Upper Dryas

The Younger Dryas (c. 12,900 to 11,700 years BP) was a return to glacial conditions which temporarily reversed the gradual climatic warming after the Last Glacial Maximum (LGM, c. 27,000 to 20,000 years BP). The Younger Dryas was the last stage of the Pleistocene epoch (c. 2,580,000 to 11,700 years BP) and it preceded the current, warmer Holocene epoch. The Younger Dryas was the most severe and long lasting of several interruptions to the warming of the Earth's climate, and it was preceded by the Late Glacial Interstadial (c. 14,670 to 12,900 BP).

The change was relatively sudden, taking place in decades, and it resulted in a decline of temperatures in Greenland by 4~10 °C (7.2~18 °F), and advances of glaciers and drier conditions over much of the temperate Northern Hemisphere. A number of theories have been put forward about the cause, and the most widely supported by scientists is that the Atlantic meridional overturning circulation, which transports warm water from the Equator towards the North Pole, was interrupted by an influx of fresh, cold water from North America into the Atlantic.

The Younger Dryas did not affect the climate worldwide. In the Southern Hemisphere and some areas of the Northern Hemisphere, such as southeastern North America, a slight warming occurred.

The Younger Dryas is named after an indicator genus, the alpine-tundra wildflower ''Dryas octopetala'', as its leaves are occasionally abundant in late glacial, often minerogenic-rich sediments, such as the lake sediments of Scandinavia.

General description and context

The presence of a distinct cold period at the end of the LGM interval has been known for a long time. Paleobotanical and lithostratigraphic studies of Swedish and Danish bog and lake sites, as in the Allerød clay pit in Denmark, first recognized and described the Younger Dryas.

The Younger Dryas is the youngest and longest of three stadials, which resulted from typically abrupt climatic changes that took place over the last 16,000 years. Within the Blytt–Sernander classification of north European climatic phases, the prefix "Younger" refers to the recognition that this original "Dryas" period was preceded by a warmer stage, the Allerød oscillation, which, in turn, was preceded by the Older Dryas, around 14,000 calibrated years BP. That is not securely dated, and estimates vary by 400 years, but it is generally accepted to have lasted around 200 years. In northern Scotland, the glaciers were thicker and more extensive than during the Younger Dryas. The Older Dryas, in turn, was preceded by another warmer stage, the Bølling oscillation, that separated it from a third and even older stadial, often known as the Oldest Dryas. The Oldest Dryas occurred about 1,770 calibrated years before the Younger Dryas and lasted about 400 calibrated years. According to the GISP2 ice core from Greenland, the Oldest Dryas occurred between about 15,070 and 14,670 calibrated years BP.

In Ireland, the Younger Dryas has also been known as the Nahanagan Stadial, and in Great Britain it has been called the Loch Lomond Stadial. In the Greenland Summit ice core chronology, the Younger Dryas corresponds to Greenland Stadial 1 (GS-1). The preceding Allerød warm period (interstadial) is subdivided into three events: Greenland Interstadial-1c to 1a (GI-1c to GI-1a).

Abrupt climate change

Since 1916 and the onset and then the refinement of pollen analytical techniques and a steadily-growing number of pollen diagrams, palynologists have concluded that the Younger Dryas was a distinct period of vegetational change in large parts of Europe during which vegetation of a warmer climate was replaced by that of a generally cold climate, a glacial plant succession that often contained ''Dryas octopetala''. The drastic change in vegetation is typically interpreted to be an effect of a sudden decrease in (annual) temperature, unfavorable for the forest vegetation that had been spreading northward rapidly. The cooling not only favored the expansion of cold-tolerant, light-demanding plants and associated steppe fauna, but also led to regional glacial advances in Scandinavia and a lowering of the regional snow line. Björck, S. (2007) ''Younger Dryas oscillation, global evidence.'' In S. A. Elias, (Ed.): ''Encyclopedia of Quaternary Science,'' Volume 3, pp. 1987–1994. Elsevier B.V., Oxford.

