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Younger Dryas
The Younger Dryas stadial, named after the alpine / tundra wildflower Dryas octopetala, and also referred to as the Big Freeze [1], was a brief (approximately 1300 +/- 70year [1]) cold climate period following the Bölling/Allerød interstadial at the end of the Pleistocene, and preceding the Preboreal 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 [2].
Younger Dryas - 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)[2] 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 [3]. 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 [5]. Nothing of the size, extent, or rapidity of this period of abrupt climate change has been experienced since [2].
Timeline of glaciation
Younger Dryas - Was the Younger Dryas global?
Although the Younger Dryas had the greatest effect in Europe, it was noted throughout the world including:
- 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.
- More dust in the atmosphere, originating from deserts in Asia.
- Drought in the Levant, perhaps motivating the Natufian culture to invent agriculture.
- The Huelmo/Mascardi Cold Reversal in the Southern Hemisphere began slightly before the Younger Dryas and ended at the same time.
However, the YD event is substantially smaller in Antarctic ice cores than in Greenland cores.
May not have been global, there is no evidence of increased glaciation in the southern hemisphere at this time.
Younger Dryas - 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 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.
Younger Dryas - 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 [2],[3],[6],[7], [8].
The end of the Younger Dryas has been dated to around 9600 BC (11550 calendar years BP, occurring at 10000 radiocarbon years BP, a "radiocarbon plateau") by a variety of methods, with mostly consistent results:
11530±50 BP -- GRIP ice core, Greenland [9]
11530+40-60 BP -- Kråkenes Lake, western Norway. [10]
11570 BP -- Cariaco Basin core, Venezuela [11]
11570 BP -- German oak/pine dendrochronology [12]
11640±280 BP -- GISP2 ice core, Greenland [6]
Younger Dryas - The Younger Dryas and the beginning of agriculture
The Younger Dryas is often linked to the adoption of agriculture in Southwest Asia [14]. 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].
See also
Other related archivesAllerød, Asia, Atlantic Ocean, BP, Blytt-Sernander, Dryas octopetala, Europe, Glaciation, Greenland, Holocene, Huelmo/Mascardi Cold Reversal, Levant, Natufian, Natufian culture, North America, Norway, Older Dryas, Pleistocene, Preboreal, Scandinavia, South America, Southwest Asia, Timeline of glaciation, Venezuela, agriculture, argon, atmosphere, carrying capacity, climate, climate change, coleopteran, cores, deglaciation, dendrochronology, deserts, forest, glaciers, ice, ice core, isotope, nitrogen, oak, periglacial, pine, radiocarbon years, snow, stadial, thermohaline circulation, tundra
 Adapted from the Wikipedia article "Younger Dryas", under the G.N U Free Docmentation License. Please also see http://en.wikipedia.org/wiki |
