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The Younger Dryas of the early Holocene . In Ireland, the period has been known as the Nahanagan Stadial, while in the UK it has been called the '''Loch Lomond Stadial'''. The Younger Dryas is also a Blytt-Sernander climate period detected from layers in north European bog peat. It is dated approximately 12,900-11,500 BP calibrated, or 11,000-10,000 BP uncalibrated. An '' Older Dryas '' stadial had preceded the Allerød, approximately 1000 years before the Younger Dryas; it lasted 300 years {Link without Title} . ABRUPT CLIMATE CHANGE The Younger Dryas saw a rapid return to glacial conditions in the higher latitudes of the Northern Hemisphere between 12,900 – 11,500 years before present (BP) in sharp contrast to the warming of the preceding interstadial Deglaciation . The transitions each occurred over a period of a decade or so [Alley et al., 1993 .Thermally fractionated Nitrogen and Argon Isotope data from Greenland Ice Core GISP2 indicates that the summit of Greenland was ~15 °C colder than today during the Younger Dryas In the UK, Coleopteran (fossil beetle) evidence suggests mean annual temperature dropped to approximately -5 °C [4 , and Periglacial conditions prevailed in lowland areas, while icefields and Glaciers formed in upland areas Nothing of the size, extent, or rapidity of this period of abrupt Climate Change has been experienced since [2 . WAS THE YOUNGER DRYAS GLOBAL? Answering this question is hampered by the lack of a precise definition of "younger dryas" in all the records. In western Europe and Greenland, the Younger Dryas is a well-defined synchronous cool period {Link without Title} . But cooling in the tropical north atlantic may have preceded this 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 ; Huybers has argued that there is fair confidence in the abscence of the yonger dryas in Antarctica, New Zealand and parts of oceania. Similarly the Southern Hemisphere cooling known as the Deglaciation Climate Reversal (DCR) began approximately 1kyr before the YD, between 14kya and 11.5 kya as noted in the Sajama ice core. The Andean climate returned to LGM conditions with colder temperatures coupled with higher precipitation (high lake stands in the altiplano). Thompson et al., 2000. Other features seen include:
CAUSES OF THE YOUNGER DRYAS The prevailing theory holds that the Younger Dryas was caused by a significant reduction or shutdown of the North Atlantic Thermohaline Circulation in response to a sudden influx of fresh water from Lake Agassiz and deglaciation in North America . The global climate would then have become locked into the new state until freezing removed the fresh water "lid" from the north Atlantic Ocean . This theory does not explain why South America cooled first. THE END OF THE YOUNGER DRYAS 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, requiring a ~7 °C warming in just a few years [8 . The end of the Younger Dryas has been dated to around 9600 BC (11550 calendar years BP, occurring at 10000 Radiocarbon Year s BP, a "radiocarbon plateau") by a variety of methods, with mostly consistent results: :11530±50 BP -- GRIP Ice core, Greenland {Link without Title} :11530+40-60 BP -- Kråkenes Lake , western Norway . {Link without Title} :11570 BP -- Cariaco Basin core, Venezuela {Link without Title} :11570 BP -- German Oak / Pine Dendrochronology {Link without Title} :11640±280 BP -- GISP2 ice core, Greenland {Link without Title} THE YOUNGER DRYAS AND THE BEGINNING OF AGRICULTURE The Younger Dryas is often linked to the adoption of agriculture in Southwest Asia It is argued that the cold and dry Younger Dryas lowered the Carrying Capacity of the area and forced the sedentary Early Natufian population into a more mobile subsistance pattern. Further climatic deterioration is thought to have brought about cereal cultivation. While there exists relative consensus regarding the role of the Younger-Dryas in the changing subsistance patterns during the Natufian , its connection to the beginning of agriculture at the end of the period is still being debated [15 . REFERENCES #Berger, W.H. (1990), The Younger Dryas cold spell – a quest for causes. ''Palaeogeography, Palaeoclimatology, Palaeoecology (Global and Planetary Change Section)'' 89, 219-237. #Alley, R.B. (2000), The Younger Dryas cold interval as viewed from central Greenland. ''Quaternary Science Reviews'' 19, 213-226. #Alley et al. (1993), Abrupt accumulation increase at the Younger Dryas termination in the GISP2 ice core, ''Nature'', 362, 527-529. #Severinghaus, J.P., ''et al.'' (1998), Timing of abrupt climate change at the end of the Younger Dryas interval from thermally fractionated gases in polar ice. ''Nature'' 391, 141-146. #Atkinson, T.C., ''et al.'' (1987), Seasonal temperatures in Britain during the past 22,000 years, reconstructed using beetle remains. ''Nature'' 325, 587-592. #Sissons, J.B. (1979), The Loch Lomond stadial in the British Isles. ''Nature'' 280, 199-203. #Alley, R.B., ''et al.'' (1993), Abrupt increase in Greenland snow accumulation at the end of the Younger Dryas event. ''Nature'' 362, 527-529. #Dansgaard, W., ''et al.'' (1989), The abrupt termination of the Younger Dryas climate event. ''Nature'' 339, 532-534. #Taylor, K.C., ''et al.'' (1997), The Holocene-Younger Dryas transition recorded at Summit, Greenland. ''Science'' 278, 825-827. #Spurk, M., ''et al.'' (1998), Revisions and extension of the Hohenheim oak and pine chronologies: New evidence about the timing of the Younger Dryas/Preboreal transition, ''Radiocarbon'' 40, 1107-1116. #Gulliksen, S., ''et al.'' (1998), A calendar age estimate of the Younger Dryas-Holocene boundary at Krakenes, western Norway, ''Holocene'' 8, 3, 249-259. #Hugheus radiocarbon and climate shifts during the last deglaciation, ''Science'' 290, 5498, 1951-1954. -- http://www.ngdc.noaa.gov/paleo/pubs/hughen2000/hughen2000.html #Friedrich, M., ''et al.'' (1999), Paleo-environment and radiocarbon calibration as derived from Lateglacial/Early Holocene tree-ring chronologies, ''Quaternary International'' 61, 27-39. -- http://www.pages.unibe.ch/products/scientific_foci/ql_dfg/friedrichabstract.html #Bar-Yosef, O. and A. Belfer-Cohen (2002), Facing environmental crisis. Societal and cultural changes at the transition from the Younger Dryas to the Holocene in the Levant. In: ''The Dawn of Farming in the Near East''. Edited by R.T.J. Cappers and S. Bottema, pp. 55-66. Studies in Early Near Eastern Production, Subsistence and Environment 6. Berlin: Ex oriente. -- #Munro, N. D., (2003), Small game, the younger dryas, and the transition to agriculture in the southern levant, ''Mitteilungen der Gesellschaft für Urgeschichte'' 12, 47–64. -- PDF SEE ALSO EXTERNAL LINKS
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