The change to glacial conditions at the onset of the Younger Dryas in the higher latitudes of the Northern Hemisphere, between 12,900 and 11,500 calibrated years BP, has been argued to have been quite abrupt. It is in sharp contrast to the warming of the preceding Older Dryas interstadial. Its end has been inferred to have occurred over a period of a decade or so, but the onset may have even been faster. Thermally fractionated nitrogen and argon isotope data from Greenland ice core GISP2 indicate that its summit was around colder during the Younger Dryas than today.

In Great Britain, beetle fossil evidence suggests that the mean annual temperature dropped to , and periglacial conditions prevailed in lowland areas, and icefields and glaciers formed in upland areas. Nothing of the period's size, extent, or rapidity of abrupt climate change has been experienced since its end.

In addition to the Younger, Older, and Oldest Dryases, a century-long period of colder climate, similar to the Younger Dryas in abruptness, has occurred within both the Bølling oscillation and the Allerød oscillation interstadials. The cold period that occurred within the Bølling oscillation is known as the intra-Bølling cold period, and the cold period that occurred within the Allerød oscillation is known as the intra-Allerød cold period. Both cold periods are comparable in duration and intensity with the Older Dryas and began and ended quite abruptly. The cold periods have been recognized in sequence and relative magnitude in paleoclimatic records from Greenland ice cores, European lacustrine sediments, Atlantic Ocean sediments, and the Cariaco Basin, Venezuela.

Examples of older Younger Dryas-like events have been reported from the ends (called terminations)
of older glacial periods. Temperature-sensitive lipids, long chain alkenones, found in lake and marine sediments, are well-regarded as a powerful paleothermometer for the quantitative reconstruction of past continental climates. The application of alkenone paleothermometers to high-resolution paleotemperature reconstructions of older glacial terminations have found that very similar, Younger Dryas-like paleoclimatic oscillations occurred during Terminations II and IV. If so, the Younger Dryas is not the unique paleoclimatic event, in terms of size, extent, and rapidity, as it is often regarded to be. Eglinton, G., A.B. Stuart, A. Rosell, M. Sarnthein, U. Pflaumann, and R. Tiedeman (1992) ''Molecular record of secular sea surface temperature changes on 100-year timescales for glacial terminations I, II and IV.'' Nature. 356:423–426. Furthermore, paleoclimatologists and Quaternary geologists reported finding what they characterized as well-expressed Younger Dryas events in the Chinese δ records of Termination III in stalagmites from high-altitude caves in Shennongjia area, Hubei Province, China. Various paleoclimatic records from ice cores, deep-sea sediments, speleothems, continental paleobotanical data, and loesses show similar abrupt climate events, which are consistent with Younger Dryas events, during the terminations of the last four glacial periods (see Dansgaard–Oeschger event). They argue that Younger Dryas events might be an intrinsic feature of deglaciations that occur at the end of glacial periods.

Timing

Analyses of stable isotopes from Greenland ice cores provide estimates for the start and end of the Younger Dryas. The analysis of Greenland Summit ice cores, as part of the Greenland Ice Sheet Project 2 and Greenland Icecore Project, estimated that the Younger Dryas started about 12,800 ice (calibrated) years BP. Depending on the specific ice core analysis consulted, the Younger Dryas is estimated to have lasted 1,150~1,300 years. Measurements of oxygen isotopes from the GISP2 ice core suggest the ending of the Younger Dryas took place over just 40~50 years in three discrete steps, each lasting five years. Other proxy data, such as dust concentration and snow accumulation, suggest an even more rapid transition, which would require about of warming in just a few years. Total warming in Greenland was .

