not have been catastropic.” and suggests a routing to the northwest during Younger Dryas.

    John A. Rayburn and James T. Teller, “Isostatic rebound in the northwestern part of the Lake Agassiz basin: Isobase changes and overflow”, Palaeogeography, Palaeoclimatology, Palaeoecology, Volume 246, Issue 1, 23 March 2007, Pages 23-30 offer new evidence for the possiblity of flow thrugh the Clearwater-Athabaska route to the arctic.

    The first article makes clear that from the Lakehurst stage to the early Morehead stage, glacial Lake Agassiz dropped 100 meters; that is a lot of water. However, it seems that Younger Drays took about 150 years to fully form and so flow may not have been that catastropically large. Another possiblity, suggested by the double outflows of General Carrera/Buenos Aires Lake

    that flow to the east and to the northwest existed simultaneously.

    Both exits require that either the ice sheet melts back, or else the ET object impacts breached the ice daming the exits. Nothing is known about the ET impacts in the Clearwater area; for the Thunder Bay area the evidence suggests the possiblity of such breaching.

    Comment by David B. Benson — 15 Jan 2009 @ 5:10 PM

  159. #157

    I am not arguing that the YD is the cooling phase of some variant of a DO-like event. But I don’t think it is a simple freshwater into the N. Atlantic event either whether caused by a comet or some other mechanism.

    I think it is more likely a variant of Ike’s post #126 suggesting a nuclear winter type event.

    The black mat is found over a wide range in N. America including areas including the Western US, the Southwestern US, and Texas. Before the event, there were clovis people and megafauna. After it, there are none. I don’t see how some other hypotheses would explain that.

    The original article on the relationship to an ET event and the YD suggests a 4km comet nucleus (comets are frequently between 1-6Km in size), possibly split in two, and hitting at an oblique angle or a complete vaporization of a smaller comet at an altitude that could impact all of N. America.

    Comment by Jim Cross — 15 Jan 2009 @ 6:09 PM

  160. Isn’t it plausible that events were leading up to the YD, and that a comet strike simply accelerated something that would have happened later anyway?

    In other words, not all such events would be triggered by comets (indeed, that could be quite rare), but this particular event might have had it’s timing influenced by comet.

    Not an expert, just musing…

    Comment by James McDonald — 16 Jan 2009 @ 12:58 AM

  161. David:
    Another way to interpret the H2O2 record is that during melting the H2O2 is released back into the atmosphere; it still leaves open the question of how the H2O2 is created, and stored in the ice record. If you take the H2O2 release scenario during melting, then during the YD initiation period there isn’t much melting going on. It in fact about 1,000 years after that we see melting — major melting due to the steepness of the curve. H2O2 has been steadily going up since 12.0kya, and infact, looks it’s stabilizing.

    Not sure how Firestone et al explain the ammonia and H2O2 observation.

    One other thing that the impact folks miss: it’s much faster to melt an ice pack from underneath, than from above (blow torch vs. a hot pan).

    Comment by Pt — 16 Jan 2009 @ 9:40 AM

  162. Interesting poster here suggesting something happended up that way about that time:

    Comment by Ray Percat — 16 Jan 2009 @ 12:00 PM

  163. Jim Cross (159) & James McDonald (160) — Thgere are several other ‘cold reversals’ (CR) during terminations. The sharpest evidence is from termination 1, which includes Younger Dryas (YD). Only YD seems to show such a great drop in methane concentration and, for termination 1, such a great drop in temperature in Greenland. It is reasonable to assume that the YDB imapcts enhanced what would have been a more ordinary size CR. See for others

    Pt (161) — Firstone et al. don’t explain the unusual chemistry, just note the correlation to the 12.9 kya YDB events. If that is when the anomalous ammonia begins, then I’ll offer the conjecture of repeated Great Plains grass fires for the 500 years it took for B. bison to completely recover from YDB. As for hydrogen peroxide,

    “Hydrogen peroxide is naturally produced as a byproduct of oxygen metabolism …” but otherwise I have not even conjectures.

    The YDB impacts I take to be shattering ice, not particularly melting it (except in the impact crater).

    Ray Percat (162) — Thank you for the notification regarding the southwest Nova Scotia impact! So from the Firestone et al. Fall 2008 AGU abstract we have a potential crater in eastern Lake Ontario (and maybe even Oman). Now we have another. (Unfortunately, I am unable to actually read the poster. Software limitations here.)

    Comment by David B. Benson — 16 Jan 2009 @ 2:48 PM

  164. David, The melting point of H2O2 (-0.41 deg C) is very close to that of water. What’s unmistakable is that there are very low H2O2 concentrations (near 0) from about 12.6 to 11.6 kya (sorry, earlier I said 12.0kya). You can check yourself for GRIP on the noaa/paleodata site. From 11.6 kya – 8 kya it increases steadily.

    Comment by Pt — 16 Jan 2009 @ 6:00 PM

  165. Pt (164) — Thank you, but I fear I am such a poor chemist that I can draw no sound inferences whatsoever from the hydrogen peroxide data.

    I do conjecture that the far North Atlantic was filled with ice during Younger Dryas. Does the H@)@ data tend to support this?

    Comment by David B. Benson — 16 Jan 2009 @ 6:34 PM

  166. Oops. H2O2 data.

    Comment by David B. Benson — 16 Jan 2009 @ 6:45 PM

  167. “Two drainage events of the [Baltic] Ice Lake occurred at Mt. Billigen in south-central Sweden and are attributed to the melting and later re-advance of glacial barriers there (Bjeorck 1995). The first event at 10,900 [radiocarbon] years, lowering the water level by 5 to as much as 15 meters. The final drainage triggered by the weakening ice giving away to 25 meters of water level, occurred in distanct steps around 10,300 [radiocarbon] years over a few year …”

    K.-C. Emeis et al., “Post=lacial Evolution of the Baltic Sea”, in G. Wefer et al. (eds), “Climate Development and History of the North Atlantic Realm”, Springer, 1999, page 210.

    Since 10,900 radiocarbon years calibrates to 12,900 calendar years, it seems probable that the Baltic Ice Lake contributed somewhat to Younger Dryas initiation.

    Comment by David B. Benson — 16 Jan 2009 @ 7:26 PM

  168. YD does not get a specific mention but a billionaire had/has investment in finding out (?)

    The search command was “sex + cause of global cooling”, Lol

    Comment by Sekerob — 17 Jan 2009 @ 3:25 AM

  169. A gold prospector would see a flake of bright shiny gold colored stuff and could plausibly assume it is gold. But this is a likely a false assumption and could lead to bad investment. It would be a plausible assumption, that needs to be tested before it could be a reasonable assumption. What we have seen is that there several sources of meltwater pulses near the start of the YD most are reported as progessive changes, none seem to have been catastrophic in nature. The releases are from general ice retreat preceding the YD, all of these were not due to an impact certainly , so why should any of them be? Where is the evidence? We have ice sheet retreat that makes sense without invoking an impact, does it make sense with an impact? A reasonable assumption needs evidence to back it up, a plausible or possible assumption still needs basic investigation.

    Comment by mauri pelto — 17 Jan 2009 @ 7:47 AM

  170. #169

    Are you disputing the evidence of any sort of ET impact near the beginning of YD? How would explain the geologic record – the ET evidence overlaid by the black mat?

    I agree that we don’t need an ET event to explain the the freshwater releases. But do we need something more than the freshwater releases to explain the YD?


    Firestone et al explain the ammonia from biomass burning. I don’t see an explanation of the H2O2 in their article.

    Comment by Jim Cross — 17 Jan 2009 @ 9:51 AM

  171. If there was a sudden massive drainage, then there ought to be sites, lots of them, in which there is extensive lake bottom covered with the black mat. Right?

    If there was an “ordinary” slow event, even one that happened to be pushed over the brink by an impact, like say one wave cresting a longterm ice/gravel dam that had been brimfull, got a notch cut in it, and then started to drain faster and cut a drainage channel so it continued to drain faster — then whatever the black fallout material is would mostly have covered either dry land or current lake water, and we would not find extensive lake bottom areas covered with black mat.

    That’s logic. Silly stuff when applied to the utterly unknown by the completely uneducated. But still, does it make any sense as a thought about where to look to distinguish a slow from fast event?

    If it were possible to find and use the black layer — heck, it ought to show up in every water and gas and oil well drilled, and in most road cuts, if anyone’s looking, shouldn’t it?

    Comment by Hank Roberts — 17 Jan 2009 @ 10:03 AM

  172. If you look at the map in this:

    You will see that the black mat sites are all over the western and southwestern US. They are not in the path of a drainage event into the N. Atlantic.

