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Revisiting the Younger Dryas

Filed under: — group @ 17 July 2010

Guest Commentary by Chris Colose

One of the most intriguing and well-studied climatic events in the past is the Younger Dryas (YD), a rather abrupt climate change between ~12.9 and 11.6 thousand years ago. As the world was slowly warming and ice was retreating from the last glaciation, the YD effectively halted the transition to today’s relatively warm, interglacial conditions in many parts of the world. This event is associated with cold and dry conditions increasing with latitude in the North, temperature and precipitation influences on tropical and boreal wetlands, Siberian-like winters in much of the North Atlantic, weakening of monsoon intensity, and southward displacement of tropical rainfall patterns. RealClimate has previously discussed the YD (here and here) however there have been a number of developments in recent years which deserve further attention, particularly with respect to the spatial characteristics and causes of the YD.

The YD is often discussed in the same context as the ‘Dansgaard-Oeschger’ events seen in the ice cores during full glacial conditions, and the ‘Heinrich events’ of layers of ice-rafted debris in North Atlantic ocean sediments. Indeed, some people occasionally refer to the YD as Heinrich event 0, but this implies that the YD cooling was caused by an ice-rafting event (probably untrue) and should be avoided.  The YD occurred last of several prominent and abrupt deglacial events including Heinrich Event 1 (~17.5 to 16 ka) which is an event contained within the Older Dryas (18 to 14.7 ka), followed by the Bølling-Allerød warm period (~14.7 to 12.9 ka) whose end then marks the start of the YD. The end of the YD can be said to be the start of the Holocene. It has been proposed that the warmings before and after the YD can be viewed as Dansgaard-Oeschger events with the YD just a regular cold (i.e. stadial) phase in between (Rahmstorf 2002, 2003). In Antarctica (~15 to 13 ka), the most featured event is that as the Younger Dryas begins, warming is occurring in Antarctica.  The cold period in Antarctica that precedes the Younger Dryas is referred to as the Antarctic Cold Reversal (ACR) (see figure, from Shakun and Carlson, 2010) and was once thought to be in phase with the YD.  They are neither directly in phase nor anti-phased with one another (see e.g. Steig and Alley, 2002).










Fig. 1. Deglacial ice core time series and insolation. (a) GISP2 δ18O (black step plot) (Blunier and Brook, 2001). (b) Byrd δ18O (grey step plot) (Blunier and Brook, 2001). (c) Insolation (Incoming Solar Radiation) for 60ºN on June 21 (black line) and for 60ºS on December 21 (dashed black line) (Berger and Loutre, 1991). The timing of the Younger Dryas (YD), Bølling/Allerød (B/A), Heinrich Event 1 (H1), Oldest Dryas (OD) and Antarctic Cold Reversal
(ACR) are denoted.

Unlike changes in global temperature (such as modern day global warming) which can be understood as a result of perturbations to the planetary energy balance, the millennial-scale climate changes during the last glaciation are viewed primarily from the lens of internal dynamics, including ice retreat and re-organizations of ocean circulation. They are not dominated by changes in global mean temperature but rather changes in temperature distribution, explained by changes in oceanic or atmospheric heat transport. In particular, proxies of deepwater formation show large reductions in the Atlantic meridional overturning circulation (AMOC) coincident with the start of the YD. This suggested weakening of overturning circulation provides immense explanatory power for the onset of the YD although no consensus has emerged concerning the trigger of the AMOC reduction. There are some radiative changes associated with millennial-scale climate change induced by the ice-albedo effect, extra dust loading out of Asia during cold snaps, as well as greenhouse gas feedbacks– although they are relatively small. However, pinning down the exact sequence of causes and effects is rather difficult since precise chronologies and global-scale reconstructions are difficult to come by prior to the Holocene.

A new study though (Shakun and Carlson, 2010) has compiled over 100 high-resolution proxy records to characterize the timing and extent of the Last Glacial Maximum (LGM) and the deglacial evolution into the Holocene, including the shorter-lived Younger Dryas. Several of the key features of the study include:

  1. The global mean cooling of the LGM relative to the peak of our current interglacial is approximately 5ºC as a minimum value. It is likely larger than this since many of the records are from the ocean which are typically less sensitive to temperature change than landmasses, and further, adiabatic cooling of marine air advected over land masses would result from the ~120 m reduction in sea level. The cooling is global in scale and largest at high latitudes, as expected from polar amplification.
  2. In contrast, during the YD, there is much more spatial heterogeneity as the North became colder and drier (increasing with latitude) while the South became warmer and wetter in the opposite sense. The global mean cooling during the YD is only ~0.6ºC .  The tropics cooled by 2.5ºC (with an error of about a degree in either direction) at the LGM, yet exhibited very little temperature change during the YD. Thus, while the YD was a global scale climate change event with widespread signatures, it was not a widespread global cooling event.