The end of the Younger Dryas has been dated to around 11,550 years ago, occurring at 10,000 BP (uncalibrated radiocarbon year), a "radiocarbon plateau" by a variety of methods, mostly with consistent results:

:

The International Commission on Stratigraphy put the start of the Greenlandian stage, and implicitly the end of the Younger Dryas, at 11,700 years before 2000.

Although the start of the Younger Dryas is regarded to be synchronous across the North Atlantic region, recent research concluded that the start of the Younger Dryas might be time-transgressive even within there. After an examination of laminated varve sequences, Muschitiello and Wohlfarth found that the environmental changes that define the beginning of the Younger Dryas are diachronous in their time of occurrence according to latitude. According to the changes, the Younger Dryas occurred as early as around 12,900~13,100 calibrated years ago along latitude 56–54°N. Further north, they found that the changes occurred at roughly 12,600–12,750 calibrated years ago.

According to the analyses of varved sediments from Lake Suigetsu, Japan, and other paleoenvironmental records from Asia, a substantial delay occurred in the onset and the end of the Younger Dryas between Asia and the North Atlantic. For example, paleoenvironmental analysis of sediment cores from Lake Suigetsu in Japan found the Younger Dryas temperature decline of 2–4 °C between 12,300 and 11,250 varve (calibrated) years BP, instead of about 12,900 calibrated years BP in the North Atlantic region.

In contrast, the abrupt shift in the radiocarbon signal from apparent radiocarbon dates of 11,000 radiocarbon years to radiocarbon dates of 10,700–10,600 radiocarbon years BP in terrestrial macrofossils and tree rings in Europe over a 50-year period occurred at the same time in the varved sediments of Lake Suigetsu. However, this same shift in the radiocarbon signal antedates the start of Younger Dryas at Lake Suigetsu by a few hundred years. Interpretations of data from Chinese also confirm that the Younger Dryas East Asia lags the North Atlantic Younger Dryas cooling by at least 200~300 years. Although the interpretation of the data is more murky and ambiguous, the end of the Younger Dryas and the start of Holocene warming likely were similarly delayed in Japan and in other parts of East Asia.

Similarly, an analysis of a stalagmite growing from a cave in Puerto Princesa Subterranean River National Park, Palawan, the Philippines, found that the onset of the Younger Dryas was also delayed there. Proxy data recorded in the stalagmite indicate that more than 550 calibrated years were needed for Younger Dryas drought conditions to reach their full extent in the region and about 450 calibrated years to return to pre-Younger Dryas levels after it ended.Partin, J.W., T.M. Quinn, C.-C. Shen, Y. Okumura, M.B. Cardenas, F.P. Siringan, J.L. Banner, K. Lin, H.-M. Hu, and F.W Taylor (2014) ''Gradual onset and recovery of the Younger Dryas abrupt climate event in the tropics''. Nature Communications. Received 10 October 2014 , Accepted 13 July 2015 , Published 2 September 2015

Global effects

In Western Europe and Greenland, the Younger Dryas is a well-defined synchronous cool period. Cooling in the tropical North Atlantic may, however, have preceded it by a few hundred years; South America shows a less well-defined initiation but a sharp termination. The Antarctic Cold Reversal appears to have started a thousand years before the Younger Dryas and has no clearly defined start or end; Peter Huybers has argued that there is a fair confidence in the absence of the Younger Dryas in Antarctica, New Zealand and parts of Oceania. Timing of the tropical counterpart to the Younger Dryas, the Deglaciation Climate Reversal (DCR), is difficult to establish as low latitude ice core records generally lack independent dating over the interval. An example of this is the Sajama ice core ( Bolivia), for which the timing of the DCR has been pinned to that of the GISP2 ice core record (central Greenland). Climatic change in the central Andes during the DCR, however, was significant and was characterized by a shift to much wetter and likely colder conditions. The magnitude and abruptness of the changes would suggest that low latitude climate did not respond passively during the YD/DCR.