    If they are caused by cold and high precipitation, then possibly the YD itself caused high precipitation in these areas. That’s why I have been asking about the other cooling events leaving black mats. Is high precipitation in the western US associated with these other events?

    The other possibility is that a comet vaporized a large part of the ice shelf sending an enormous amount of water into the atmosphere over these areas. Another possibility is a very large comet actually grazed the atmosphere and shed a large amount of water from itself. That’s a very remote possibility but it would leave no impact crater.

    Comment by Jim Cross — 17 Jan 2009 @ 12:18 PM

  173. Correction: the Copyright date of the book mentioned in comment #167 in 2002.

    Jim Cross (172) — The conditions for forming black mats may involve a combination of temperatures and precipitation such that other cold reversals (CR) don’t form such. For example, the global temperatures were higher during the 8.2 kya CR.

    As I see the evidence reported on this thread so far, the ET object was most likely not a comet; there is evidence of materials highly similar to those picked up from the lunar surface, for example.

    All — Just to set in perspective, the Baltic Ice Lake final discharge, here given as 25 m drop at 11,600 years BP, was about 7,800 km^3 discharging 0.12 to 0.25 Sv:….57..355J

    and, interestingly, about 300 years before the end of Younger Dryas,

    with some helpful maps:

    From Hui Jiang, Nils-Olof Svensson and Svante Björck, “Meltwater Discharge to the Skagerrak–Kattegat from the Baltic Ice Lake during the Younger Dryas Interval”, Quaternary Research, Volume 49, Issue 3, May 1998, Pages 264-270:

    the flow was directed into the Nordic Sea.

    I estimate the initial volume, at the time of YD initiation, at about 1,400—4200 km^3. I take this as equally catastropic, so a flow of about 0.02 Sv to 0.13 Sv.

    This is to be compared to the 9500 km^3 from the 100 m drop of Lake Agassiz. If this took 100 years, the flow was but 0.003 Sv; if 10 years, then 0.03 Sv. I tentatively conclude the contribution from the first release of the Baltic Ice Lake was important, expecially as it likely all went directly to the Nordic Sea.

    Comment by David B. Benson — 17 Jan 2009 @ 3:19 PM

  174. David – Assuming that increases to H2O2 concentrations are indicative of melting (I’m ~80% sure), and that decreases to H2O2 concentrations are indicative of freezing (about ~20 % sure), I counted 4 melting/freezing cycles before YD initiation. The number of cycles corresponds to the d18O record. At 12.651 kya H2O2 is zero (no melting) and stays at a very low value (near 0) for another 1,000 years until 11.587 kya, after which time it shoots up to 28.25 ppb at 11.552 kya. The H2O2 has been on steady increase to present with melt-freeze fluctuations much stronger than before YD. Implication: whatever phenomena caused the plunge into YD, isn’t the one that caused the northern hemisphere to get out of the freeze and keep the current warming trend going.

    Hopefully someone else will shed light on how melting of sea ice can be distinguished from land ice. I see lots of ice in the NA and no melting at all during YD.

    Comment by Pt — 17 Jan 2009 @ 4:14 PM

  175. Pt (174) — Data from

    (previously cited) shows a local maximum temperature at 13.588 kya

    13.588 -38.4303

    followed by a declining trend to

    12.92 -45.9021

    where it falls off a cliff

    12.887 -48.0428

    with the next local minimum being about 182 years later:

    12.705 -50.13

    I hold the view that YD cold was caused by the abrupt change in THC caused by all the fresh water being dumped into the North Atlantic by drawdown of proglacial Lake Agassiz (LA) and the Baltic Ice Lake (BIL). This is hardly original with me and agrees with what is thought to have occurred at the later 8.2 kya CR.

    One unique aspect of YD, IMO, is the depth of the cold. Via methane concentrations, it was the deepest draw down (of methane) in three terminations by a factor of about 2.5 greater than the next, during termination 3.

    Also, rather remarkably, both LA and BIL began dumping water at very close to the same time. While having the two ice dammed lakes fail with a few centuries of another another might not be very surprising, the synchrony of the failures and at the same time as the YDB event is, at the least, suspicious.

    Be that as it may, do say more about “I see lots of ice in the NA … during YD.” Is this more than just hydrogen peroxide evidence?

    Comment by David B. Benson — 17 Jan 2009 @ 5:00 PM

  176. Eric;

    It is curious that one of the YD impact authors, Allen West , has also claimed that an assortment of mammoth tusks and other fossils from Alaska and Siberia incorporate embedded iron micrometeorites blasted into the upper surfaces of the unfortunate megafauna like so many shotgun pellets.

    This is not exactly consonant with a comet rich in carbonaceous chondrites, but i have yet to see the PNAS paper and their arguments for nanodiamond identification . Both the evidence from Tunguska, and elaborate modeling of Rayleigh-Taylor instability at hypersonic interfaces on atmospheric entry suggests that such strikes can result in complete disruption of comets at ~100 kilometer elevations, but that does not explain the geographic spread of the hypothesized swarm, leaving tidal disruption as with Hale-Bopp as the obvious alternative .

    I confess to having a small dog on the sidelines of this catfight- see Naturwissenschaften . Feb. 2006:…..93…88S

    I also commented on the Mammoth Headache issue:

    Comment by Russell Seitz — 17 Jan 2009 @ 5:57 PM

  177. Russell Seitz (176) — Thanks for commenting and I’ll hope you will continue to contribute here. My reading of the evidence is that the ET object was an ‘asteroid’, of sorts, not a comet. I could well be wrong about this.

    All — Here is a suggested central line of the path of the ET object, Edmonton (YEG) to Berlin (TXL):

    I conjecture that along this path there were repeated air explosions, sending impactors of many different sizes in, roughly, an ellipse along the route. In any case, nandodiamonds are found in Alberta and black mats in England, Belgium, Germany and Poland; hence my suggested routing. Using the base site

    you can choose your own great circle route.

    Regarding this one, while I could not find a suitable prarie provinces map, the great circle route goes close to the Clearwater-Athabasca proglacial Lake Agassiz overflow route; possibly the ice dam heare was seriously damaged. Note it also goes close enough to south-central Sweden that it could be the Baltic Ice Lake dam was also damaged. But more, note that various edges of LIS, GIS, the Iceland Ice Fields and the Fennoscandinavian Ice Sheet could well be so damaged that substantial quantities of ice quickly (within 180 years) made their way to the sea.

    Comment by David B. Benson — 17 Jan 2009 @ 6:49 PM

  178. R. Seitz, I have been told that any mammoth so pelted with micro-meteorites would have been burned/blown to smithereens by a shock wave by the time these micrometeorites arrived. Mr. West proposes a highly complicated theory of why this would not be the case which involves “tunneling effects” of impactors.

    Also, the dates on the tusks don’t match up, some going back to well before the alleged impact. Therefore it is proposed that the animals where this has been seen might have been died and been buried in ice 1,000s of years earlier, but been partially exposed again at the time of impact.

    Comment by bigcitylib — 17 Jan 2009 @ 8:14 PM

  179. bigcitylib (178) — That would agree with the comments above suggesting a ‘comet’ impact around 38,000 years ago.

    Comment by David B. Benson — 17 Jan 2009 @ 8:42 PM

  180. I believe I can show what mechanism caused the Carolina Bay (older than 38,000 BP) and the Younger Dryas (12900 PB) burn marks. The evidence supports a semiperiodic mechanism which rules out comets.

    This article in Wikipedia discusses the dating of the analysis of the Carolina Bay burn marks and notes they are roughly 38,000 BP.

    An impact hypothesis can not explain the geographic dispersion of the burn marks (geographically dispersed in the Younger Dryas case, i.e. Throughout North America and in multiple sites in Europe) and concentrated in along New Jersey to North Carolina coast, for the Carolina Bay marks.

    Look at the Carolina Bay marks. (See link below.) How can impacts cause overlapping burn marks? How can impacts create burn marks that are elliptical up to 10 km long with no impact craters? Why do the Carolina Bay marks have an axis that points in the North West direction?

    The impact hypothesis also can not explain the observed climate changes of the Younger Dryas. Abrupt cooling. Stays cold for around 1000 years and then abruptly warms and returns to interglacial.

    “Many radiocarbon dates, which were obtained from organic matter preserved within undisturbed sediments, which fill Carolina Bays, are greater than 14,000 BP radiocarbon in age. The finite radiocarbon dates range in age from 440 ± 50 to 27,700 ±2,600 BP radiocarbon in age (Whitehead 1981, Gaiser et al. 2001). Some samples are so old that they contained insufficient radiocarbon for dating, which results in “greater than dates”. For example, samples from sediments filling Carolina Bays have been dated at greater than 38,000 to 49,550 BP radiocarbon years (Frey 1955, Brooks et al. 2001).”