    Fig. 2. Magnitude of the glacial-interglacial temperature change relative to absolute latitude. (Shakun and Carlson 2010)Fig. 3. Magnitude of the Younger Dryas temperature change. Map of the Younger Dryas temperature anomaly (a). Circle denotes the size of the temperature change. Blue is cooling, red warming (Shakun and Carlson 2010).

  3. The timing of the LGM and peak interglacial is synchronized between hemispheres on orbital timescales, which the authors attribute primarily to the global radiative forcing provided by CO2. As has been noted in the past, the CO2 lags the onset of deglaciation in most records, as this is paced by summer insolation changes. However the CO2 still acts as the dominant temperature-change influence throughout the deglacial period and provides an effective means to communicate temperature anomalies to the tropics. On the other hand, the YD exhibits the well-known bipolar see-saw effect which involves a reduction in northward heat transport, which warms the South. The see-saw is best expressed in the mid to high latitudes, although the see-saw model is a poor descriptor for the tropical variability.

The see-saw effect during millennial-scale climate changes has been confirmed before (also discussed at RealClimate in the context of the somewhat similar Dansgaard-Oeshger events) and is consistent with modeling efforts of the climate evolution during the last deglaciation, including Liu et al., 2009 (discussed here) who show that current state-of-the-art models can simulate the magnitude of abrupt climate changes well.

So what caused the reduction in the AMOC?

The most prevalent concept for slowing AMOC involves a reduction in the surface water density at the ocean surface via adding freshwater into the ocean. The preferred location is primarily the North Atlantic, which is a key point for deep ocean convection. The original idea for this to cause a YD-event was proposed in 1976 by Johnson and McClure, and involved the opening of eastern Lake Agassiz outlet via northward retreat of the Laurentide Ice Sheet out of Lake Superior. This re-routed drainage from the Mississippi to the St. Lawrence River.

There is a difference between the diversion of continental runoff from the Mississippi River (routing) and the relatively fast pro-glacial lake drainage to a new level (flooding). In contrast to the Johnson and McClure paper, many recent studies have focused on short-lived floods, although the re-routing mechanism might be a necessary, and in fact primary ingredient (Carlson et al., 2007; Carlson and Clark, 2008) in accord with modeling studies which require a persistent forcing to substantially alter AMOC (Meissner and Clark, 2006).

Evidence of a specific flood water pathway at the right time has proven to be elusive. No clear evidence exists for a flood event into the Atlantic, though evidence discussed by Murton et al. (2010) for an Arctic pathway has recently emerged.

There has also been interest in the prospect of a comet impact during the YD triggering a flood (e.g., Firestone et al., 2007 discussed previously at RC) although subsequent work has suggested that their results are not robust (Surovell et al., 2009), and it is likely that the impacts a comet would have on atmospheric chemistry, particularly the formation of nitrate and ammonium, is inconsistent with observations in ice core records (Melott et al., 2010). Further, the problem with a comet impact still remains — how could it generate a continuous freshwater forcing? Because of dicey evidence and no predictive ability, the comet hypothesis has not gained much favour.

Recently another hypothesis has been put forward: The Younger Dryas, instead of being a freak occurrence, is instead a key (and normal) part of the deglaciation process. This was most clearly expressed in a new paper by Broecker et al (2010) ( including George Denton and Richard Alley). Their main point is that a catastrophic flood or comet would only serve as a trigger for an event that was already primed to happen. Evidence for this comes primarily for the existence of YD-like events during previous deglaciations, notably from Chinese stalagmite data (Cheng et al., 2009) who looked at monsoon patterns in the past. In particular, a YD-like event shows up during Termination III (~ 245 ka) and possibly Termination IV, which share similar characteristics to the YD. The finding of many events with characteristics like the YD further provides evidence against the necessity of comet-impact hypothesis. However, this concept doesn’t negate the need to understand the mechanisms for the YD or its potential predecessors. Whether it was primed to happen or not, what actually happened and how is still of great interest.

Broecker et al (2010) cite Lowell et al (2005) and Fisher et al (2008) to justify their reason for why the flood hypothesis is unappealing, but further work done by Carlson et al. (2007), Carlson and Clark (2008), and Carlson et al. (2009) provides newer support for the re-routing hypothesis. Furthermore, while Broecker et al. emphasize the lack of evidence for a catastrophic event, if the slower re-routing hypothesis is correct, then the lack of evidence for a sudden flood is irrelevant. This may very well be the mechanism that is common to previous deglacial events.

The existence of events similar to the YD in the more distant past has been proposed before (Carlson, 2008). By analyzing paleo-methane concentrations, Carlson (2008) also noted that events similar to the YD happened during T III and possibly earlier deglaciations (see figure, from Broecker et al 2010).






