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The Older Dryas was a stadial (cold) period between the Bølling and Allerød interstadials (warmer phases) approximately 14,000 years ago (BP), towards the end of the Pleistocene epoch. Its age is not well defined, with estimates varying by 400 years, but its duration is agreed to have been around two centuries. The gradual warming since the last glacial maximum 27-24,000 years BP has been interrupted by two cold spells, the Older Dryas having been followed by the Younger Dryas (c. 12,900-11,650 BP). In northern Scotland the glaciers were thicker and deeper during the Older than the succeeding Younger Dryas, and there is no evidence of human occupation of Britain. In north-west Europe an earlier Oldest Dryas is also recognised. They are named after an indicator genus, the arctic and alpine plant Dryas, the remains of which are found in higher concentrations in deposits from colder periods. The Older Dryas was a variable cold, dry Blytt-Sernander period, observed in climatological evidence in only some regions, depending on latitude. In regions where it is not observed, the Bølling-Allerød is considered a single interstadial (warm) period. Evidence of the Older Dryas is strongest in northern Eurasia, particularly part of North Europe roughly equivalent to Pollen zone Ic. ==Dates== In the Greenland oxygen isotope record, the Older Dryas appears as a downward peak establishing a small, low-intensity gap between the Bølling and the Allerød. This configuration presents a difficulty in estimating its time, as it is more of a point than a segment. But which point, and how long a segment should be assigned? The segment is small enough to escape the resolution of most C-14 series; i.e., the points are not close enough together to find it.〔(Perry, Charles A., Hsu, Kenneth A.; Geophysical, archaeological, and historical evidence support a solar-output model for climate change ). Section: Model Timeline Calibration, Proceedings of the National Academy of Sciences.〕 One approach to the problem assigns a point and then picks an arbitrary segment. You might read that the Older Dryas is "centered" near 14,100 BP or is 100 to 150 years in duration "at" 14,250 BP. A second approach finds C-14 or other dates as close to the end of the Bølling and the beginning of the Allerød as possible and selects end points based on them. This type of date is of the form, for example, 14,000-13,700 BP. The best approach attempts to include the Older Dryas in a sequence of points as close together as possible (high resolution), or within a known event. For example, pollen from the island of Hokkaidō in Japan records a Larix pollen peak and matching sphagnum decline at 14,600-13700 BP. In the White Sea a cooling occurred at 14,700-13,400/13,000, which resulted in a readvance of the glacier in the initial Allerød. In Canada, the Shulie Lake phase (a readvance) is dated to 14,000-13,500 BP. On the other hand, varve chronology in southern Sweden indicates a range of 14,050-13,900 there.〔Edwige Pons-Branchu, Climatic control on speleothems growth. High precision U/Th dating of speleothems from South and East of France.〕 Capturing the Older Dryas through high resolution dating continues to be of concern to researchers in climatology. 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「Older Dryas」の詳細全文を読む スポンサード リンク
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