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The Younger Dryas comet-impact hypothesis: gem of an idea or fool’s gold?

There was a paper in Science last week that has gotten quite a bit of press. It reports further evidence in support of the idea that the Younger Dryas — a distinct period towards the end of the last ice age when the deglaciation in the Northern Hemisphere was interrupted for a period of about 1300 years — was caused by a barrage of comets hitting North America.

When the first papers on this came out last year, we expressed skepticism. We remain skeptical and our reasons remain unchanged. But we think it is worth saying a bit more on this, because the reporting on this issue has largely ignored just how big an idea this is, and therefore how much more work would need to be done before it could be taken very seriously.

For background, see the good article by Kenneth Chang in the New York Times, which, however, does not address our main concerns with the hypothesis.

The brief history is that in 2007, Firestone and others published an article in PNAS showing evidence of various materials that may be diagnostic of extraterrestrial origin (and hence an impact) in layers of sediment dating to 12.9 thousand years ago, just before the beginning of the Younger Dryas cold event. Now, in a Brevia piece in Science Kennett and others show further evidence: “abundant nanodiamonds in sediments dating to 12.9 ± 0.1 thousand calendar years before the present at multiple locations across North America.”

According to Richard Kerr’s news item that accompanies the article in Science, at least some experts are skeptical that Kennett and others have really found nanodiamonds, or that, even if they have, they are necessarily evidence of an impact. But we don’t claim enough expertise in nanodiamond detection or interpretation to have an opinion on this aspect, so let’s give them the benefit of the doubt. Suppose there really was an impact (or impacts) at the right time in the right place. We’d still be skeptical that this was a trigger for the Younger Dryas.

Among our reasons for skepticism (again, see our earlier post on this) there is a basic statistical problem. The problem is — and this context is missing from most if not all of the articles we’ve seen on this — that explaining the Younger Dryas in terms of an impact leaves all the other rapid climate change events (the so-called “Dansgaard-Oeschger events”) of the last glacial period unexplained.* 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.

We recognize that it isn’t entirely an either/or situation. Indeed, the suggestion appears to be that a cometary barrage causes various kinds of havoc, including the ice sheet collapse that led to ocean circulation change (the most well-evidenced proximal cause of rapid climate change). But 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, especially given that it would still be necessary to show that the ice sheet would care about comets, which we also consider unlikely (see the good discussion — particularly Mauri Pelto’s comments — on this over at the Open Mind Blog). On the other hand, if abrupt climate changes don’t happen on their own — if they only happen due to extraterrestrial causes — then one would want to see evidence of impacts for at least a few more of them, not just one. That would be a truly exceptional paradigm-breaking discovery, going against just about everything we think we know about the system.

We emphasize that we are not saying “the Younger Dryas can’t have been caused by a comet, unless all the Dansgaard-Oeschger events were caused by comets”. We’re saying that we see no need to invoke such an hypothesis, so the level of proof required for this extraordinary idea will need to be extraordinarily strong. So far, it doesn’t appear that that is the case.

Think about it. 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. Now that would be news. Perhaps further research by Kennett, Firestone and others will indeed show that to be the case. We’re not, however, holding our breath.

———-
*Not to mention that there is an event similar to the Younger Dryas at the end of at least one other glacial period, “termination III” (see e.g. Carlson et al., 2008).

252 Responses to “The Younger Dryas comet-impact hypothesis: gem of an idea or fool’s gold?”

  1. 51
    Bob North says:

    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.

  2. 52
    ike solem says:

    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: http://www.sciencedaily.com/releases/2004/10/041001092938.htm 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”.

  3. 53
    David B. Benson says:

    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

    http://www.lpl.arizona.edu/impacteffects/

    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.”

    from

    http://en.wikipedia.org/wiki/Younger_Dryas

    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.”

    from

    http://en.wikipedia.org/wiki/Lake_Agassiz

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

  4. 54
    William Astley says:

    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.

  5. 55
    Jim Eager says:

    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?

  6. 56
    David B. Benson says:

    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.

  7. 57
    Vernon says:

    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.

  8. 58
    David B. Benson says:

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

    http://www.sciencedaily.com/releases/2008/07/080702160950.htm

    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.

  9. 59
    David B. Benson says:

    From

    http://www.ngdc.noaa.gov/mgg/greatlakes/lakeontario_cdrom/html/gmorph.htm#a8

    “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.”

  10. 60
    Jim Eager says:

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

  11. 61
    Jim Eager says:

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

    Captcha: Arctic signifi-

  12. 62
    Hank Roberts says:

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

  13. 63
    Mark says:

    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.

  14. 64
    Mark says:

    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.

  15. 65
    David B. Benson says:

    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.

  16. 66
    Mark says:

    and on 63 I made a boo boo.

    Darnit. It’s been a long week…

  17. 67
    spilgard says:

    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:

    http://www.physorg.com/news62952904.html

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

  18. 68
    David B. Benson says:

    Tunguska, not what I wrote before.

    We all make mistakes.

  19. 69
    Jim Eager says:

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

  20. 70
    Jim Cross says:

    #56, 49

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

    Hank,

    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:

    http://www.google.com/search?q=novice++web+browser+help&btnG=Search

  21. 71
    William Astley says:

    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.

    http://www.pnas.org/content/104/41/16016/suppl/DC1#F7

    Quote:
    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). …

    Quote:
    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.

  22. 72
    William Astley says:

    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.

    http://www.pnas.org/content/104/41/16016/suppl/DC1#F7

    Quote:
    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). …

    Quote:
    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.

  23. 73
    Cathe P says:

    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.

  24. 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.

  25. 75
    Nick Gotts says:

    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.

  26. 76
    Mark says:

    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.

  27. 77
    Andre says:

    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) http://www.iceandclimate.nbi.ku.dk/publications/papers/pdfs/242.pdf 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

  28. 78
    Ray Ladbury says:

    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.

  29. 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.

  30. 80
    ike solem says:

    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?

    http://www.ncdc.noaa.gov/paleo/pubs/monnin2001/fig1-lg.jpg

    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.

  31. 81
    D iversity says:

    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.

  32. 82
    Hank Roberts says:

    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.

    http://lgmacweb.env.uea.ac.uk/lequere/publications.html

    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).

  33. 83
    Ray Percatr says:

    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:
    http://www.pnas.org/content/105/18/6520

  34. 84
    David B. Benson says:

    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

    and

    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.

  35. 85
    David B. Benson says:

    “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:

    http://adsabs.harvard.edu/abs/2008AGUFMPP13C1472F

  36. 86
    John Mashey says:

    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.

  37. 87
    John Lang says:

    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.

  38. 88
    David B. Benson says:

    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

    http://en.wikipedia.org/wiki/Jomon

    and the Natufians

    http://en.wikipedia.org/wiki/Natufian_culture

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

  39. 89
    Jim Cross says:

    #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?

  40. 90
    David B. Benson says:

    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).

  41. 91
    Jerry says:

    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.

  42. 92
    John Mashey says:

    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 :-)

  43. 93
    David B. Benson says:

    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.

  44. 94
    David B. Benson says:

    “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.”

    from

    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”

    http://bulletin.geoscienceworld.org/cgi/content/abstract/116/5-6/729

  45. 95
    Jim Cross says:

    #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.

  46. 96
    ike solem says:

    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:

    http://www.nature.com/nature/journal/v339/n6225/abs/339532a0.html

    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.

  47. 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.

  48. 98
    ccpo says:

    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.

    Cheers

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

  49. 99
    jcbmack says:

    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.

  50. 100
    Ray Ladbury says:

    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.


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