Fig. 4. Major events surrounding Termination III. (A) shows Vostok temperature deviation (purple) and CH4 (blue) records (Suwa and Bender, 2008). (B) shows EPICA/Dome C (EDC) δD (orange) and CH4 (blue) records (Loulergue et al., 2008). (C) is the Vostok CO2 record (Petit et al., 1999). (D) is the absolute-dated Asian Monsoon record from Sanbao Cave, China (Cheng et al., 2006). (E) and (F) show IRD and inferred seawater δ18O records from marine core ODP-980 (McManus et al., 1999). Both Vostok and EDC timescale were shifted in order to correlate the abrupt jump of the last portion of CH4 in ice cores to the abrupt monsoon jump in panel (D) (Cheng et al., 2009). The ODP-980 records are on original timescales. Two Weak Monsoon Intervals (WMI) are marked by yellow background. Termination III events, analogous to the YD, B/A, ACR and MI are labeled: YD III, B/A III, ACR III and MI-III. Figure is simplified from that in Cheng et al. (2009). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article).

As a conclusion, over the last couple of years, there has now been growing evidence that an event similar to the YD is not “unique” but instead is a common theme across various deglacial events; this provides evidence against the necessity for a “catastrophic trigger,” and while it may be the case that a comet or some other catastrophe occurs at each termination, that seems improbable.


References
Broecker, W.S., Denton G.H., Edwards L.R., Cheng H., Alley R.B., Putnam A.E., 2010. Putting the Younger Dryas cold event into context. Quaternary Science Reviews , 29, 1078-1081
Carlson, A.E., 2008. Why there was not a Younger Dryas-like event during the Penultimate Deglaciation: Quaternary Science Reviews, v. 27, p. 882-887
Carlson, A.E., and Clark, P.U., 2008. Rapid climate change and Arctic Ocean freshening: Comment: Geology, v. 36, p. e177
Carlson, A.E., Clark, P.U., and Hostetler, S.W., 2009. Comment: Radiocarbon deglaciation chronology of the Thunder Bay, Ontario area and implications for ice sheet retreat patterns: Quaternary Science Reviews, v. 28
Cheng, H., Edwards, R.L., Broecker, W.S., Denton, G.H., Kong, X., Wang, Y., Zhang, R., and Wang, X., 2009. Ice Age Terminations: Science, v. 326, p. 248–252
Firestone, R.B., et al., 2007. Evidence for an extraterrestrial impact 12,900 years ago that contributed to the megafaunal extinctions and the Younger Dryas cooling: Proceedings of the National Academy of Sciences of the United States of America, v. 104, p. 16016–16021

Fisher, T.G., Yansa, C.H., Lowell, T.V., Lepper, K., Hajdas, I., Ashworth, A., 2008. The chronology, climate, and confusion of the Moorehead Phase of glacial Lake Agassiz: new results from the Ojata Beach, North Dakota. Quaternary Science Reviews, 27, 1124–1135

Johnson, R.G., McClure, B.T., 1976. A model for Northern Hemisphere continental ice sheet variation. Quaternary Research 6, 325–353
Liu, Z., Otto-Bliesner, B., He, F., Brady, E., Thomas, R., Clark, P.U., Carlson, A.E., Lynch-Stieglitz, J., Curry, W., Brook, E., Erickson, D., Jacob, R., Kutzbach, J., and Chen, J., 2009. Transient Climate Simulation of Last Deglaciation with a New Mechanism for Bølling-Allerød Warming: Science, v. 325, p. 310-314
Lowell, T.V., Waterson, N., Fisher, T., Loope, H., Glover, K., Comer, G., Hajdas, I., Denton, G., Schaefer, J., Rinterknecht, V., Broecker, W., and Teller, J., 2005, Testing the Lake Agassiz meltwater trigger for the Younger Dryas: EOS (Transactions, American Geophysical Union), v. 86, p. 365–373
Meissner, K.J., and Clark, P.U., 2006. Impact of floods versus routing events on the thermohaline circulation: Geophysical Research Letters, v. 33, L15704
Melott, A.L., Thomas, B.C., Dreschhoff, G., and Johnson, C.K., 2010. Cometary airbursts and atmospheric chemistry: Tunguska and a candidate Younger Dryas event, Geology, v. 38, 355–358
Murton J.B., Bateman M.D., Dallimore S.R., Teller J.T., Yang Z., 2010. Identification of Younger Dryas outburst flood path from Lake Agassiz to the Arctic Ocean, Nature, 464, 740-743
Shakun, J.D., and Carlson, A.E., 2010. A Global Perspective On Last Glacial Maximum to Holocene Climate Change: Quaternary Science Reviews.
Steig, E.J., and Alley, R.B. Phase relationships between Antarctic and Greenland climate records. Annals of Glaciology 35: 451-456 (2002).


92 Responses to “Revisiting the Younger Dryas”

  1. 51
    David B. Benson says:

    Proglacial Lake Agassiz stopped draining to the Mississippi about at the onset of YD. There is no evidence of drainage to the east at that time and as Chris Colose mentioned, now evidence of drainage to the Arctic Ocean. This would force more sea ice out Fram Strait, hleping to hamper deepwater formation.