    “The cometary theory, on the other hand, popular among earth scientists of the 1930s and 40′s, is that the Bays are the result of an encounter between North America and a low density comet exploding above or impacting with the Laurentide Ice Sheet ~12,900 years ago [4]. Supporting evidence includes the failure of “wind and wave” theories to satisfactorily account for a number of the peculiar features of Carolina Bays, including the recent identification of markers suggestive of an extraterrestrial connection, the alignment of bays with points over the Great Lakes, and their tendency to overlap one another from east to west. Extraterrestrial markers include microspherules, magnetic grains with extraterrestrial chemistry, carbon spherules suffused with nanodiamonds, and levels of iridium sixty times background levels.”

    Look closely at the Carolina Bay marks, figure 7 in this link. Could a impact create those marks?

    Carolina Bays. The Carolina Bays are a group of »500,000 highly elliptical and often overlapping depressions scattered throughout the Atlantic Coastal Plain from New Jersey to Alabama (see SI Fig. 7). They range from ≈50 m to ≈10 km in length (10) and are up to ≈15 m deep with their parallel long axes oriented predominately to the northwest. ….All of the Bay rims examined were found to have, throughout their entire 1.5- to 5-m sandy rims, a typical assemblage of YDB markers (magnetic grains, magnetic microspherules, Ir, charcoal, soot, glass-like carbon, nanodiamonds, carbon spherules, and fullerenes with 3He). …

    Comment by William Astley — 18 Jan 2009 @ 10:40 AM

  181. Summary, imagery, and links in a good blog post here:

    Don’t miss the great excerpt quoted there introduced with:

    “In 1933, the photo above kicked off the Carolina Bay debate. Edna Muldrow wrote The Comet that Struck the Carolinas in Harpers Monthly that year…..”

    Comment by Hank Roberts — 18 Jan 2009 @ 12:21 PM

  182. I retract the nonsense I mistakenly posted in comment #179. Please ignore that bit of muddled thinking.

    I am ignoring the Carolina Bays simply because of the enigmatic nature of the evidence. There is ample decent evidence for YDB without even looking at the Bays.

    Comment by David B. Benson — 18 Jan 2009 @ 2:03 PM

  183. Here is what might be found, if looked for thoroughly enough: “Characterisation of a Small Crater-like Structure in SE Bavaria, Germany”

    which is short, well illustrated and altogether nicely done. Unfortunately, it is surely too recent to be one of the YDB impacts; it is rather the size (11 m diameter) is suggestive.

    Comment by David B. Benson — 18 Jan 2009 @ 4:59 PM

  184. Brushy Creek, LA: “Possible Meteorite Impact Crater in St. Helena Parish, Louisiana”

    might just possibly be the right age for association with YDB. The article is moteworthy in explaining the Carolina Bays as siliciclastic karst.

    Comment by David B. Benson — 18 Jan 2009 @ 5:23 PM

  185. Carolina bays:
    and here:


    Comment by Ray Percat — 18 Jan 2009 @ 9:02 PM

  186. According to

    (with well done sketch maps), YDB occurred when Lake Ontario was covered by an ice lobe of LIS. So the Charity Shoal Impact Crater in eastern Lake Ontario

    is a potentially a YDB impact.

    For much more speculative coincidence, note the German volcano

    located in what seems to be Germany’s only active volcanic zone, is dated by one source as erupting in 12,930 BP.

    Comment by David B. Benson — 19 Jan 2009 @ 5:09 PM

  187. A few belated thoughts… I’m still with Eric’s original post in maintaining some skepticism, although the debate has been terrific and I learned a lot just by reading through this. In the end, I think the Science paper is getting a lot of press mainly because it is being sold as “the cause” for both the YD and the megafauna extinction. And parts of that all-encompassing explanation don’t fit.

    To me, the most interesting part of the megafauna debate is its space- and time-transgressive nature. That is, the decline of megafauna on several (but not all) continents and its coincidence with the arrival of humans at various times on the various continents. The YD coincides roughly with the extinction and the end of Clovis in N.America, and thus with the proposed comet imapct. But it doesn’t correspond well with the main extinctions in Australia, Eurasia, or South America.

    So if you want an explanation for Pleistocene-Holocene megafauna extinctions you don’t need or even really want an “event” isolated in time or space. That’s why human impact, whether hunting or introducing pests/diseases, is such an attractive explanation – people can move around and spread the cause to match the extinction pattern. Natural environmental change can do that too but you get into some fairly suspicious coincidences with the human history.

    This theme is really similar to Eric’s original point about the Younger Dryas being similar to the DO events and to reversals during earlier terminations. You don’t *need* an isolated, unique cause to explain the YD, so if you propose one you are really making the story more complicated, not less.

    Same goes for the Agassiz flood theme. Yes there is a big flood coinciding with the start of the YD and therefore with the proposed impact. But what about the other Agassiz floods, not to mention the other similar proglacial outburst floods that have been well-documented elsewhere.

    Now, none of this proves that there was no impact or it couldn’t have been a factor. I just feel more comfortable not invoking an extraterrestrial event where one isn’t needed.

    Comment by Mike B — 19 Jan 2009 @ 10:50 PM

  188. The following is a comparison of the Greenland Ice Temperatures to the Antarctic Ice Temperatures for the last 140 kyrs.

    The warm interstadial periods are evident in the Greenland data and are followed by a return to the very cold glacial period. Note the abrupt end to the last interglacial period the “Eemian” Interglacial. This what we are trying to explain. There is a cyclic powerful forcing mechanism. The Younger Dryas burn marks were not created by an asteroid or comet impact.

    The same forcing mechanism that created the Younger Dryas cooling is evident through out the last glacial period. The effect of the mechanism changes due to specific variables which are not apparent unless you think about what the factors which affect the specific mechanism. (The mechanism is not ocean currents.)

    A hint is some thing external is forcing the geomagnetic field. The effect of the forcing is dependent on the seasonal timing of aphelion (whether the forcing impacts the Northern or Southern Hemisphere) and the amount and location of ice on the planet’s surface. (Ice is an insulator. The continental crust is slight conductive. The effect is diffused if the event occurs over ocean.)

    This paper discusses the analysis that shows there is geomagnetic field variance. The geomagnetic field intensity is 7 to 10 times greater during the interglacial than the glacial period. (The inclination does effect the forcing but the forcing is external not internally driven.)

    Remember there is a very large change in cosmogenic isotopes at the start of the Younger Dryas which is consistent with an abrupt change in the geomagnetic field. (There are cosmogenic isotope changes at the other abrupt climate changes.)

    This paper is also hint. Something is causing a change in nuclear decay time between aphelion and perihelion. Note there is an observed lag in the change in nuclear decay rates, which rules out neutrino variance as the cause of the nuclear decay rate change, as neutrinos travel at the speed of light.

    The geomagnetic field (internal) resists a step change. When equilibrium is reached the effect (whether reinforcing or suppressing is dependent on the geomagnetic field’s polarity at the time of the event and the hemisphere where the event occurs.)

    The complication in discovering the mechanism is connecting the evidence the geomagnetic field changes cyclically if one does not look for an external forcing mechanism (you must see and think of the burn marks at look at geomagnetic field variance data and think of what periodicity is required to explain the abrupt climate changes and adjust for orbital precision which changes the hemisphere where the event occurs.)

    Curiously (from a standpoint of why didn’t someone point out this phenomenon before) or as would be expected, there is astronomical evidence(it is a fundamental stellar process) to support what is happening and the specific forcing mechanism was considered as one of the possible solar nuclear reactions.

    Comment by William Astley — 19 Jan 2009 @ 11:09 PM

  189. #187

    You say:

    “Now, none of this proves that there was no impact or it couldn’t have been a factor. I just feel more comfortable not invoking an extraterrestrial event where one isn’t needed.”

    Me either but the question is whether something else is needed to explain the length and severity of the YD. You seem to be ruling out freshwater draining into the N. Atlantic and an ET event. What cause do you have?

    Regarding the megafauna. No doubt the megafauna were in decline at the start of the YD due to human hunting. However, you miss one critical point made by Firestone et al. It is not just that the megafauna go extinct roughly around the time of the YD. Megafauna and clovis evidence exists in the strata below the ET boundary but is completely absent in the strata above. The layer is a sharp divide.

    Comment by Jim Cross — 20 Jan 2009 @ 7:48 AM

  190. The layer is a sharp divide.

    Indeed it is, which is all the more important reason not to ignore something which now demonstrably did occur.

    We now know that hemispherically catastrophic impact events occur on roughly millennial timescales, If it further is indicated that catastrophic and explosive hypervelocity impacts can and do occur, which then can evenly distribute millions of smallish hypersonic impactors over a very wide area (both hemispherically and regionally) on the order of tens of millennia, that is further cause for concern for the civilization and universe you reside in.

