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The Younger Dryas, from c. 12,900 to c. 11,700 years ago (BP), was a sharp decline in temperature over most of the northern hemisphere, at the end of the Pleistocene epoch, immediately preceding the current warmer Holocene. It was the most recent and longest of several interruptions to the gradual warming of the Earth's climate since the severe Last Glacial Maximum, c. 27,000 to 24,000 BP. The change was relatively sudden, taking place in decades, and resulted in a decline of 2 to 6 degrees Celsius, advances of glaciers and drier conditions, over much the temperate northern hemisphere. It is thought to have been caused by a decline in the strength of the Atlantic meridional overturning circulation, which transports warm water from the equator towards the North Pole, and which in turn is thought to have been caused by an influx of fresh cold water from North America into the Atlantic. The Younger Dryas was a period of climatic change, but the effects were complex and variable. In the southern hemisphere, and some areas of the north such as the south-eastern United States, there was a slight warming. The presence of a distinct cold period at the end of the Late Glacial interval has been known for a long time. Paleobotanical and lithostratigraphic studies of Swedish and Danish bog and lake sites, e.g. Allerød clay pit in Denmark, first recognized and described the Younger Dryas.〔〔Andersson, G. (1896). ''Svenska växtvä rldens historia.'' P. A. Norstedt & Söner, Stockholm〕〔Hartz, N., and V. Milthers (1901) ''Det senglacie ler i Allerød tegelværksgrav.'' Meddelelser Dansk Geologisk Foreningen 8:31–60.〕 The Younger Dryas is named after an indicator genus, the alpine-tundra wildflower ''Dryas octopetala''. Leaves of ''Dryas octopetala'' are occasionally abundant in the Late Glacial, often minerogenic-rich, lake sediments of Scandinavian lakes. The Younger Dryas is the youngest and longest of three stadials that resulted from typically abrupt climatic changes that took place over the last 16,000 calendar years.〔Mangerud, J., S.T. Andersen, B.E. Berglund, and J.J. Donner (1974) ''Quaternary stratigraphy of Norden, a proposal for terminology and classification.'' Boreas. 3:109-128.〕 Within the Byltt-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 BP. This is not securely dated, and estimates vary by 400 years, but it is generally accepted that it lasted around 200 years. In northern Scotland the glaciers were thicker and more extensive than during the Younger Dryas. The Older Dryas, in turn, is preceded by another warmer stage, the Bølling oscillation that separates it from a third and even older stadial. This stadial is often, but not always, known as the Oldest Dryas. The Oldest Dryas, occurred approximately 1,770 calendar years before the Younger Dryas and lasted about 400 calendar years. According to the GISP2 ice core from Greenland, the Oldest Dryas occurred between about 15,070 and 14,670 calendar years BP.〔Stuiver, M., P.M. Grootes, and T.F. Braziunas (1995) ''The GISP2 (δ) climate record of the past 16,500 years and the role of sun, ocean, and volcanoes.'' Quaternary Research 44:341–354.〕 In Ireland, the Younger Dryas has also been known as the Nahanagan Stadial, while in the United Kingdom 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 three events: ''Greenland Interstadial-1c to 1a (GI-1c to GI-1a)''.〔Björck, S. M.J.C. Walker, L. Cwynar, S.J. Johnsen, K.-L. Knudsen, J.J. Lowe, and B. Wohlfarth, (1998) ''INTIMATE Members, 1998. An event stratigraphy for the Last Termination in the North Atlantic region based on the Greenland Ice Core record: a proposal by the INTIMATE group.'' Journal of Quaternary Science 13:283–292.〕 ==Abrupt climate change== Since 1916 and the onset and later refinement of pollen analytical techniques and a steadily growing number of pollen diagrams, palynologists have concluded that the Younger Dryas represented a distinct period of vegetational change in large parts of Europe during which warmer climate vegetation was replaced by generally cold climate – glacial plant succession that often contained ''Dryas octopetala''. This drastic change in vegetation is typically interpreted to be an effect of a sudden decrease in (annual) temperature, unfavorable for the previously rapidly northward-spreading forest vegetation. This cooling not only favored the expansion of cold-tolerant, light-demanding plants and associated steppe fauna, it also led to regional glacial advances in Scandinavia and a lowering of the regional equilibrium line altitude (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 12,900–11,500 BP in calendar years has been argued to have been quite abrupt. in sharp contrast to the warming of the preceding Older Dryas interstadial. It has been inferred that its end occurred over a period of a decade or so,〔 〕 but the onset may have been faster. Thermally fractionated nitrogen and argon isotope data from Greenland ice core GISP2 indicate that the summit of Greenland was approximately colder during the Younger Dryas〔 than today. In the UK, coleopteran (beetle) fossil evidence suggests that mean annual temperature dropped to , 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 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, which occurred the Bølling oscillation, is known as the intra-Bølling cold period (ICBP) and the cold period, which occurred in the Allerød oscillation, is known as the intra-Allerød cold period (ICAP ). Both of these cold periods are comparable in duration and intensity with the Older Dyras and began and ended quite abruptly. These cold periods have been recognized in sequence and relative magnitude in paleoclimatic records from Greenland ice cores, European lacustrine sediments, Atlantic Ocean sediments, and Cariaco Basin, Venezuela.〔Yu, Z. and U. Eicher (2001) ''Three amphi-Atlantic century-scale cold events during the Bølling-Allerød warm period.'' Géographie physique et Quaternaire. 55:171–179.〕 Examples of older Younger Dryas-like events have been reported from the ends, which are called ''terminations'',〔Note: The relatively rapid change from cold, glacial conditions to warm interglacials are called ''terminations''. They are numbered from the most recent termination as ''I'' and with increasing value, e.g. ''II'', ''III'', and so forth, into the past. Termination I is the end Marine Isotope Stage 2 (MIS2); Termination II is the end of Marine Isotope Stage 6 (MIS6); Termination III is the end of Marine Isotope Stage 8 (MIS8); Termination III is the end of Marine Isotope Stage 10 (MIS10), and so forth. For an example, go see (Pleistocene glacial terminations triggered by synchronous changes in Southern and Northern Hemisphere insolation: The insolation canon hypothesis. ) by K.G. Schulz and R.E. Zeebe.〕 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.〔Bradley, R. (2015) ''Paleoclimatology: Reconstructing Climates of the Quaternary, 3rd ed.''Academic Press: Kidlington, Oxford ISBN 978-0-12-386913-5〕 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, then the Younger Dryas is not the unique paleoclimatic event, in terms of size, extent, and rapidity, as it has been 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.〔Chen, S., Y. Wang, X. Kong, D. Liu, H. Cheng, and R.L. Edwards. (2006) ''A possible Younger Dryas-type event during Asian monsoonal Termination 3.'' Science China Earth Sciences. 49(9):982-990.〕 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. They argue that Younger Dryas events might be an intrinsic feature of deglaciations that occur at the end of glacial periods.〔〔Sima, A., A. Paul, and M. Schulz (2004) ''The Younger Dryas—an intrinsic feature of late pleistocene climate change at millennial timescales.'' Earth Planetary Science Letters. 222:741-750.〕〔Xiaodong, D., Z. Liwei, and K. Shuji (2014) "A Review on the Younger Dryas Event. Advances in Earth Science." 29(10):1095-1109.〕 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Younger Dryas」の詳細全文を読む スポンサード リンク
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