    But even more fresh water was stored in proglacial lakes furhter east. I found (and linked in a comment on the Clovis Comet thread here on RC, near the end) a paper demonstrating massive flooding down the Mohawk/Hudson where the fresh water would then effect the Labrador deepwater formation. The occurred at about the same time as the onset of YD.

    But more. The Baltic Ice Lake drained, with the fresh water going to the Nordic Sea, again at about the same time as the onset of YD, possibly causing a salinity crisis there. The amount of water from this first drainage is poorly characterized sine a subsequent flooding removed evidence at the outflow.

    Now all three of these locations were set to flood at any time and it might be simply coincidence that all went nearly simultaneously. YD-III looks enough milder to me to suggest that the proglacial ice lakes at that termination were less simultaneous. Which, combined with other mysteries, still leaves open the Clovis Comet possiblity, although that might have been a meteor shower; there is some suggestive evidence buried under the east end of Lake Ontario.

  2. 52

    OT but what’s happened to the updates on Ice Volume? Seems they have quit just when its getting interesting? Is the value so far away from the model that the model can no longer be considered valid?

    http://psc.apl.washington.edu/ArcticSeaiceVolume/IceVolume.php

    An update would be informative, eh!

    N

  3. 53
    Thomas says:

    Brian @36, my estimation of 100M of head needed already accounted for the factor of ten due to ice being approx 90% as dense as water. Roughly speaking 10M of water is one atmosphere of pressure.
    Also if the water overflows an ice dam, might thermal plus erosive effects enlarge the outlet? We do have modern smaller scale analogies, I believe Lake George in Alaska is dammed every year and breaks out every summer.

  4. 54

    There is no evidence of drainage to the east at that time

    I just love it when people use the term ‘no evidence’.

    No. None. Nada. Will you reconsider, just a little bit?

  5. 55
    Hank Roberts says:

    The notion “don’t concern yourself about climate change, it’s not something we can control” is clearly wrong, we know how to control it:
    http://www.google.com/search?q=alley+big+control+knob

    We know we’ve turned it up; we know how to turn it down, and we know the kinds of precautions we should have been taking from experience:
    http://nobelprize.org/nobel_prizes/chemistry/laureates/1995/crutzen-lecture.pdf

  6. 56
    Brian Dodge says:

    “Also if the water overflows an ice dam, might thermal plus erosive effects enlarge the outlet?” Sure.
    Its clear from the geologic record of the Missoulan flood left in the Washington scablands that glacial dams can hold back very large lakes and when they fail they do so catastrophically. The way I envision it occurring is that as the ice age ends, large glacial/ice cap structures that have accumulated over thousands of years to great thickness block drainages. the extended preceding cold and thickness of the ice insure that the base is frozen to the underlying geology, and plastic flow has filled all the contours so it’s sealed.

    As the climate warms, water begins to accumulate behind the dam. When the meltwater reaches significant depth(i.e., >900 meters for a 1km high dam), the hydrostatic differential starts to deform and lift the ice dam, and cause failures along planes of weakness – the ice will have been accumulating as snow but sliding and deforming downhill, flowing down the drainage – even where the plastic flow has closed any gaps, there will be weaker areas, and these will fail first. I think that because of geothermal heat and possibly frictional heating, combined with the fact that the top of a thick glacier will be cooler because of the adiabatic lapse rate, the bottom will be warmer and weaker, and will be where the failure will first occur. The Greenland Ice sheet is warmer at the bottom than the top[1], and lakes have been observed beneath Antarctica[2].

    The lifting and lubrication of the glacier bottom will cause faster flow, and more stress and failure of the moving ice. With a large volume of water available, the glacial dam could rapidly transform into a megaslushy and then a flood.

    The relatively low relief [3] that separates south flow to the Mississippi and north flow to the Red River and the Mackenzie River would have limited the depth of a lake forming at the southern edge of the melting ice sheet at the end of the last ice age. I surmise that an ice dam failure that would release this into the Arctic would have been caused by the thickness of the glacial cap decreasing rather than the depth of the lake increasing. I wonder what sea level was at the beginning of the Younger Dryas – low enough to close the Bering Straight? – and also how thick the Arctic sea ice was. I suppose it’s possible that the fresh water flowed over the surface of the ice and into the Fram Straight, with the thermal energy contained in the water melting snow on the surface and the decrease in albedo caused by wetting the surface increasing the total flow of freshwater into the North Atlantic.

    [1] http://www.sciencemag.org/cgi/content/full/282/5387/268/F1
    [2] http://www.sciencedaily.com/releases/2008/01/080115173541.htm
    [3] Edmonton, Alberta sits almost on the Arctic/Hudson bay divide at an elevation of 668 meters; Fargo, North Dakota, on the Red River of the North, which drains into Lake Winnipeg & then into Hudson’s Bay is 275 meters above sea level. Looking at the maps at http://atlas.nrcan.gc.ca, it looks like the low spot on the divide is about 500 meters.