    What we need to be doing is simulating the effects of high incidence, low grazing angle, explosive impacts of large carbonaceaous bodies on the surface of the Earth, since we now have evidence it occurs regularly.

    Comment by Thomas Lee Elifritz — 20 Jan 2009 @ 10:24 AM

  191. Rahmstorf, S., 1995: Bifurcations of the Atlantic thermohaline circulation in response to changes in the hydrological cycle. Nature, 378, 145-149, found that a fresh water flux of as little as 0.06 Sv sufficed, in model studies, to induce a THC transition event.

    From comment #173, Early BIL flux was about 0.02 Sv to 0.13 Sv directly into the Nordic Sea. Possibly this sufficed. From the same comment, drop from Lockhart phase LA to early Morehead phase LA added anywhere between 0.3 Sv and only 0.003 Sv, presumably mostly into the Arctic (Teller et al., 2005, full citation in comment #158). Additional fresh water flux came from melt of the southeastern portion of LIS and probably the southwestern portion of FIS, but no estimate of such fluxes is suggested.

    This suggests that between the two (nearly) simultaneous dischanges there was sufficient flux to initiate Younger Dryas by a THC transition.

    Comment by David B. Benson — 20 Jan 2009 @ 2:35 PM

  192. Summarizing, at 12,9 ka five events occurred nearly simultaneously:
    (1) ET objects(s) air explode or impact (YDB);
    (2) Early BIL discharges into the Sagerrak and on to the Nordic Sea;
    (3) Lakehurst phase LA stops discharging into the Mississippi River basin and begins dropping about 100 meters to the early Morehead phase;
    (4) Abrupt temperature drop of over 2 K begins in central Greenland, initiating Younger Dryas (YD) cold reversal (CR);
    (5) Laacher See volcano erupted.

    In the prior post, we see that (2) & (3) likely explain (4). What remains to be explained is the unusual duration and intensity of the YD CR. The interval of the YD CR is the second longest in three terminations (from comment #152, termination 3 had a 1900 year YD-like CR). It is over twice as intense as that event, the second most intense in three terminations, as measured by decreases in methane. Other indications of the extreme nature of the YD CR are given in prior comments.

    The YDB hypothesis (YDBH) is that (1) initiated (2) and (3) so that these two drainages occurred nearly simultaneously, not seveeral centuries apart as might otherwise be the case, as I suppose in somewhat the similar termination 3 event. This simultaneous influx of fresh water in the far North Atlantic and Arctic Ocean caused the extreme cold in that region (and thus in Europe).

    But what about (5)? Checking

    mostly likely just coincidence, although I suppose a bolide from YDB might have impacted just there, setting off the eruption. One might reserch this small point.

    Other possible indicators tending to confirm YDBH would be an impact crater of the right age in Lake Vattern, Sweden, and similarly for lakes, large and small, across most of Canada. Modeling of large fluxes from Early BIL and first LA transition would also be helpful to better constrain the flux requirement for THC trnasition.

    Whether YDBH is more firmly supportd by additional evidence or not, it is a productive hypothesis in that it suggests lines of further reaearch, a few of which are mentioned in the previous paragraph.

    Comment by David B. Benson — 20 Jan 2009 @ 5:30 PM

  193. Astley, I don’t think the magnetic intensity is as significant as the change in direction of the geo-magnetic field. The Be-10 signal gets larger during the YD cold period in the GRIP ice record, and more pronounced with much large cycling prior to YD in the antartic ice record. Is this due to solar activity, geomagnetic, or a transition to a fall out mechanism?

    [Response: It may well be due to changes in the deposition of the 10Be itself - which can be affected by rainfall and circulation changes independently of any production change (Field et al, 2006). - gavin]

    Comment by Pt — 20 Jan 2009 @ 6:47 PM

  194. In reply to PT 193#

    The observed Younger Dryas change can not be explained by a simple increase or decrease of the solar heliosphere (i.e. Change in the solar magnetic cycle). The GCR modulation by the sun directly is not capable based on the hypothesized mechanism of the extreme affect and long term affect on planetary temperature that is observed for the Younger Dryas. (1000 years of cooling.)

    There is evidence to support the hypothesis that a semiperiodic external event forces the geomagnetic field. A long term large change in geomagnetic field would based on the hypothesized mechanisms significantly force planetary temperature. For example:

    Is the geodynamo unstable?

    “Recent palaeomagnetic studies suggest that excursions of the geomagnetic field, during which the intensity drops suddenly by a factor of 5 to 10 and the local direction changes dramatically, are more common than previously expected. The `normal’ state of the geomagnetic field, dominated by an axial dipole, seems to be interrupted every 30 to 100 kyr; it may not therefore be as stable as we thought.”

    “Recent studies suggest that the Earth’s magnetic field has fallen dramatically in magnitude and changed direction repeatedly since the last 700,000 years ago (Langereis et al. 1997; Lund et al. 1998). These important results paint a rather different picture of the long term behavior of the field from the conventional one of a steady dipole reversing at random intervals; instead the field appears to spend up to 20% of its time in a weak non-dipole state (Lund et al. 1998).”

    As I said I believe the forcing mechanism is the sun can provide data to support that hypothesis. (See my comment above.) The resultant effect depends on the hemisphere were the event occurs which is controlled by precision (21 kyr cycle. Perihelion of the earth orbit controls the hemisphere where the event occurs.) and the planet’s inclination (41 kyr cycle. Greater the inclination at the time of the event the greater the effect.). It appears the effect is greater for greater orbital eccentricity (100 kyr cycle).

    On a long term basis the position of the continents also affects the resultant which would explain the long term period when there were no magnetic field reversals. (i.e. All the continental crust is either north or south of the equator and the effect is greatest over continental crust as the conductive ocean dissipate the event. As the event is periodic the planet is similar to a capacitor with a charge difference core to surface. This explains anomalous charge releases during earth quakes and other peculiar electric phenomena which is observed when the solar charge forcing is stronger. The phenomenal is what is causing sprites which are electrical discharges from the top the earth’s atmosphere to space.

    Based on this hypothesis the North Carolina burn marks would be due to restrike.

    Comment by William Astley — 20 Jan 2009 @ 10:30 PM

  195. William and David,

    This paper is referenced through one of postings linked to by this posting. Have either of you looked at it?

    Basically, it says you don’t need a comet to explain freshwater draining into the N. Atlantic or exotic geomagnetic theories to explain the YD.

    It is, however, just a model but does correlate with the climate record,

    Comment by Jim Cross — 21 Jan 2009 @ 7:46 AM

  196. David B. Benson (DBB) -

    You should define (YSD) your acronyms (YA)or avoid them altogether (ATA) when they are not necessarily readily apparent (NNRA). It would help the average reader (AR) follow your train of thought (ToT) more readily.

    BTW, this has been one of the more interesting posts in a quite a while and I do appreciate the many references and other useful information that you and many others have brought to the table.


    Comment by Bob North — 21 Jan 2009 @ 10:02 AM

  197. Bob North (195) — In some comment or other, I did. Missing a repeat in comments #191 and #192 are

    LIS: Laurentide Ice Sheet
    FIS: Fennoscandinavian Ice Sheet
    LA: Lake Agassiz
    BIL: Baltic Ice Lake

    Jim Cross (196) — I still don’t see the YD CR as being at all like a DO event, which is warming followed later by cooling in Greenland. Do note that the corresponding event in termination 3 methane records lasted 1900 years, which rather spoils the supposed 1500 year ‘cycle’. What I see in both cases is a THC transition event, to cooler conditions, followed later by a THC transition event to warmer conditions.

    I am uncertain about the other evidencve to be found during generally cold conditions, but YD CR is clearly from terrestrial evidence an interruption in the general northern hemisphere pattern of following orbital forcing to warmer, interglacial conditons. The main cause, as I see it, is glacial meltwater finally triggering the THC down transition. The North Atlantic finally recover enough to trigger the THC up transition. The only role for the YDB extraterrestrial object encounter is to cause both ice dams, one for BIL, the other for LA, to fail simultanteously, a coindcidence for which there is evidence. I infer from the length of the corresponding event in termination3 tha such simultanaity did not occur for the drainage of large proglacial lakes at that deep time.

    Going on to briefly consider the 8.2 kya event, the same THC down transition followed later by recovery matches well with the final drainage of proglacial Lake Agassiz-Objibway. Sorry, but I just don’t see any reality in the Bond 1500 year ‘cycle’.