Effects of the Younger Dryas were of varying intensity throughout North America. In western North America, its effects were less intense than in Europe or northeast North America; however, evidence of a glacial re-advance indicates that Younger Dryas cooling occurred in the Pacific Northwest. Speleothems from the Oregon Caves National Monument and Preserve in southern Oregon's Klamath Mountains yield evidence of climatic cooling contemporaneous to the Younger Dryas.

Other features include the following:

* Replacement of forest in Scandinavia with glacial tundra (which is the habitat of the plant ''Dryas octopetala'')
* Glaciation or increased snow in mountain ranges around the world
* Formation of solifluction layers and loess deposits in Northern Europe
* More dust in the atmosphere, originating from deserts in Asia
* A decline in evidence for Natufian hunter gatherer permanent settlements in the Levant, suggesting a reversion to a more mobile way of life
* The Huelmo–Mascardi Cold Reversal in the Southern Hemisphere ended at the same time
* Decline of the Clovis culture; while no definitive cause for the extinction of many species in North America such as the ''Columbian mammoth'', as well as the ''Dire wolf'', ''Camelops'', and other Rancholabrean megafauna during the Younger Dryas has been determined, climate change and human hunting activities have been suggested as contributing factors.Brakenridge, G. Robert. 2011
''Core-Collapse Supernovae and the Younger Dryas/Terminal Rancholabrean Extinctions''.
Elsevier, Retrieved 23 September 2018 Recently, it has been found that these megafauna populations collapsed 1000 years earlier.

North America

East

The Younger Dryas is a period significant to the study of the response of biota to abrupt climate change and to the study of how humans coped with such rapid changes. The effects of sudden cooling in the North Atlantic had strong regional effects in North America, with some areas experiencing more abrupt changes than others. A cooling and ice advance accompanying the transition into the Younger Dryas between 13,300 and 13,000 cal years BP has been confirmed with many radiocarbon dates from four sites in western New York State. The advance is similar in age to the Two Creeks forest bed in Wisconsin.

The effects of the Younger Dryas cooling affected the area that is now New England and parts of maritime Canada more rapidly than the rest of the present day United States at the beginning and the end of the Younger Dryas chronozone. Proxy indicators show that summer temperature conditions in Maine decreased by up to 7.5 °C. Cool summers, combined with cold winters and low precipitation, resulted in a treeless tundra up to the onset of the Holocene, when the boreal forests shifted north.

Vegetation in the central Appalachian Mountains east towards the Atlantic Ocean was dominated by spruce (''Picea'' spp.) and tamarack (''Larix laricina)'' boreal forests that later changed rapidly to temperate, more broad-leaf tree forest conditions at the end of the Younger Dryas period. Conversely, pollen and macrofossil evidence from near Lake Ontario indicates that cool, boreal forests persisted into the early Holocene. West of the Appalachians, in the Ohio River Valley and south to Florida rapid, no-analog vegetation responses seem to have been the result of rapid climate changes, but the area remained generally cool, with hardwood forest dominating. During the Younger Dryas, the Southeastern United States was warmer and wetter than the region had been during the Pleistocene because of trapped heat from the Caribbean within the North Atlantic Gyre caused by a weakened Atlantic meridional overturning circulation (AMOC).

Central

Also, a gradient of changing effects occurred from the Great Lakes region south to Texas and Louisiana. Climatic forcing moved cold air into the northern portion of the American interior, much as it did the Northeast. Although there was not as abrupt a delineation as seen on the Eastern Seaboard, the Midwest was significantly colder in the northern interior than it was south, towards the warmer climatic influence of the Gulf of Mexico. In the north, the Laurentide Ice Sheet re-advanced during the Younger Dryas, depositing a moraine from west Lake Superior to southeast Quebec. Along the southern margins of the Great Lakes, spruce dropped rapidly, while pine increased, and herbaceous prairie vegetation decreased in abundance, but increased west of the region.