  7. 57
    Didactylos says:

    Nigel: The PIOMAS graph was updated just a few days ago, on the 17th. What exactly are you complaining about?

    They won’t be able to revalidate the model until a few years of Cryosat-2 data is available.

  8. 58
    Thomas says:

    Thanks Brian. Your statement about glacial ice being warmer at depth is true near the center of an icecap. Nearer the edges, in an abalation zone ice flow is from up glacier, where the climate is colder than near its edges -especially its equator ward edges. Because ice is nearly incompressible, the adiabatic heating of ice under compression is pretty low. So in these zones, I think the colder ice is advected at depth, and the near surface is relatively warm, reflecting the warmer climate at the ablation zone. The same ice sheet can have both positive and negative thermal gradients with depth. In any case there is no disagreement about the potential for catstrophic release of dammed up water.

  9. 59
    Aaron Lewis says:

    Re 18: Hank,
    Certainly, ice sheets can support a lot of ice and be very thick. Ice under a lot of ice tends to be cold and strong. However, ice in contact with water is warm and weak. Ice does not do such as good job of supporting liquid water as the moulins on Greenland demonstrate.

    Re 27: Nick, If ice is in contact with liquid water, then the ice is at its melting point. Any depression of its melting point means that it – melts (if it has a source for the heat of fusion.)

    Re 35: Hank, Physics and observations in Greenland and France suggest that 6 meters is greatest head of liquid water that ice can support. Look around Greenland. These days Greenland has lots of pools of melt water, some kilometers across. How deep are they? None of the pools of melt water on the surface of the ice are deeper than 6 meters. What happens when they become deeper than 6 meters? They fall through the ice – hundreds or thousands of meters of ice. I brought this up to the RC group a few years and go and they agreed that some physicist had made such theoretical calculations, but said nobody had actually observed the phenomenon. Well, now we have multiple good observations of big (liquid) lakes falling through thick ice.

    Re 36/56: Brian, A 1 km thick ice sheet will dam and retain a lake of water that is just over 6 meters deep. Then, the water will punch a hole (moulin) through the ice dam. (Just as ponds of melt water punch moulins through the Greenland Ice Sheet.) We really have to change our thinking about Lake Missoula, and etc. Those floods had to start as mostly ice slurries, mobilized by small amounts of water from shallow, super glacial lakes, which fell through thick ice to form high energy streams of water. Then, the ice melted as it moved downhill. This has unpleasant implications as we consider failure modes for extant ice sheets.

  10. 60
    Hank Roberts says:

    > PIOMAS
    I’m still seeing last month’s chart too (dated 6/18) as of right now.

    Are you seeing a 7/17 chart? I’m looking at
    http://psc.apl.washington.edu/ArcticSeaiceVolume/images/BPIOMASIceVolumeAnomalyCurrent.png

  11. 61
    flxible says:

    Hank – That link shows a chart labeled “Last day 1020-07-17″ at bottom left. Refresh your browser, or clear the cache.

  12. 62
  13. 63
    Lawrence Coleman says:

    Just perusing the uni-bremen site re: arctic ice sheet. Looks like the N.E passage will be open shortly with the N.W maybe at the most a month away again. You could probably quite safely steer an ice breaker though the icy mush of the N.E passage right now.
    By the looks of it the summer of 2010 still has the potential to set a new record in extent of ice melt.

  14. 64
    E.P. Grondine says:

    It appears that Lake Agassiz emptied to the north, its path there opened by a larger piece of the comet impact.

    If you are ever able to look at the temperatures of the Pacific Current at the west coast of North America, the mechanism behind the data will probably become clear to you. Cooler water led to less snow falling in North America, and more sunlight absorbed.

    E.P. Grondine
    Man and Impact in the Americas

  15. 65
    Sekerob says:

    Re #60 Hank Roberts said:20 July 2010 at 10:06 PM “can you see this”.

    Yes, the front page shows 6.18 and the one of about 2 weeks ago increasing the anomaly from about -10,700 km cubic to about -11,250 never appeared. So here we are 1 month later and still/back on about 10,700 km cubic missing. JAXA is speeding up again too dropping away from the 2009+2006 track.

    For the interested, Walt Meier of NSIDC had a guest post a few weeks ago up over on WUWTBOH was comparing his past Sea Ice Volume work with PIOMAS. Interesting, my interpretation being that he gave the thumbs up to the PIOMAS effort.

  16. 66
    Hank Roberts says:

    Today, the 7/17 PIOMAS chart appears for me too. Odd delay, I had yesterday flushed caches and rebooted. Maybe they have several servers.

  17. 67
    David B. Benson says:

    Thomas Lee Elifritz (54) — I’ve read the attempts to develop evidence for Lake Agassiz outflow to the east at the onset on YD; no evidence found. Several of the papers are linked on the Clovic Comet thread here on RC. A summary is available in Wally Broecker’s “The Great Ocean Conveyor” (2010).

    Brian Dodge (56) — Bering Strait was effectively closed at the onset of YD. Not that it mattered much.

