This is a follow up posting to Younger Dryas -The Rest of the Story!
Guest post by Don J. Easterbrook
Dept. of Geology, Western Washington University.
The Younger Dryas was a period of rapid cooling in the late Pleistocene 12,800 to 11,500 calendar years ago. It followed closely on the heels of a dramatically abrupt warming that brought the last Ice Age to a close (17,500 calendar years ago), lasted for about 1,300 years, then ended as abruptly as it started. The cause of these remarkably sudden climate changes has puzzled geologists and climatologists for decades and despite much effort to find the answer, can still only be considered enigmatic.
The Younger Dryas interruption of the global warming that resulted in the abrupt, wholesale melting of the huge late Pleistocene ice sheets was first discovered in European pollen studies about 75 years ago. Terrestrial plants and pollen indicate that arboreal forests were replaced by tundra vegetation during a cool climate. This cool period was named after the pale yellow flower Dryas octopetella, an arctic wildflower typical of cold, open, Arctic environments. The Younger Dryas return to a cold, glacial climate was first considered to be a regional event restricted to Europe, but later studies have shown that it was a world-wide event. The problem became even more complicated when oxygen isotope data from ice cores in Antarctica and Greenland showed not only the Younger Dryas cooling, but several other shorter cooling/warming events, now known as Dansgaard-Oerscher events.
The Younger Dryas is the longest and coldest of several very abrupt climatic changes that took place near the end of the late Pleistocene. Among these abrupt changes in climate were: (1) sudden global warming 14,500 years ago (Fig. 1) that sent the immense Pleistocene ice sheets into rapid retreat, (2) several episodes of climatic warming and cooling between ~14,400 and 12,800 years ago, (3) sudden cooling 12,800 years ago at the beginning of the Younger Dryas, and (4) ~11,500 years ago, abrupt climatic warming of up to 10º C in just a few decades. Perhaps the most precise record of late Pleistocene climate changes is found in the ice core stratigraphy of the Greenland Ice Sheet Project (GISP) and the Greenland Ice Core Project (GRIP). The GRIP ice core is especially important because the ages of the ice at various levels in the core has been determined by the counting down of annual layers in the ice, giving a very accurate chronolgoy, and climatic fluctuations have been determined by measurement of oxygen isotope ratios. Isotope data from the GISP2 Greenland ice core suggests that Greenland was more than~10°C colder during the Younger Dryas and that the sudden warming of 10° ±4°C that ended the Younger Dryas occurred in only about 40 to 50. years.
Figure 1. Temperature fluctuations over the past 17,000 years showing the abrupt cooling during the Younger Dryas. The late Pleistocene cold glacial climate that built immense ice sheets terminated suddenly about 14,500 years ago (1), causing glaciers to melt dramatically. About 12,800 years ago, after about 2000 years of fluctuating climate (2-4), temperatures plunged suddenly (5) and remained cool for 1300 years (6). About 11,500 years ago, the climate again warmed suddenly and the Younger Dryas ended (7).
Radiocarbon and cosmogenic dating of glacial moraines in regions all over the world and abrupt changes in oxygen isotope ratios in ice cores indicate that the Younger Dryas cooling was globally synchronous. Evidence of Younger Dryas advance of continental ice sheets is reported from the Scandinavian ice sheet, the Laurentide ice sheet in eastern North America, the Cordilleran ice sheet in western North America, and the Siberian ice sheet in Russia. Alpine and ice cap glaciers also responded to the abrupt Younger Dryas cooling in both the Northern and Southern hemispheres, e.g., many places in the Rocky Mts. of the U.S. and Canada, the Cascade Mts. of Washington, the European Alps, the Southern Alps of New Zealand, and the Andes Mts. in Patagonia of South America.
Figure 2. Temperature fluctuations over the past 15,000 years showing the abrupt cooling during the Younger Dryas and other warming and cooling periods, the Oldest Dryas (cool), Bölllng (warm), Older Dryas (cool), Allerød (warm), InterAllerød (cool), and Younger Dryas (cool).
Figure 3. Oxygen isotope record from the Greenland ice core showing an abrupt temperature drop 12,800 years ago, 1300 years of cool climate, and sudden warming 11,500 years ago.