Rocky Mountains

Effects in the Rocky Mountain region were varied. In the northern Rockies, a significant increase in pines and firs suggests warmer conditions than before and a shift to subalpine parkland in places. That is hypothesized to be the result of a northward shift in the jet stream, combined with an increase in summer insolation as well as a winter snow pack that was higher than today, with prolonged and wetter spring seasons. There were minor re-advancements of glaciers in place, particularly in the northern ranges, but several sites in the Rocky Mountain ranges show little to no changes in vegetation during the Younger Dryas. Evidence also indicates an increase in precipitation in New Mexico because of the same Gulf conditions that were influencing Texas.

West

The Pacific Northwest region experienced 2 to 3 °C of cooling and an increase in precipitation. Glacial re-advancement has been recorded in British Columbia as well as in the Cascade Range. An increase of pine pollen indicates cooler winters within the central Cascades. On the Olympic Peninsula, a mid-elevation site recorded a decrease in fire, though forest persisted and erosion increased during the Younger Dryas, suggesting cool and wet conditions. Speleothem records indicate an increase in precipitation in southern Oregon, the timing of which coincides with increased sizes of pluvial lakes in the northern Great Basin. Pollen record from the Siskiyou Mountains suggests a lag in timing of the Younger Dryas, indicating a greater influence of warmer Pacific conditions on that range, but the pollen record is less chronologically constrained than the aforementioned speleothem record. The Southwest appears to have seen an increase in precipitation, as well, also with an average 2 °C of cooling.

Effects on agriculture

The Younger Dryas is often linked to the Neolithic Revolution, with the adoption of agriculture in the Levant. . The cold and dry Younger Dryas arguably lowered the carrying capacity of the area and forced the sedentary early Natufian population into a more mobile subsistence pattern. Further climatic deterioration is thought to have brought about cereal cultivation. While relative consensus exists regarding the role of the Younger Dryas in the changing subsistence patterns during the Natufian, its connection to the beginning of agriculture at the end of the period is still being debated.

Sea level

Based upon solid geological evidence, consisting largely of the analysis of numerous deep cores from coral reefs, variations in the rates of sea level rise have been reconstructed for the postglacial period. For the early part of the sea level rise that is associated with deglaciation, three major periods of accelerated sea level rise, called ''meltwater pulses'', occurred. They are commonly called
* ''meltwater pulse 1A0'' for the pulse between 19,000~19,500 calibrated years ago;
* ''meltwater pulse 1A'' for the pulse between 14,600~14,300 calibrated years ago;
* ''meltwater pulse 1B'' for the pulse between 11,400~11,100 calibrated years ago.
The Younger Dryas occurred after meltwater pulse 1A, a 13.5 m rise over about 290 years, centered at about 14,200 calibrated years ago, and before meltwater pulse 1B, a 7.5 m rise over about 160 years, centered at about 11,000 calibrated years ago. Finally, not only did the Younger Dryas postdate both all of meltwater pulse 1A and predate all of meltwater pulse 1B, it was a period of significantly-reduced rate of sea level rise relative to the periods of time immediately before and after it.

Possible evidence of short-term sea level changes has been reported for the beginning of the Younger Dryas. First, the plotting of data by Bard and others suggests a small drop, less than 6 m, in sea level near the onset of the Younger Dryas. There is a possible corresponding change in the rate of change of sea level rise seen in the data from both Barbados and Tahiti. Given that this change is "within the overall uncertainty of the approach," it was concluded that a relatively smooth sea-level rise, with no significant accelerations, occurred then. Finally, research by Lohe and others in western Norway has reported a sea-level low-stand at 13,640 calibrated years ago and a subsequent Younger Dryas transgression starting at 13,080 calibrated years ago. They concluded that the timing of the Allerød low-stand and the subsequent transgression were the result of increased regional loading of the crust, and geoid changes were caused by an expanding ice sheet, which started growing and advancing in the early Allerød about 13,600 calibrated years ago, well before the start of the Younger Dryas.