    Comment by David B. Benson — 21 Jan 2009 @ 3:16 PM

  198. > hemispherically catastrophic impact events
    > occur on roughly millennial timescales

    That’d be one hemisphere catastrophically damaged every how often, on average? With what kind of error bar?

    Comment by Hank Roberts — 21 Jan 2009 @ 4:29 PM

  199. Why Younger Dryas (YD) is not like DO events.

    DO events, occurring during a ‘glacial’ between interglacials, consist of

    (1) a sharp down transition in temperature;
    (2) an interval of cold temperature;
    (3) a sharp up transition in temperature;
    (4) a period of warm, but gradually declining temperatures.
    These are supposed to occur quasi-periodically with a period of about 1500 years.

    Ordinarily, the sequence (3), (4), (1) is said to be the DO event; this makes no difference for quasi-periodic phenomena.

    The sharp down transition of YD occurred at 12.9 ka. This was followed by a period of cold, but generally warming temperatures. The sharp up transition occured at 11.6 ka as I read it from the GISP2 temperature reconstructions by Alley (previously cited). Note that is 1600 years, already longer than the presumed DO total period. Thereafter the temperatures continue, on average, to go up, without cold reversals for some time.

    In particular, these go up until the 8.2 ya sharp down transition in temperature. Time elapsed from YD down transition to 8.2 ya down transition is 4,700 years, not in accordance with the DO event period.

    Comment by David B. Benson — 21 Jan 2009 @ 6:32 PM

  200. #197


    I don’t understand why you are having such a hard time grasping this warming/cooling thing?

    The idea is that during glacial and early Holocene times, there are rapid warmings. It is the basic Milankovitch forcing augmented by CO2 increases. The warmings lead to meltings of ice which flush into the N. Atlantic causing the shutdown of the THC. That leads to coolings. These things go together.

    My only question is whether this cycle explains completely the YD or whether some ET event augmented the YD.

    Comment by Jim Cross — 21 Jan 2009 @ 6:42 PM

  201. Here is the NOAA Paleoclimatology Program page on Younger Dryas:

    Excellant graphic.

    Jim Cross (200) — I understand the warming and cooling quite well, thank you. Your second paragraph is quite a good summary, but there is no ‘cycle’ as is evidenced by the methane records for the two prior terminations as well as the details of termination 1; I suggest ‘oscillation’ as a more neutral term.

    Your question is precisely what I set forth in comment #192, augmented by comment #191 and other prior comments, as the YDB hypothesis. It is based on a quite remarkable (near) simultaneous occurrance of events across North America and northern Europe. From the NOAA page linked above, it seems that greatly enhanced flow out the St. Lawrence may need to be added to the list of simultaneous events.

    Anyway, please do read comments #191 and #192 again. That YDB augmented YD to some extent can hardly be doubted. The hypothesis is that YDB significantly augmented YD.

    Comment by David B. Benson — 21 Jan 2009 @ 7:29 PM

  202. Correction to comment #199: 12.9 – 11.6 = 1300 years, almost as long as the enitre presumed period for DO cycling of 1500 years. In any event, the temperature in central Greenland continued to improve until the 8l2 ka CR event.

    Comment by David B. Benson — 21 Jan 2009 @ 8:54 PM

  203. We are still at square one for evidence on an impact affecting any ice sheet or inducing any rapid drainage event. I have seen not one shred of field data supporting the idea. We still have lots of data recording the fluctuations of the ice sheets, the lake levels of the impounded proglacial lakes. Still no sign. You underestimate the amount of effort and research that has gone into reconstructing the behavior of the LIS and FIS, literally lifetimes of digging and mapping. Something catastrophic will leave evidence.

    Comment by mauri pelto — 21 Jan 2009 @ 9:47 PM

  204. Something catastrophic will leave evidence.

    It has, a black mat demarcation layer in the statigraphy, nanodiamonds, spherules, various shocked materials, crater fields, the quest for evidence has just begun. We are in the all out speculation phase of this result, as it should be. Already the Peruvian impact, Burckle crater and the Holocene impact working group have revolutionized our thinking, and this is just another line of evidence.

    How much evidence do you think the Tunguska event would have left after 12,900 years? We’re lucky to get anything and the fact that we are still gleaning evidence after 12,900 years itself is evidence of the size of the thing, especially considering that most of that evidence was immediately involved, altered and removed by subsequent catastrophic flooding events shortly thereafter.

    I for one am grateful for the heads up on these things.

    Comment by Thomas Lee Elifritz — 22 Jan 2009 @ 10:15 AM

  205. mauri pelto (203) — The most immediate piece of field evidence might well be Charity Shoal Impact Crater, Lake Ontario. Substantiating this would require a better age determination (if possible) together with a better understanding of flow to Gulf of Saint Lawrence during YD.

    I certainly have an appreciation of the field work required and a deep admiration for those who collect it. Indeed, all that work has collected the evidence of several (nearly) simultaneous events across North America and northern Europe.

    Thomas Lee Elifritz (204) — Well stated!

    On the matter of possible flow from Lak Agassiz at YD initiation,
    the Fall 2008 AGU abstract

    Characterizing the Discharge Features of Glacial Lake Agassiz during the Post-Marquette Period Using Marine Seismic-Reflection Methods

    Gary, J L, Wattrus, N J, Colman, S M, Voytek, E B

    demonstrates appropriate sediments in Lake Superior for a latter stage of Lake Agassiz drainage. A similar abstract from the Fall 2007 AGU meeting by the first three authors indicates an intent to do the work described above and then continue, further south, to look for similar features (if any) for the YD stage drainage. So by the next meeting we might actually have some decent data regarding the widely surmised, but unsupported, drainage to the east during YD.

    The graph from NOAA found in the link in comment #201 and directly linked as

    suggests a flow to the Gulf of St Lawrence of an astounding 0.15 Sv maintained for over 1300 years! I would certainly like to see the paper(s) supporting this graph, but just now have no good idea what search terms to use. In any case, I don’t see how Lake Agassiz, Lake Warren, etc. could maintain such a large and steady flow. More melting of eastern LIS than I had realized, I suppose.

    Comment by David B. Benson — 22 Jan 2009 @ 1:50 PM

  206. Figure 6 in
    G. Larson and R. Schaetzl
    REVIEW: Origin and Evolution of the Great Lakes
    J. Great Lakes Res. 27(4):518–546
    Internat. Assoc. Great Lakes Res., 2001
    indicates the extent of LIS at 13 ka, just before YD initiation. This tends to indicate a great difficulty with drainage of Lake Agassiz to the east, too much ice in the way. On the othr hand, it confirms that Lake Ontario was filled to overflowing with ice, helping to advance the idea that the Charity Shoal Impact was at YD onset and caused flow to the east to substantially increase.

    The first several abstracts from the Fall 2005 AGU meeting support strong flows into the Gulf of St Lawrence at YD initiation althugh the first also suggests the alternate routing through the arctic. In any case, it seems there were substantial flows:

    Comment by David B. Benson — 22 Jan 2009 @ 5:17 PM

  207. David,
    Won’t a large discharge of freshwater accelerate the THC? Using simple hydrostatics: if you have two columns of water of equal depth next to each other and the one on the right has more freshwater, won’t the pressure at the bottom of the left column be higher than the one on the right? (p=h*rho*g) I’m trying to picture how all this freshwater flow is supposed to effect a diffusion (i.e., concentration) limited system that’s several thousand meters deep near the ocean floor.

    Comment by Pt — 22 Jan 2009 @ 5:27 PM

  208. Pt (207) — As I understand it from the paper linked earlier by SteveF and the Rahmstorf paper cited in comment #191, a sufficiently large freshwater flux (greater than 0.06 Sv) into the North Atlantic causes THC to switch to the ‘colder’ mode, that is, avoiding the Nordic Sea and so causing cooling in Greenland and northern Europe.

    Comment by David B. Benson — 22 Jan 2009 @ 5:55 PM

  209. Pt, this may help

    Usual advice; click “recent” and evaluate partly by number of citing articles that carry a paper’s ideas forward.

    Comment by Hank Roberts — 22 Jan 2009 @ 7:20 PM

  210. John A. Rayburn, Peter L. K. Knuepfer and David A. Franzi, “A series of large, Late Wisconsinan meltwater floods through the Champlain and Hudson Valleys, New York State, USA”, Quaternary Science Reviews, Volume 24, Issue 22, November 2005, Pages 2410-2419, show that flood events went further south, as in the title, during early Younger Dryas. There was no ice in Lake Ontario, so I suppose the dates in the review paper cited in comment #206 are radiocarbon dates. Sigh. This now suggests that the Charity Shoal Impact was long before YDB, but not necessarily. In any case, there was substantial flow into the North Atlantic from the proglacial lakes east of Lake Agassiz and the suthors state that this was much larger than any (presumed) Lake Agassiz addition.