The Younger Dryas had multiple glacial advances and retreats
The Younger Dryas was not just a single climatic event. Late Pleistocene climatic warming and cooling not only occurred before and after the YD, but also within it. All three major Pleistocene ice sheets, the Scandinavian, Laurentide, and Cordilleran, experienced double moraine-building episodes, as did a large number of alpine glaciers. Multiple YD moraines of the Scandinavian Ice Sheet have long been documented and a vast literature exists. The Scandinavian Ice Sheet readvanced during the YD and built two extensive end moraines across southern Finland, the central Swedish moraines, and the Ra moraines of southwestern Norway(Fig. 4). 14C dates indicate they were separated by about 500 years.
Figure 4. Double Younger Dryas moraines of the Scandinavian Ice Sheet.
Among the first multiple YD moraines to be recognized were the Loch Lomond moraines of the Scotish Highlands. Alpine glaciers and icefields in Britain readvanced or re-formed during the YD and built extensive moraines at the glacier margins. The largest YD icefield at this time was the Scotish Highland glacier complex, but smaller alpine glaciers occurred in the Hebrides and Cairngorms of Scotland, in the English Lake District, and in Ireland. The Loch Lomond moraines consist of multiple moraines. Radiocarbon dates constrain the age of the Loch Lomond moraines between 12.9 and 11.5 calendar years ago.
Multiple Younger Dryas moraines of alpine glaciers also occur throughout the world, e.g., the European Alps, the Rocky Mts., Alaska, the Cascade Range, the Andes, the New Zealand Alps, and elsewhere.
Figure 5. Double Younger Dryas moraines at Titcomb Lakes in the Wind River Range of Wyoming.
Implications
The multiple nature of YD moraines in widely separated areas of the world and in both hemispheres indicates that the YD consisted of more than a single climatic event and these occurred virtually simultaneously worldwide. Both ice sheets and alpine glaciers were sensitive to the multiple YD phases. The GISP2 ice core shows two peaks within the YD that match the glacial record. The absence of a time lag between the N and S Hemispheres glacial fluctuations precludes an ocean cause and is not consistent with the North Atlantic Deep Ocean Water hypothesis for the cause of the Younger Dryas, nor with a cosmic impact or volcanic origin.
Both 14C and 10Be production rates in the upper atmosphere changed during the YD. 14C and 10Be are isotopes produced by collision of incoming radiation with atoms in the upper atmosphere. The change in their production rates means that the Younger Dryas was associated with changes in the amount of radiation entering the Earth’s atmosphere, leading to the intriguing possibility that the YD was caused by solar fluctuations.
Why the Younger Dryas is important
What can we learn from all this? The ice core isotope data were hugely significant because they showed that the Younger Dryas, as well as the other late Pleistocene warming and cooling events could not possibly have been caused by slow, Croll-Milankovitch orbital forcing, which occurs over many tens of thousands of years. The ice core isotope data thus essentially killed the Croll-Milankovitch theory as the cause of the Ice Ages.
In an attempt to save the Croll-Milankovitch theory, Broecker and Dention (1990) published a paper postulating that large amounts of fresh water discharged into the north Atlantic about 12,800 years ago when retreat of the Laurentide ice sheet allowed drainage of glacial Lake Agassiz to spill eastward into the Atlantic Ocean. They proposed that this large influx of fresh water might have stopped the formation of descending, higher-density water in the North Atlantic, thereby interrupting deep-water currents that distribute large amounts of heat globally and initiating a short-term return to glacial conditions. If indeed that was the case, then the Younger Dryas would have been initiated in the North Atlantic and propagated from there to the Southern Hemisphere and the rest of the world. Since that would take time, it means that the YD should be 400-1000 years younger in the Southern Hemisphere and Pacific areas than in the Northern Hemisphere. However, numerous radiocarbon and cosmogenic dates of the Younger Dryas all over the world indicate the cooling was globally synchronous. Thus, the North Atlantic deep current theory is not consistent with the chronology of the Younger Dryas.
The climatic fluctuations before and after the Younger Dryas, as well as the fluctuations within it, and the duration of these changes are not consistent with a single event cause of the YD. Neither cosmic impact or volcanic eruptions could produce the abrupt, multiple climatic changes that occurred during the late Pleistocene.