  18. 68

    It appears that Lake Agassiz emptied to the north, its path there opened by a larger piece of the comet impact.

    Ed, the existence of the comet impact is by no means assured. The continent and pole was covered by large ice sheets, periodically waxing and waning for two million years or so. The continental geography hasn’t changed all that much in that time besides the subsiding and rebounding of the lithosphere :

    http://en.wikipedia.org/wiki/Post-glacial_rebound

    which in the area of interest could have been as great as several hundred meters. Since interglacials are defined by the more or less disappearance of the ice sheet on the continent, more or less all of the ice makes it to some ocean somewhere as fresh water, over a period of time. The question is the amount of fresh water runoff forcing, and over what time period the forcing operates, in order to achieve the observed climatic trigger event. The invocation of cosmological catastrophism may or may not be required, depending on the details of the models describing the effect, and the cometary hypothesis would be much better accepted if there was some definitive evidence for its occurrence, and that remains yet to be demonstrated. So far the only thing in its favor is the detection of nanodiamonds downwind (Greenland) from a presumably more localized impact near somewhere near Lake Nipigon, and the veracity of those claims have yet to be demonstrated. There does appear to be a small blip in the atmospheric nitrogen at the time, but again the resolution of the ice proxies prohibits any definitive statement one way or the other, just like the lake bed and Channel Island proxies do not definitively exclude a more ice and volatile rich impacts at a localized site, contrary to the claims of the authors and the media. The availability of highly resolved ice and sediment proxies should be forthcoming shortly which should be able to make some definitive statements on this interesting problem, which is interesting precisely because of its multidisciplinary applications to a wide variety and planetary – geological, biological and anthropological issues involved. Even without invoking a cometary impact, the terrain in and around the entire Nipigon basin is worth another penetrating look.

  19. 69
    Prokaryotes says:

    More on north–south seesaw behavior.

    http://www.pnas.org/content/107/27/12091.abstract

  20. 70
    E.P. Grondine says:

    Re: number 51 – The Kiscoty-Llyodminster structure needs to be investigated immediately, but the funding for this is $0. You’re right about http://cosmictusk.com – the First Peoples accounts of this impact event may be found there and help to limit the YD transition solution space.

  21. 71
    Brian Dodge says:

    “Evolution of melt pond volume on the surface of the Greenland Ice Sheet” W. A. Sneed and G. S. Hamilton, GEOPHYSICAL RESEARCH LETTERS, VOL. 34, L03501, doi:10.1029/2006GL028697, 2007
    “Liestїl et al. [1980] describe an emptied surface lake on Brїggerbreen, Svalbard, with a length of ~200 m and a maximum depth of ~10 m. Echelmeyer et al. [1991] report on a number of surface lakes in the upper portion of Jakobshavns Isbrae drainage basin with surface areas from a few tens of square meters up to 10 km2 and depths ranging from <1 m to 20 m in crevasses.”

    In “Glacier Lakes and Outburst Floods In the Nepal Himalaya”, T. YAMABA1 & C. K. SHARMA2 Snow and Glacier Hydrology (Proceedings of the Kathmandu Symposium, November 1992). IAHSPubl. no. 218,1993. the authors describe measurements taken on Imja glacial lake. The debris covered Ice core dam at the outlet end is retaining a lake with a maximum depth of 98 meters and an average depth of 47 meters. They say ” “The ice body under the end moraine is a fossil ice of Imja Glacier,” which was deposited between the 15th and 19th centuries.

  22. 72

    no evidence found.

    Really, when people say that, it just makes my day.

  23. 73
    Chris G says:

    OT, but regard to the CRU and DOE funding, I amongst many spurious ones, I found a hit that appears to be credible. Summary: US DoE has been funding the CRU, and halted this pending the outcome of invetigation(s). Not settled still,

    “DoE spokeswoman Stephanie Mueller said: “The Department is not currently funding them, but they do have a renewal application under review that is proceeding through the normal application process. ”

    http://www.bbc.co.uk/news/uk-england-essex-10712286

    Otherwise, Inhofe was potentially linked to a litigation hold from the DoE(?) regarding the preservation of documents.

  24. 74
    Chris G says:

    On topic, I’d give this a blue or black rating. It’s definitely not a double black; that’s reserved for when I have to dust off memories of eigenvectors and which greek letter means what mathematical concept.

    So, what I’m walking away with is that there may be a repeating pattern that the YD is an example of. If you surmise that the mechanics of ice sheet disintegration as they are warmed is pretty much the same for any ice sheet, and that the follow-on effects (such as increased freshwater flow into northern oceans) will be likewise similar, given location and land-form similarities, this is entirely plausible. Any random events, such as a comet impact, or relative timings of Asian, European, or North American sheet collapse, would affect the particular shape and magnitude, but not significantly affect the relative timing and general shape.

