Posted on 06/21/2012 10:11:38 AM PDT by Ernest_at_the_Beach
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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fyi
What is wrong with using standard Gregorian dates, or Hebrew years, or something that is NOT a relative measurement. /rant off
Thanks for posting this. I've always been interested in the Younger Dryas era and the following warming.
/johnny
Maybe someone comments at WUWT on the article.
Do you love the first graph that has wild temperature swings, and there is a teensy weensy blip with an arrow pointed at the present with a label “global warming”? Also, note that the temperature of the little ice age appears to be higher than our current temperature.
Still meditating.
Pet peeve. Sorry. I did like the article.
/johnny
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Andrew says:
Don-It’s true that events like the younger dryas do not appear to be orbitally forced, but good agreement has been shown between the rate of change of global ice volume and the near Arctic circle summer insolation. It is certainly clear that some “spikes” of rapid change exceed what would be expected from Milakovitch alone, but I wouldn’t be so quick to say that the Milankovitch theory of the glaciations is “dead” at all. The long term correlation is quite good. We just need to invoke other factors for some episodes that deviate from the right Milankovitch model.
This is all very fascinating to me.....more from the comments:
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Dennis Cox says:
The article states:
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.
In point of fact, since the given astronomical model for the Younger Dryas Impact Hypothesis is the progressive disintegration of the progenitor of the Taurids, as described in The Structure, and Evolution of the Taurid Complex by D.I. Steel et al. and proposed in W.M. Napier’s Paleolithic Extinctions and the Taurid Complex, it should be noted that the YDIH as written does not propose a single event at all.
The progenitor of the Taurid family of objects is thought to have entered the inner solar system, and a very short period elliptical orbit that crossed the orbits of all the planets of the inner solar system sometime between 20,000 and 30,000 YA. The astronomical data on the Taurids is as good as anything you can dig up with a shovel, and trowel. And that evidence indicates the 50 to 100 km wide Taurid Progenitor object immediately began to breakup as soon as it entered the inner solar system.
The the Earth’s passage though the debris from the progressive breakup of the Taurid Progenitor would have resulted in devastating impact showers, and storms of varying intensity twice a year for thousands of years, both before, and after, the start of the Younger Dryas.
The evidence so far is implying that the event at the start of the YD that produced a global impact layer comparable only to the Cretaceous/Tertiary boundary layer that marks the extinctions of the dinosaurs 65 million YA was only the worst of many annual cluster airburst events of varying intensity over a period that lasted for many millennia.
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Jim Clarke says:
What I find most interesting is that, according to the Greenland ice core (figure 1) Greenland has been about 3-5 degrees C warmer than it is now for most of the Holocene! All those gloom and doom studies about the rapidly melting Greenland ice sheet are maybe just picking up on the very beginning of a return to Holocene average!
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has no institution ever considered that there might be an irregular rotational hotspot in the earth’s core that gives more heat to the southern hemisphere during glaciations, and that it may be the cause of deglaciations? This is the case with some of Saturn’s moons Enceladus, where the southern tip is excessively warm, as a result of its core, and has more heat coming out of the southern cap, whilst the north is frozen. Geo-thermal events must have a considerable, if hitherto unstudied impact, particulary submarine.
Its one of climatology’s great weaknesses that the entire earth is not studied, and only the atmposphere
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You miss the big picture: when I looked at the first figure I recognised it looks like a diagram of an engine where somebody turned first time the key and it rumbled a couple of times before it died.
That’s the Younger Dryas break.
If you look at the Pleistocene 10Be levels in Figure 2 of the link I provided in my previous comment (Russian Journal of Earth Sciences) you see a rapid decline of 10Be (denoting increased solar activity) as it warms and then a very sudden and deep increase in 10Be production during the YD which would normally indicate a sudden and deep reduction in solar magnetic field activity. There is also, of course, another possible cause. To assume that 10Be production is a good proxy for solar magnetic activity one assumes that GCRs are constant. Maybe they aren’t. Maybe we are buffeted from time to time by “gusts” of dense cosmic rays or maybe sometimes pass through a “stream” of them where there are just more of them. The point is that it could be a combination of both things working at the same time. Maybe we get more GCRs hitting the atmosphere because the solar system is passing through an area with more GCRs and not because the sun changed. The GCRs are subject to being channeled by magnetic phenomena in the Galaxy.
So while we do know that solar magnetic activity can modulate GCRs reaching Earth, to assume that is the ONLY thing that chances those numbers might be a bit naive. Maybe we are bombarded from time to time with what amounts to shock waves or gusts of these particles. Maybe we find ourselves in streams of them. But one things does seem clear: for most of the Pleistocene the 10Be production was much higher than it has been during the Holocene. Why? I guess we will find out “shortly”.
The YD can be looked at as a cool interval or two successive warm intervals, the first short, the second long (the Holocene). The 2 warm periods of different length paradigm makes more sense. Just before the YD were deep glacial conditions and the YD was just a drop back to current glacial “normality” after an abortive jump to interglacial conditions. The first spike was an abortive interglacial, the second a “successful” one.
Indeed, if we remind ourselves that these very short abortive interglacial spikes occurred regularly throughout the glacial period, then the YD cool interval disappears as an anomaly, it is just an abortive interglacial spike that just happened to occur shortly before the “successful” interglacial rize which – unlike the abortive spikes – held on stably to the interglacial attractor rather than falling away from it.
Further, if we consider glacial and interglacial being alternate attractors in a nonlinear/nonequilibrium climate system, then abortive interglacial spikes and the less frequent interglacial rises which “stick” are an expected and normal behaviour.
Thus the YD is not in any way a “problem” except a problem of imagination and paradigm of the observer. The need for every upward or downward wiggle of earths climate history to have some discreet and unique external forcing comes from ignorance of quasi-chaotic systems and a deficient paradigm. It is even slightly absurd to imagine the climate system to be so passive.
Earth’s climate can change BY ITSELF.
OK it might be entrained in a simple or complex way by external periodic forcing, but it is not slavishly forced. Hunting for the magic celestial rhythm is futile.
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“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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Michael g. Wallace says:
Nice article. In response to some other comments, I don’t see how anyone can claim to see a correlation between Milankovitch and past glacial oscillations. There are many intervals of time over the past several 100K glacial cycles where orbital forcing is the opposite of the global temperature trend. It’s no surprise that Principal Component Analyses applied to Milankovitch don’t match up with PCA applied to O18. I have seen a paper by Roe which shows a better match of these, but only when some nonlinear cause and effect assumptions are applied. When you invoke nonlinear causation, you can prove anything.
Also amusing how people don’t seem to worry about squaring Milankovitch with pre Quaternary climate patterns, going back hundreds of millions of years, where no ice ages have been identified. Orbital cycling likely persisted throughout that period, but for some reason that no scientist has been able to explain, or has bothered to try to explain, Milankovitch impacts were apparently nil.
I think the description of the sudden warming period (over decades) should by itself ‘kill’ any claims that current warming and sea level rise rates are unprecendented. And that should ‘kill’ any certainty about anthropogenic catastrophic climate change. But for some reason it won’t.
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Dennis Cox says:
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.
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.
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Just for fun:
New Evidence Supports Cosmic Impact Theory
http://www.archaeorama.com/archaeology/cosmic-impact-theory/ P>
Rather revealing study in comparisons of warming and cooling cycle intensities.
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