    Lev Tarasov & W.R. Peltier, “Arctic freshwater forcing of the Younger Dryas cold reversal”, Nature 435, 662-665 (2 June 2005)

    offer the unusual hypothesis that it was a substantial melt from Keewatin ice dome, writing “The underlying source of this strong discharge into the Arctic Ocean is the large Keewatin ice dome, whose existence at the Last Glacial Maximum was recently confirmed through analyses of space geodetic and absolute gravity constraints. The strength of local sourcing is evident in that even with the removal of all runs that have northwest drainage of Lake Agassiz at -12.8 kyr, ensemble discharge into the Arctic Ocean still dominates, with a 1sigma range of 1.1–1.5 dSv.” The authors also indicate a rather basic hydrological problem with massive dischanges from other sources (in the supplimental information), a problem fully in accordance with what is observed in earthquake-caused submarine turbidity currents: dirty water sinks.

    In any case, this paper offers a different source for fresh water injection into the Nordic Sea, called in GIN sea in the paper.

    Comment by David B. Benson — 22 Jan 2009 @ 8:07 PM

  211. Andrew G. Couch and Nick Eyles, “Sedimentary record of glacial Lake Mackenzie, Northwest Territories, Canada: Implications for Arctic freshwater forcing”, Palaeogeography, Palaeoclimatology, Palaeoecology, Volume 268, Issues 1-2, 10 October 2008, Pages 26-38, show that glacial Lake Mackenzie cleaned dirty meltwater so that the flow into the Arctic might have been clean enough to remain on the surface. The authors explain why their methods would not detect any large, sudden increases in flow. The dates of the study are about right for all of YD. The Turner et al. paper previously cited references a paper showing some decreased salinity at around the right time for YD. In any case, but not mentioned in these papers, rafted ice would still have the effect of forcing sea ice out Fram Strait into the Nordic Sea.

    Similarly for rafted ice down the Hudson and later out to the Gulf of St Lawrence, irrespective of what portiion of the enhanced flow was dirty. (we’ll lump these together as the mid-Atlantic.)

    For the Baltic Ice Lake (BIL), both outbursts only removed about 10 m of bedrock, from a study read long ago and my own brief observations of the flow channel. So first the smaller 5–15 m drop and then the final 25 m drop did not remove much of the rock; the water was quite clean. Much of it, together with rafted ice, then made its way into the Nordic Sea.

    The conjectured explanation for the great intensity of YD is the combination of ice from the Arctic and meltwater/ice from Early BIL, together with whatever contribution the mid-Atlantic provided. That these events occurred nearly simultaneously, rather than spread over centuries, is taken to explain why YD at 1300 years was shorter than the less intense and longer (1900 years) similar interval in termination 3.

    Comment by David B. Benson — 23 Jan 2009 @ 8:58 PM

  212. The increased fresh water flow into the Arctic during the pre-YD stage of deglaciation, from sources all around the Arctic, caused about 700 years of gradually declining temperatures in central Greenland just before the YD temperature plunge. This appears to be ever increasing volumes of sea ice pushed south out of Fram Strait, but not enough to significantly affect methane concentrations nor to cause a THC transition. I have not located any evidence of catastrophic flows to the Arctic down the MacKenzie River at any time near 12,900 calendar years ago.

    From the Rayburn et al. paper cited in comment #210, “We estimate the steady-state meltwater discharge into the North Atlantic to be 0.3–0.6 Sv. The first flood event was a combined Iroquois/Vermont outflow at around 10,900 14C yr BP that released 700 km3 of meltwater into the North Atlantic through the Hudson Valley with an estimated discharge of 1.1 Sv. A second outflow event released 2500 km3 through the Hudson Valley shortly after the first event. Finally, approximately 1500 km3 was released to the North Atlantic through the Gulf of St. Lawrence at the incursion of the Champlain Sea about 150–300 years later.” we see that the mid-Atlantic region provided substantial flux beginning at about the right time. Previous comments noted the catastrophic release from the Early Baltic Ice Lake at about the same time as YD onset in central Greenland and the near simultaneous switching of proglacial Lake Agassiz drainage from the the south to another outlet. In my just prior comment, it is proposed that the extra freshwater fluxes from BIL and mid-Atlantic, together with rafted ice, were then enough to cause a THC transition to the “cold” regime, evidenced by the central Greenland sudden temperature drop.

    The YDB hypothesis is that these nearly simultaneous terrestrial changes were started by the YDB encounter with one or more ET objects.
    Jim Cross in comment #95 suggested the possiblity of considrable melt.
    I used the impact calculator for a 15 km comet of the density of Shoemaker-Levy 9 to obtain “Energy before atmospheric entry: 5.88 x 10^23 Joules = 1.41 x 10^8 MegaTons TNT” and the target ice “melted or vaporized is 3680 km^3 = 883 miles^3″. That volume is comparable to the volumes of meltwater released from various glacial lakes at YD onset. This size is purely conjectural and there is no additional evidence, AFAIK, of suddenly increased fluxes. This paragraph is only to suggest the possiblity of catastrophically large increased melt triggering the three nearly simultaneous terrestrial events.

    Comment by David B. Benson — 24 Jan 2009 @ 6:55 PM

  213. Comet impact theory disproved

    New data, published today, disproves the recent theory that a large comet exploded over North America 12,900 years ago, causing a shock wave that travelled across North America at hundreds of kilometres per hour and triggering continent-wide wildfires.

    Dr Sandy Harrison from the University of Bristol and colleagues tested the theory by examining charcoal and pollen records to assess how fire regimes in North America changed between 15 and 10,000 years ago, a time of large and rapid climate changes.

    Their results provide no evidence for continental-scale fires, but support the fact that the increase in large-scale wildfires in all regions of the world during the past decade is related to an increase in global warming.

    Well, there’s that.

    Comment by Jim Galasyn — 26 Jan 2009 @ 9:23 PM

  214. So what’s in that layer of black stuff then?

    Comment by Hank Roberts — 26 Jan 2009 @ 9:45 PM

  215. I’ve seen a lot of poor to crappy science reporting, but that one takes the cake. I’m guess the paper cited doesn’t even mention the hypothesized extraterrestrial impact.

    Comment by Thomas Lee Elifritz — 26 Jan 2009 @ 11:36 PM

  216. Re #193, Field’s paper doesn’t reproduce the artic nor antarctic Be-10 ice concentrations going back well beyond the YD. They do however generate a rule of thumb that says that 10% of Be-10 concentration change causes +/- 1 W/m2 change in radiative forcing due to 2XCO2, volacanoes (internal factors). If you look at the Be-10 concentration from antarctic going back from 30 kya-15 kya that’ll put the forcing at -60 W/m2, obviously not possible with internal factors. The artic Be-10 starts major deviation from the antartic around 30 kya and then converges back together about prior to the YD time period. If there was external (solar, geomagnetic field meander) forcing that initiated the YD cooling, it began at least 10 kya before the actual event.

    [Response: You have it backwards. Changes in climate cause changes in 10Be in those experiments, not the other way around. - gavin]

    Comment by Pt — 26 Jan 2009 @ 11:37 PM

  217. David, Do you have a ref. that shows the path of the deep conveyor belt? There are millions of km3 of ocean water (ca. 35k ppm) to deal with, and a few thousand freshwater km3 seems like a drop in the bucket. Perhaps you/someone else can explain how the THC gets into a cold spell.

    Comment by Pt — 26 Jan 2009 @ 11:45 PM

  218. Not much in the way of detail here, but Sandy Harrison from the University of Bristol dumps some cold water over this. No continent wide forest firesto be found.

    Comment by bigcitylib — 27 Jan 2009 @ 10:19 AM

  219. If you believe that paper ‘disproves’ anything, you need to find a career outside of science. That headline in no way resembles the content of the cited paper, and the content of the cited paper doesn’t even address the hypothesized extraterrestrial impact. The fact that this kind of science reporting even occurs in the United States is a clear indication of how far the level of science education has fallen in the America of today.

    Comment by Thomas Lee Elifritz — 27 Jan 2009 @ 11:50 AM

  220. 219. You may be right. The story is rather sketchy and I have not got hold of the actual paper. But it SEEMS to be taken directly from the University of Bristol news release.

    Comment by bigcitylib — 27 Jan 2009 @ 12:19 PM

  221. Alright, now I do have the paper, but haven’t read the entire thing. From the intro:

    We also test the hypothesis that
    a comet impact initiated continental-scale wildfires at 12.9 ka; the
    data do not support this idea, nor are continent-wide fires indicated at any time during deglaciation. There are, however, clear
    links between large climate changes and fire activity. Biomass
    burning gradually increased from the glacial to the beginning of
    the Younger Dryas. Although there are changes in biomass burning
    during the Younger Dryas, there is no systematic trend.