  25. 75
    Chris G says:

    Still thinking. If there really is a repeated pattern of gradual warming followed by a sudden dip (linked to AMOC changes), then final, inter-glacial warming, I’m thinking this lends more credence to the idea that ice sheets, probably because of their albedo, are self-stabilizing up to a point, but when some threshold is crossed, they collapse rapidly.

  26. 76
    John Pollack says:

    Lakes Superior, Huron, Michigan, and Ontario all contain regions well below sea level. These could not have been excavated by water erosion, only glacial scour. This implies that during the evolution of these lakes, an ice sheet was able to displace a great depth of water down to the bedrock. I see nothing at all to prevent an ice dam from holding back a deep lake, then eventually giving way as the climate warmed and the ice sheet shrank.

  27. 77
    Chris Colose says:

    Chris G,

    Also, the characteristics of the escape from glaciation depend on the solar insolation. T1 is similar to T3 in this regard, but their doesn’t really appear to be a YD-like event during the last deglacial event ~120 kya. Carlson (2008) attribute this to higher insolation and consequently, more rapid Northern Hemisphere ice sheet retreat and AMOC reduction. If re-routing occurred in a simlar way to the the YD then freshwater influx would have occurred in an already suppressed AMOC, resulting in little climatic impact.

  28. 78
    David B. Benson says:

    Thomas Lee Elifritz (72) — Happy to have made you day, but rather than such pointless comments, why don’t you go read the relevant literature yourself?

  29. 79

    I read it all the time. What, pray tell, do you think happened up in Nipigon before after the Younger Dryas? Surely some or all of that water made its way to the ground, to the oceans and to the atmosphere somewhere?

    It’s the volumes, the time evolution and the periods that I am interested in. The sequences and the chronologies of the flooding events. The results are all around me here, I can’t possibly miss them. If the ‘science was settled’ then we wouldn’t be discussing it, would we. Why just a few years ago we were discussing the previous termination issues and I do remember posting a previous paper by Siddall et al. So you can hardly accuse me of not keeping up.

    That being said, the ice line was approaching Nipigon before the reversal, a meltoff as rapid as that would be a sight to see of you had a spare few thousand years. It’s much better to watch it speed up to 100 years, no?

  30. 80
    David B. Benson says:

    Thomas Lee Elifritz (79) — At the time of YD onset Lake Nipigon was under the Laurentide Ice Sheet. Here is an easily locatable paper:
    http://adsabs.harvard.edu/abs/2007AGUFMOS33A1001W
    stating that the (quite interesting) Nipigon phase of proglacial Lake Agassiz evolution was quite a bit later.

    This thread is about an event, YD onset, with a highly precise dating (+- 50 years) and several enigmatic aspects; several of those were addressed in the comments of the Clovis Coment thread here are RC but there is also one more which Chris Colose did not address: the noted archaeologist C. Vance Haynes Jr. has recently written a report which largely eliminates most of the purported evidence for a Clovis Comet (or other form of bolide). Too bad, because such an event would at one swoop explain the very sudden onset of YD, the extirpation of many (but not all) species of megamammels and the abrupt end of Clovis culture everywhere in North America. This doesn’t mean that there wasn’t a “meteor swarm”, but so far there is nothing definite, AFAIK. This is in conterdistinction to antlers from 43 kya blasted with micrometeorites found fairly recently in Siberia; nothing similar, of any age, found in North America AFAIK. If found and dated to within a few year of YD onset, that would be conclusive as far as I’m concerned. Of course, there is the meteor crater under the east end of Lake Ontario, “of Holocene age”, which could be more precisely dated; that would help one way or the other.

  31. 81

    That’s why I’m interested, most of the recent work indicates that a lot of the big catastrophic glacial lake discharges occurred after the Younger Dryas, and the chronology has changed somewhat to rule out the Moorehead phase, etc. I read all those guys, Leverington, Jakobsson etc… The problem is extremely interesting even without the comet. There was massive megaflooding bursting out all over it seems, but nothing quite adds up yet because the time period is quite a bit earlier than most of the mayhem. On top of all that sea level measurements tell us something slightly different, that water had to be coming from somewhere. So maybe it was slow forcing in the beginning that just reached a resonance and then once it was forced out of that the massive flooding mixed it up enough to set it back to more or less a warming trend again. Or maybe it was something else completely. That’s why I just check in here once and a while.

    I’d really like to see something definitive come out of this, and it’s getting closer all the time. I’m patient, and I found a great vacation spot out of this anyways, so I’m good.

  32. 82
    James Staples says:

    I’m wondering what I missed in Oceanography Class about O-18. Is it that warmer SEAWATER abosorbs more of the Heavier Isotope, while warmer FRESHWATER does not?
    In Fig. 4, regarding the T III Event, that would seem to be the indication – though I don’t recall being taught that, in 1982.
    Go figure – that was 1982!
    BTW: If you respond, could you also provide a Link for the ‘Arctic Pathway’ you mentioned? I’ve been active discusing, on several Forums, the role of the AMOC in Climate Change Scenarios, and I’d like to be able to speak as intelligently as possible about it. Haven’t seen a thing in Science, Nature, New Scientist, anywhere.