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Related articles
- Younger Dryas -The Rest of the Story! (wattsupwiththat.com)
- New evidence of Younger Dryas extraterrestrial impact (wattsupwiththat.com)
- Catching up with the Younger Dryas: do mass-extinctions always need impacts? (skepticalscience.com)
- Study Finds New Evidence Supporting Theory of Extraterrestrial Impact (sott.net)
- The Great Ice Meltdown and Rising Seas: Lessons for Tomorrow (giss.nasa.gov)
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Or during the optimum of the last interglacial which was even warmer than it was in the Holocene. Virtually every plant species alive today was alive during that period and survived it with only few exceptions of species that might have appeared since that time.
Don….I was with you until you stuck that “global warming” into the end of the graph. sorta throws some doubt on the other assumptions.Or it could just be me 🙂
I doubt very much that the Lake Agazziz related cooling would take 400+ years to spread from north to south.
A super efficient stone age man’s carbon capture scheme ??
Excellent article, well presented, but I agree with Dennis Cox, no one can exclude impact or near miss from outer space. There is enough physical evidence to warrant consideration and modification as our knowledge grows. That is all part of the joy of science. Keep an open mind.
rbateman, I am still undecided on the flood myths. Is it due to the black sea flooding with the Dardenelles opening, or just due to glaial retreat? Or is it both. Interestingly, agriculture is though to have started in Asia Minor; maybe it started on farms 600 foot below current Black Sea levels, and tthe locals escaped the “deluge”. Of further note, the Biblical version involves Mount Arrarat, also in Asia Minor. I never did understand that as a kid; the Jews never lived in Asia Minor; now it makes more sense.
“Both 14C and 10Be production rates in the upper atmosphere changed during the YD. 14C and 10Be are isotopes produced by collision of incoming radiation with atoms in the upper atmosphere. The change in their production rates means that the Younger Dryas was associated with changes in the amount of radiation entering the Earth’s atmosphere, leading to the intriguing possibility that the YD was caused by solar fluctuations.”
At about this time there was a magnetic reveral (the Gothenberg). Changes in the magnetic field of the Earth could have changed the amount of radiation entering the Earth’s atmosphere.
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Stephen Wilde says:
June 19, 2012 at 1:40 pm
The solar periodicity seems to be around 500 years as per MWP to LIA to date and the thermohaline circulation is around 1000 years.
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OK, but what could be the mechanism for a 500 yr solar periodicity? And is there any evidence?
The sun is an extremely well-mixed & uniform plasma ball (compared to differentiated planets & their oceans and/or atmospheres). Yes, there are the fairly well-understood magnetic cycles in the convection zone, but these cycle back to ~same conditions every cycle. I’m not sure the sun’s uniformity allows for longer “cycles”. Of course, there’s the gradual compression & heating of the core as it gets denser, but that’s not important for the timescales in question.
The abrupt warming after the YD at GISP2 was 10c in one decade according to this article from NOAA.
http://www.ncdc.noaa.gov/paleo/abrupt/data4.html
Clearly our present small warming of 0.7c over the last 100+ years is not unusual or unprecedented.
Also our present slight warming comes at the end of a minor ice age and after one of the coldest periods of the Holocene.
This recovery is normal and to be expected. It is not unusual or unprecedented at all.
But there are scores of these warmings and coolings throughout the glacials and interglacials, so what were the NATURAL causes during those times.,
Great article. Peridot has already commented that Dryas octopetala has white flowers (and it has eight petals, hence the name). But there is another Dryas species, Dryas dummondii, which grows for example in the Canadian Rockies and has yellow flowers that nod. Do we know which species the Younger Dryas is named for? I have always assumed its for D. octopetala, but I don’t really know. D. octopetala is a wonderful arctic-alpine plant in the rose family, occurring at or near sea level as one goes north, eg as Peridot mentions in western Ireland, but needing more elevation as one goes south, eg its in the Wallowas Mountains in Oregon and in the Rockies in Montana and elsewhere. It likes limestone, at least in nature, and I wonder if that impacted its distribution during the “Younger Dryas”.
Doesn’t Figure 1 have a typo error? It lists the Medieval Warm Period and Little Ice age incorrectly, those should be at 800 and 200 years BP marks. The larger warm period and cool period on #9 and #10 are the late Roman warm period and subsequent Dark Ages cool period that occur prior to the MWP and LIA.