    Comment by bigcitylib — 27 Jan 2009 @ 12:35 PM

  222. Pt (217) — Here is a link to a starter page on THC:

    Also see the Rahmstorf 1995 paper cited in a previous comment; its available from his publications page.

    Regarding the supposed “continent-wide” fires, this seems rather irrelevant and also appears to contradict the ammonia data from NGISP. Another example of the difficulty of determining what actually happened at such a deep time.

    bigcitylib (221) — Can you provide a link to the paper? I don’t seem to be able to locate it.

    Looking at Stefan Rahmstorf’s THC fact sheet, I noticed that the Labrador Sea is important. I’ll now offer the suggestion that mid-Atlantic meltwater and rafted ice shut down the Larador Sea deepwater formation.

    In any case, I think I misstated the situation in the Arctic. There is some evidence for increased freshwater in the Arctic at the time interval in question.

    Comment by David B. Benson — 27 Jan 2009 @ 2:35 PM

  223. Ok, then it’s extremely poor science. How do you test a hypothesized pinpoint chronological impact event by looking at gradual changes over centuries and millennia?

    In short, you don’t. If you want to refute (not disprove, since it hasn’t even been ‘proven’ yet, and science doesn’t ‘prove’ anything anyways) the impact hypothesis, which rests upon the observed impact crater fields, the strong statigraphic demarkation layer, and the observed particulate anomalies in the observed statigraphic layer, then you work on refuting any one of those lines of evidence, you don’t present some hazy result about increase wildfires associated with global warming.

    This paper is a really great example of bad science.

    When you google the paper title, it gives you this link :

    Look at the graph. Nothing extraordinary there. No mention of the comet hypothesis, no glaring evidence of extreme events which could lay down such a dramatic layer of charcoal, nothing much there at all. What we have is someone using a controversy to publicize a science paper.

    Comment by Thomas Lee Elifritz — 27 Jan 2009 @ 2:50 PM

  224. David and others interested. It was emailed to me by Either email her directly or, failing that, email me at

    Comment by bigcitylib — 27 Jan 2009 @ 3:16 PM

  225. Re: 223.


    Re*fute”\ (r?*F3t”), v. t. [imp. & p. p. Refuted; p. pr. & vb. n. Refuting.] [F. r['e]futer, L. refuteare to repel, refute. Cf. Confute, Refuse to deny.] To disprove and overthrow by argument, evidence, or countervailing proof; to prove to be false or erroneous; to confute; as, to refute arguments; to refute testimony; to refute opinions or theories; to refute a disputant.

    There were so many witnesses in these two miracles that it is impossible to refute such multitudes. –Addison.

    Syn: To confute; disprove. See Confute.

    Webster’s Revised Unabridged Dictionary, © 1996, 1998 MICRA, Inc.

    Comment by Garry S-J — 27 Jan 2009 @ 5:19 PM

  226. #214 Hank

    This article

    that I referred to earlier, states this about the black mats:

    “Of the 97 geoarchaeological sites of this study that bridge the Pleistocene-Holocene transition (last deglaciation), approximately two thirds have a black organic-rich layer or “black mat” in the form of mollic paleosols, aquolls, diatomites, or algal mats with radiocarbon ages suggesting they are stratigraphic manifestations of the Younger Dryas cooling episode 10,900 B.P. to 9,800 B.P. (radiocarbon years).”

    Basically, this is saying that it is layer of decayed organic matter formed in wet soils. If correct, it has nothing to do with burning.

    Comment by Jim Cross — 27 Jan 2009 @ 5:27 PM

  227. bigcitylib (224) — Thanks, but Thomas Lee Elifritz in comment #223 provided a link to a summary based on the paper. After reading the link, I largely agree with TOmas’s commentary.

    Comment by David B. Benson — 27 Jan 2009 @ 5:27 PM

  228. David,

    If you would prefer to base you opinion on the summary…

    Comment by bigcitylib — 27 Jan 2009 @ 6:54 PM

  229. So you are citing the dictionary in your analysis of a scientific result. Ok. That will always get me going.

    I have no problem with the result of the paper, which is intuitively – global warming, more wildfires. However, what I am questioning is linking this centennial and decadal result, spread over millennia, to a geologically instantaneous hypothesis, when it doesn’t address any of the lines of evidence which have been developed in support of that hypothesis. I am even more opposed to the press coverage of this result. This kind of thing happens all the time, each time for dramatic than the previous.


    See what I mean? I think it would be great if there never was a Clovis impact, and that perhaps the stressed Clovis population simply resorted to huge wildfires for hunting in some sort of post glacial feeding frenzy.

    Is that the truth? Is the black mat simply the result of extensive ice sheet flooding, followed by extreme climate change, drought and wildfires? Why do the hypothesized crater fields still point to Lake Michigan? What about the nanodiamonds? What about the associated extraterrestrial particulate layer and the geological markers? All of the evidence I can gather about this press release is that it seems a public relations ploy.

    Too late, the feeds have picked it up, it’s everywhere. This is going to be more fun sorting out than the PETM.

    Meanwhile, science trudges on.

    Comment by Thomas Lee Elifritz — 27 Jan 2009 @ 8:08 PM

  230. Re #216. Fields used GCMs along with modifications to solar forcing. The generated the 10% Be-10 => 1 W/m2 based on these simulations.

    What’s critical to Be-10 deposition are sulfate ions in the upper troposphere. If you plot [SO4] along with the Be-10 concentrations, there’s a direct relationship between Be-10 & [SO4] for many thousand years prior to YD cooling, but not so for Greenland Be-10 which has an inverse relationship. The basic point here is that there a lead in over several thousand years (for example increased volcanic activity) that may have caused the YD cooling.

    [Response: Which [SO4] record are you using to determine this? Be10 scavenging by aerosols doesn’t depend strongly on volcanic activity because there is always enough aerosol to pick up whatever Be10 is produced. The idea that volcanism thousands of years before caused the YD is rather dubious though. – gavin]

    Comment by Pt — 28 Jan 2009 @ 11:34 AM

  231. Re #222, David thanks. There’s positive and negative flows into/out of the NA. There’s an “equilibrium” point (at 0 Sv) that is critical in these studies (well mixed conditions are also assumed throughout — resistance due to water bouyancy may make this reasonable; which means the system may be more stable). Will get back to you on this later.

    Comment by Pt — 28 Jan 2009 @ 11:40 AM

  232. Re#230. See the [SO4] record in Taylor dome and GRIP. I believe sulphate in aerosols are required to precipitate out Be-10 from the upper troposphere. There’s increases [SO4] activity in both records over 1000s of years prior to YD cooling. If increased volcanic activity is not particularly palatable, then there’s dimethlysulphate from ocean dwelling algae (but I don’t see how SO4 could be puffed off that high into the atmosphere), or if that is not likely, then if we go by too much internal forcing (using the 10% rule from Field) means coincidence between external forcing (including geomagnetic field meander) and and incr. sulfate activity.

    [Response: You are correlating [SO4] in Taylor Dome with 10Be in GRIP? I don’t see the connection (or the peer reviewed paper that shows this?). Why doesn’t it show up if you do the correlation with [SO4] at GRIP? And how do you know that both aren’t simply responding to the large climate changes that characterised the last ice age? Can we see some real work here…? – gavin]

    Comment by Pt — 28 Jan 2009 @ 12:50 PM

  233. Thomas Lee Elifritz (229) — Actually, I have some doubts about the conclusion that more warming implies more wildfires in North America during ther period studied. Foremost is that North America was inhabited by Paleoamericans at that deep time. There is direct evidence from later time that the hunter-gathers in North America intensionally set forest fires. See, for example, “Ancient Forests”. I have no evidence, one way or the other, about whether the Paleoamericans did the same or how significant such would be.

    The other, more minor, objection is that it seems that much of the lower 48 suffered drought throughout (much of) YD. I didn’t bother to keep a link to the paper seems it seems remote from the primary purpose of this thread. The relevance here is that this would tend to help explain the flat slope of the graph during YD; no temperatures required.

    All in all, and having looked at other limnological studies with some care, it seems the paper in question implies essentially nothing regarding YDB.

    Comment by David B. Benson — 28 Jan 2009 @ 2:54 PM

  234. In retrospect now I can see why the paleofire group would want to hitch their star to the holocene impact group. The misreporting of it can only increase exposure of this remarkable and relevant problem to the drunken masses.

    Having not read the paper, I don’t know what sort of proxies they are using to develop their correlations, but they seem insufficiently resolved to detect brief intense widespread impact induced fires or even if that would be the largest relevant forcing issues on what was to come.