  33. 83
    Hank Roberts says:

    James, you asked for help finding this:

    “evidence discussed by Murton et al. (2010) for an Arctic pathway”

    Putting the name and date into Google Scholar finds it:
    http://www.nature.com/nature/journal/v464/n7289/abs/nature08954.html

  34. 84
    James Staples says:

    Thanks Hank Roberts! One of these days I’ll switch to Google – right now, I’m hoping Bill Gates will back a certain project of mine; so Bing it is!
    Yes, yes, I know, Monsieur Moderator – nothing to do with…..

  35. 85
    flxible says:

    83 & 84: Actually scroogle is better and no adverts – try it, you’ll like it. :)

  36. 86
  37. 87

    James,

    It is horses for courses. For general searches use Bing or Google, but for scientific papers use Google Scholar.

    Cheers, Alastair.

  38. 88
    E.P. Grondine says:

    Re: #68, #85

    Tom, Roger, for the latest research on the YD impacts go to http://cosmictusk.com.

    The nano-diamonds found at the boundary layers are from cometary impact, period. End of debate on whether a cometary impact occured. See in particular Napier’s paper there.

    The First Peoples memories of this cometary impact event likely will provide some limit on the solution space, in particular the Mohawk account. Other limits on the YD solution space are provided by determining the areas where they survived the event, the numbers killed, and the new foods they ate and animals they hunted.

    E.P. Grondine
    Man and Impact in the Americas

  39. 89
    Andy Breckenridge says:

    ‘There was massive megaflooding bursting out all over it seems, but nothing quite adds up yet’ (81)
    I love this line. It’s true that’s there’s much published on ‘megafloods’, but the reality is they’re mostly hypothetical. There’s little concrete evidence for these Agassiz floods, and the best evidence for megafloods post-dates the YD. Absence of evidence is of course not evidence of absence.
    The Murton et al paper ignores the most recent ice margin reconstructions for the NW Agassiz outlet (via the Clearwater River). Their hypothesis necessitates an undocumented ice retreat/advance cycle over that height of land. This is also what would be required to permit an eastern drainage into Superior during the YD. Each hypothesis (NW or E drainage) has equal merit in my opinion. The well documented Moorhead low in the Agassiz basin establishes the fact that Agassiz was re-routed either E or NW (or both?) at some time during the YD.
    Evidence that supports YD eastern drainage includes a gray clay bed in Lake Michigan that’s roughly YD in age called the Wilmette bed that looks similar to younger sediments deposited during eastern Agassiz drainage. There are gray lacustrine sediments in western Lake Superior that pre-date the Marquette Advance (10 14C BP). The gray clay has a provenance north of the Superior basin, and requires substantial ice margin retreat in the Superior basin during or before the YD. There’s also interesting/unexplained YD aged excursions in the d18O records from the Champlain Sea, Huron, and Erie that may be related to drainage.
    Similar re-routing events for precursors to glacial Lake Agassiz during previous glacial terminations should have also occurred. I wonder if the Clearwater Valley isn’t in part a pre-Wisconsin landform. Ouimet Canyon near Nipigon may by another pre-Wisconsin landform carved during an early episode of glacial lake drainage to Superior.
    This riddle of Agassiz routing will eventual be solved because continuous sediment records exist in the Great Lakes, and a Lake Agassiz fingerprint will be found. Until then, we’ll continue to have floods bursting all over.

  40. 90
    kevinride says:

    Recent mapping of a number of raised beach ridges on the north coast of Greenland suggests that the ice cover in the Arctic Ocean was greatly reduced some 6000-7000 years ago. The Arctic Ocean may have been periodically ice free.

    ”The climate in the northern regions has never been milder since the last Ice Age than it was about 6000-7000 years ago. We still don’t know whether the Arctic Ocean was completely ice free, but there was more open water in the area north of Greenland than there is today,” says Astrid Lyså, a geologist and researcher at the Geological Survey of Norway (NGU).

    http://www.globalwarmingsurvivalcenter.com/

  41. 91
    E.P. Grondine says:

    Andy –

    In my opinion, one reason nothing has added up yet is because no one previously considered the role of impact in releasing Lake Agassiz. In my opinion, there also seems to be a research bias looking toward Atlantic data rather than Pacific data, and in particular north polar data.

    The recent Canadian research on the northward drainage may bring new interest in the Alaskan and Siberian “mucks”, and their distribution and dating. We may also get some really good northern Pacific coast data as well. Afterwards, in my opinion, things will finally add up.

    The real question for me is how long it is going to take for this process to unfold.

    E.P. Grondine
    Man and Impact in the Americas

  42. 92
    E.P. Grondine says:

    There is one more point that must be mentioned. Anyone working with Alaskan and Siberian mucks needs to keep in mind that there were two other large recent iron asteroid impacts in the region.


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