Causes

The current theory is that the Younger Dryas was caused by significant reduction or shutdown of the North Atlantic "Conveyor" – which circulates warm tropical waters northward – as the consequence of deglaciation in North America and a sudden influx of fresh water from Lake Agassiz. Geological evidence for such an event is not fully secure, but recent work has identified a pathway along the Mackenzie River that would have spilled fresh water into the Arctic and thence into the Atlantic. The global climate would then have become locked into the new state until freezing removed the fresh water "lid" from the North Atlantic. However, simulations indicated that a one-time-flood could not likely cause the new state to be locked for 1,000 years. Once the flood ceased, the AMOC would recover and the Younger Dryas would stop in less than 100 years. Therefore, continuous freshwater input was necessary to maintain a weak AMOC for more than 1,000 years. Recent study proposed that the snowfall could be a source of continuous freshwater resulting in a prolonged weakened state of the AMOC.

An alternative theory suggests instead that the jet stream shifted northward in response to the changing topographic forcing of the melting North American ice sheet, which brought more rain to the North Atlantic, which freshened the ocean surface enough to slow the thermohaline circulation. There is also some evidence that a solar flare may have been responsible for the megafaunal extinction, but that cannot explain the apparent variability in the extinction across all continents.

Impact hypothesis

The controversial Younger Dryas impact hypothesis claims that a comet impact or air burst occurred in North America about 12,900 years ago as the mechanism that initiated the Younger Dryas cooling.

Among other things, findings of melt-glass material in sediments in many places in around 60 localities in the United States, and in countries as distant as Mexico and Syria have been reported. The researchers argue that the material, which dates back nearly 13,000 years, was formed at temperatures of as the result of a bolide impact. They argue that these findings support the Younger Dryas Boundary (YDB) hypothesis, that the bolide impact occurred at the onset of the Younger Dryas. The hypothesis has been questioned by some scientists as a misinterpretation of data., although other scientists still support the comet impact hypothesis while others question the colleagues that have rejected the hypothesis.

After a review of the sediments found at the sites, new research has found that the sediments claimed by hypothesis proponents to be deposits resulting from a bolide impact date from much later or much earlier times than the proposed date of the cosmic impact. The researchers examined 29 sites commonly referenced to support the impact theory to determine if they can be geologically dated to around 13,000 years ago. Crucially, only three of those sites actually date from then.

Kinzie, ''et al.'' (2014) looked at the distribution of nanodiamonds produced during extraterrestrial collisions: 50 million km² of the Northern Hemisphere at the YDB were found to have the nanodiamonds. Only two layers exist showing these nanodiamonds: The YDB 12,800 calibrated years ago and the Cretaceous-Tertiary boundary, 65 million years ago, which, in addition, is marked by mass extinctions.

New support for the cosmic-impact hypothesis of the origin of the YDB was published in 2018. It postulates Earth's collision with one or more fragments from a larger (over 100 km diameter) disintegrating comet (some remnants of which have persisted within the inner solar system to the present day). Evidence is presented consistent with large-scale biomass burning (wildfires) following the putative collision. The evidence is derived from analyses of ice cores, glaciers, lake- and marine-sediment cores, and terrestrial sequences.

Evidence that adds further to the credibility of this hypothesis includes extraterrestrial platinum, which has been found in meteorites. There are multiple sites around the world with spikes in levels of platinum that can be associated with the impact hypothesis, of which at least 25 are considered significant. Although most of these sites are found in the Northern Hemisphere, a study conducted in October 2019 has found and confirmed another site with high platinum levels located in the Wonderkrater area north of Pretoria in South Africa. This coincides with the Pilauco site in southern Chile which also happens to contain high levels of platinum as well as rare metallic spherules, gold and high-temperature iron that is rarely found in nature and suspected of originating from airbursts or impacts. These Southern Hemisphere high platinum zones further support the credibility of an impact event.