Very good article Dr. Easterbrook. A couple of comments that in part are echoing Dennis Cook’s comments: 1) Why would an absence of a time lag negate the idea of a cosmic impact event? 2) Perhaps you might wish to check out a couple of references provided below that suggest the Younger Dryas may have taken hold in a year or even less. To me the most plausible way that such a rapid onset could takje place is through a significant cosmic event (likely a comet encounter). That said I ma becoming more and more confinced of the importance of solar variations to our climate, but not on the rapid scale that was displayed in the Younger Dryas Thank-you again Don, signed Rod Chilton.
78) J. P. Steffenson et al., “High-Resolution Greenland Ice Core Data Show Abrupt Climate Change Happens in a Few Years,” (2008): Science 321, 680-683.
79) K. Ravillious” Ice Age Took Hold in Less than a Year,” (2009): New Scientist, 10.
Ice floats. It probably didn’t freeze all the way to the bottom. Could have certainly had an impact on currents, though, and there is evidence that it did. During the Pleistocene there was apparently a rather large area in the Southeastern Pacific that had no circulation for thousands of years during the glaciation. There was also a large increase in brine rejection from the increasing ice pack and this very dense, very cold water settled in that area and just sat. The restarting or speeding up or changing of ocean circulation could cause a lot of this really cold water to start moving around. What impact that might have will probably have to wait until 100K years or so for us to discover.
Sorry Dennis I of course mean Cox not Cook
phlogiston says:
June 19, 2012 at 3:06 pm
I never thought I would agree with the Phlogiston theory 😉
Don, this is asserted but not explained. Just how is this time lag not consistent with either the NADOW or an impact scenario? How precise are the time increments of time that this is based on? Wouldn’t an impact cause non-lagged effects?
Steve Garcia
@crosspatch June 19, 2012 at 2:50 pm:
Just thinking out loud, but couldn’t the impact of a very large (~1.0 to ~2.0 km) cometary impactor as suggested possibly screw up the atmospheric shield, mimicking a change in the solar magnetic field? This posits a mechanism that is not now on the radar, I know. But it might also provide a mechanism for the sudden warming at the end of the YD – that due to the atmosphere returning to its previous condition, the shield could be reintroduced on a short time scale.
IOW, might the impact, if big enough, with a plume/fireball large enough, suddenly change the chemistry of the atmosphere? And of so, what would it take to reverse that effect, and how short of time might it happen in? We do know the O18 changed. What else? Perhaps ozone? What else?
I would point out that any hypothesis that tries to explain the YD onset or its end should also be required to explain the sudden increase of global temps at the end of the Allerod. (…at least as shown in the O18 and Be10 data) We have two big rises and one big decline. The impact hypothesis is addressing the decline, but everyone throws up their hands about the two increases. Those might be the elephant in the room.
Steve Garcia
J. Philip Peterson says:
June 19, 2012 at 2:56 pm
This graph must be wrong because it doesn’t show the hockey stick.
Turn your screen upside down. Problem fixed.
Interesting shoe print in figure 5.
Oh! My bad. LOL
crosspatch says:
June 19, 2012 at 2:50 pm
To assume that 10Be production is a good proxy for solar magnetic activity one assumes that GCRs are constant. Maybe they aren’t.
The 10Be we measure is determined both by production [about 2 ounces per year total over the globe] and by deposition. The latter depends strongly on climate.
I think the graph is labelled wrong. The MWP and LIA should be moved one hump to the right. I think the arrow for the MWP is pointing instead at the Roman Warm Period.
Hmmm,
On the contrary, I’m thinkin’ that an event that produced a global impact layer is perfectly consistent with “The absence of a time lag between the N and S Hemispheres glacial fluctuations”. So while that absence of a time lag between the two hemispheres is not consistent with the Deep Ocean Water hypothesis, it is consistent with a cosmic event of sufficient magnitude that it simultaneously emplaced high energy blast-effected materials into a global stratigraphic layer.
Just for fun:
New Evidence Supports Cosmic Impact Theory
http://www.archaeorama.com/archaeology/cosmic-impact-theory/
Couldn’t the YD events be the result of a cosmic impact, as accumylating evidence supports, followed by a Maunder-type solar minimum which slows the recovery? Eliminating an impact as a part of the puzzle may be shortsighted as sh*t happens.