    Comment by Thomas Lee Elifritz — 28 Jan 2009 @ 6:17 PM

  235. Re #232. As [SO4]_Taylor Dome (Antartica) goes up starting around 30 kya, the [Be-10]_Taylor Dome goes up until about the YD period. [SO4]_GRIP goes up starting around 30 kya, the [Be-10] goes down. The [Be-10] signature in Taylor Dome and GRIP diverge around 30 kya. [SO4] in both places going up suggests global [SO4] activity (most probably volcanic since [SO4] must get to the upper troposphere to bring down Be-10. The question you ask is why there’s an inverse relationship between [SO4] and [Be-10] in GRIP. One possible explanation is that the earth’s magnetic field in the north deflected the energetic [Be-10] from Greenland; the poles of the earth can meander independent of each other.
    I don’t understand your last question.

    Comment by Pt — 29 Jan 2009 @ 11:22 PM

  236. The last part should say “energetic protons.”

    Comment by Pt — 29 Jan 2009 @ 11:25 PM

  237. Having just reviewed some of the basics of this remarkable problem of the Quaternary – Pleistocene – Holocene extinctions, I noticed that someone might want to update this excellent wiki page with relevant Holocene ET impact hypotheses pros and cons.

    Sea level is always a good climate indicator, and I noticed a graph somewhere outlining sea levels during the previous interglacial, where two sharp reversals are shown, much more dramatic than the Younger Dryas. Plus that interglacial was warmer, and fairly abrupt as well.

    Not sure of the proxies they use, but here is a paper where I was only tangentially logistically involved in :

    There are some nice links at the bottom of the page. It’s seems to me we are in the same situation as the Clovis.

    Humans, playing with fire, overhunting, changing the landscape, climate changing, ice sheets melting, and the occasional large impact, on the order of hundreds to thousands to tens of thousands of years. Now speed that up the modern world, as in geologically instantaneous. Or even worse, a really bad day. I’m sure glad we’ve got NOAA and NASA to protect us.

    Clovis culture must have been spectacular.

    Comment by Thomas Lee Elifritz — 30 Jan 2009 @ 2:19 PM

  238. Thomas Lee Elifritz (227) — If you find the link to a paper on sea level reversals during termination 2, kindly post it. Such reversals would seem to disagree with the methane record analyzed in Carlson et al., 2008, linked in the footnote to Eric’s orginal essay beginning this thread. I, at least, see no easy way to reconcile such evidence.

    Comment by David B. Benson — 30 Jan 2009 @ 5:29 PM


    Impacts are going to be different, but there appears to be some nontrivial oceanic climate energy coupling between hemispheres during reversals and oscillations.

    That might explain some of the discrepancies.

    Comment by Thomas Lee Elifritz — 30 Jan 2009 @ 10:45 PM

  240. I don’t think they are invoking an impact theory to explain the Younger Dryas. Rather, the Younger Dryas simply lends concomitant evidence of an impact theory supported by many legs.

    Comment by Marc — 1 Feb 2009 @ 12:25 AM

  241. Gavin, is “Pt” known to you as someone who could show some real work, as you ask, to support any of the various notions “Pt” is posting?

    Or are you being polite? I can’t tell if these posts are just throwing words out and saying maybe there’s some explanation hidden in them.

    Comment by Hank Roberts — 1 Feb 2009 @ 12:16 PM

  242. The impact is invoked to help explain the ‘extinctions’ because other lines of evidence have various problems.

    If you throw an unusual impact into the mix, then all of the conflicting evidence starts to make sense. Any one of them is sufficient to cause the megafauna extinctions given enough time – man, climate and impacts. The problem is the abruptness and the regional nature of the events.

    The question still remains – what really happened. The farther back you go, the harder those questions get. What is remarkable about this situation, is that nothing much has changed in the intervening 12,900 years, except now we’re doing it again on a much larger scale. One big impact and all of your problems will be instantly reorganized into a scramble for survival. That’s how unprepared the human race is for the inevitable.

    The funny thing is, it doesn’t need to be this way. We could easily prepare ourselves for anything thrown at us if only we were to quit breeding uncontrollably, and even that’s a relatively (i.e. easy) simple problem to solve.

    Comment by Thomas Lee Elifritz — 1 Feb 2009 @ 2:41 PM

  243. Hank, H2O2 can be created by an oxygen radical from the ozone cycle via: H2O + 1/2 O2(.) = H2O2. The reaction typically requires Fe(III)-ligand intermediate, supplied by dust particles. So the H2O2 signal from melting can be amplified by the “dust” reaction.

    Looking at the H2O2 in the ice record from the arctic(which goes back 14.5 kya – 8 kya) — and interpreting it in terms of the dust contribution, there are weak 400 year cycles prior to the YD onset, nothing during the YD period, followed by a rapid increase within a few decades (ice melting in the NA), a gradual increase after that, followed by a strong up slope starting circa 9.5 – 10kya. The 400 cycles might be due to ITCZ movement which may have led to the wet conditions in the Sahara during YD cooling. The slope increase (ca. 9.5-10 kya) is probably due to the onset of extreme dry conditions in the Sahara which has been attributed to orbital tilt and perihelion timing.

    There’s bound to be a coupling between events in the tropics/semi-tropics and the rapid cooling in the NA at YD onset: no heat pump (via moisture transport) from the tropics (as alternate to THC?).

    Comment by Pt — 3 Feb 2009 @ 10:17 PM

  244. Hey, Pt, you may be the world’s expert on this stuff, but without citations to published work I — or likely any other amateur reader here — can’t tell if you’re just throwing words on the page, or riffing from your last paper in Nature or Science. Got sources for any of that?

    ReCaptcha says:
    “fanatics NOTED”

    Comment by Hank Roberts — 3 Feb 2009 @ 11:43 PM

  245. Hank, Chill. If you want to read peer reviewed, read the journals. This is a blog for crying out loud. I’m not selling anything. Take it for what it’s worth.

    Comment by Pt — 4 Feb 2009 @ 11:40 PM

  246. For those of you interested in bolide and impact physics and statistics, the analysis of these has moved to Mars :

    The atmosphere is very thin, but the dust covering makes finding these things very easy.

    Of course, we’ve also seen what happens at Jupiter :

    The moon is an ideal experimental testing ground with no atmosphere at all, but the noise level is very high ”

    Captcha : net problem

    Comment by Thomas Lee Elifritz — 6 Feb 2009 @ 1:47 PM

  247. And to complete the connection between humans playing with fire and climate, we have these reports out of Australia :

    Comment by Thomas Lee Elifritz — 8 Feb 2009 @ 9:44 PM

  248. A possible Younger Dryas Laurentide ice sheet impact remnant feature transposed onto glacial runoff terrain near Lake Nipigon :

    Nipigon Feature

    What I see here is a typical large butterfly feature with a smaller vaguely oval feature along the northwest edge, and lots of catastrophic flooding.

    Comment by Thomas Lee Elifritz — 12 Feb 2009 @ 1:41 AM

  249. A butterly? I do not see the real find here, looks like fools gold still. I encourage you to revisit the field reports from Tom Lowell, that comment on the lack of an outlet in this particular locale.

    Comment by mauri pelto — 12 Feb 2009 @ 11:06 AM

  250. I’ve read all those papers, mostly Peter Barnett. I don’t object to the general analysis that most of the Nipigon events postdate the Younger Dryas event, I just thought if there was a big bolide impact on the ice sheet at 12,900 BP in that area, traveling Northwest to Southeast, then perhaps there might be some evidence of it, and thus I decided to look. That is the only thing that stands out.

    The ‘butterfly’ is a result of Tunguska simulations. Most of the traditional Nipigon flows were east and west of Nipigon, and certainly there were many flows after any presumed Younger Dryas flow. Sea level rise is nearly continuous during the entire deglaciation, there are no obvious reversals. We already know that the YD outflows were not especially remarkable, there has to be something else, and this is yet another example of ‘something’. Of course, it could be nothing.

    And it was so much fun looking, and rewarding to spot something.

    Comment by Thomas Lee Elifritz — 12 Feb 2009 @ 12:04 PM

  251. Sufficiently related to be worth linking:

    “Climate ‘Flickering’ Ended Last Ice Age In North Atlantic Region”

    Comment by David B. Benson — 18 Feb 2009 @ 4:36 PM

  252. Dallas Abbott’s 536 AD Comet Impact Hypothesis

    I have prepared a short abstract of my Lake Nipigon astroblemish discovery for wider dissemination, since nobody appears interested.

    Darwin’s Valentine

    Captcha : trips center-field


    Comment by Thomas Lee Elifritz — 25 Feb 2009 @ 12:20 PM

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