Laacher See eruption hypothesis

The Laacher See volcano erupted at approximately the same time as the beginning of the Younger Dryas, and has historically been suggested as a possible cause. Laacher See is a maar lake, a lake within a broad low-relief volcanic crater about 2 km (1.2 mi) diameter. It is in Rhineland-Palatinate, Germany, about 24 km (15 mi) northwest of Koblenz and 37 km (23 mi) south of Bonn. The maar lake is within the Eifel mountain range, and is part of the East Eifel volcanic field within the larger ''Vulkaneifel''. This eruption was of sufficient size, VEI 6, with over 20 km³ (2.4 cu mi) tephra ejected, to have caused significant temperature change in the Northern Hemisphere.

The hypothesis was dismissed based on the timing of the Laacher See Tephra relative to the clearest signs of climate change associated with the Younger Dryas Event within various Central European varved lake deposits. This set the scene for the development of the Younger Dryas Impact Hypothesis and the meltwater pulse hypothesis. Interest was revived in 2014 when research placed the eruption of the Laacher See volcano at 12,880 years BP, coinciding with the initiation of North Atlantic cooling into the Younger Dryas. Although the eruption was about twice the size as the 1991 eruption of Mount Pinatubo, it contained considerably more sulfur, potentially rivalling the climatologically very significant 1815 eruption of Mount Tambora in terms of amount of sulfur introduced into the atmosphere. Evidence exists that an eruption of this magnitude and sulfur content occurring during deglaciation could trigger a long-term positive feedback involving sea ice and oceanic circulation, resulting in a cascade of climate shifts across the North Atlantic and the globe. Further support for this hypothesis appeared as a large volcanogenic sulfur spike within Greenland ice, coincident with both the date of the Laacher See eruption and the beginning of cooling into the Younger Dryas as recorded in Greenland. The mid-latitude westerly winds may have tracked sea ice growth southward across the North Atlantic as the cooling became more pronounced, resulting in time transgressive climate shifts across northern Europe and explaining the lag between the Laacher See Tephra and the clearest (wind-derived) evidence for the Younger Dryas in central European lake sediments.

However, in 2021, further research precisely dated the eruption to 200 ± 21 years before the onset of the Younger Dryas, consequently ruling out this hypothesis The same study also concluded that the onset of the Younger Dryas took place synchronously over the entire North Atlantic and Central European region.

Although the timing of the eruption appeared to coincide with the beginning of the Younger Dryas, and the amount of sulfur contained would have been enough to result in substantial Northern Hemisphere cooling, the hypothesis has not yet been tested thoroughly, and no climate model simulations are currently available. The exact nature of the positive feedback is also unknown, and questions remain regarding the sensitivity to the deglacial climate to a volcanic forcing of the size and sulfur content of the Laacher See eruption. However, evidence exists that a similar feedback following other volcanic eruptions could also have triggered similar long-term cooling events during the last glacial period, the Little Ice Age, and the Holocene in general, suggesting that the proposed feedback is poorly constrained but potentially common.

It is possible that the Laacher See eruption was triggered by lithospheric unloading related to the removal of ice during the last deglaciation, a concept that is supported by the observation that three of the largest eruptions within the East Eifel volcanic field occurred during deglaciation. Because of this potential relationship to lithospheric unloading, the Laacher See eruption hypothesis suggests that eruptions such as the 12,880 year BP Laacher See eruption are not isolated in time and space, but instead are a fundamental part of deglaciation, thereby also explaining the presence of Younger Dryas-type events during other glacial terminations.

See also

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* Huelmo–Mascardi Cold Reversal
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* Meltwater pulse 1B
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* Beaufort Gyre reversal

Footnotes

References

External links

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Blytt–Sernander system
Nordic Stone Age
Palynology
Last Glacial Maximum
Stone Age Europe
10th millennium BC
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11th millennium BC