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 cold climate period following the Bölling/Allerød interstadial at the end of the Pleistocene, and preceding the Preboreal of the early Holocene.

Contents

1 Abrupt climate change 2 Was the Younger Dryas global? 3 Causes of the Younger Dryas 4 The end of the Younger Dryas 5 References 6 External links

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. Thermally fractionated nitrogen and argon isotope data from Greenland ice core GISP2 indicates that the summit of Greenland was ~15 ºC colder than today during the Younger Dryas [3]. In the UK, coleopteran (fossil beetle) evidence suggests mean annual temperature dropped to approximately -5 ºC [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].

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.

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 South America having 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 ºC 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]

References

1. Berger, W.H. (1990), The Younger Dryas cold spell – a quest for causes. Palaeogeography, Palaeoclimatology, Palaeoecology (Global and Planetary Change Section) 89, 219-237.
2. Alley, R.B. (2000), The Younger Dryas cold interval as viewed from central Greenland. Quaternary Science Reviews 19, 213-226.
3. 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.
4. Atkinson, T.C., et al. (1987), Seasonal temperatures in Britain during the past 22,000 years, reconstructed using beetle remains. Nature 325, 587-592.
5. Sissons, J.B. (1979), The Loch Lomond stadial in the British Isles. Nature 280, 199-203.
6. Alley, R.B., et al. (1993), Abrupt increase in Greenland snow accumulation at the end of the Younger Dryas event. Nature 362, 527-529.
7. Dansgaard, W., et al. (1989), The abrupt termination of the Younger Dryas climate event. Nature 339, 532-534.
8. Taylor, K.C., et al. (1997), The Holocene-Younger Dryas transition recorded at Summit, Greenland. Science 278, 825-827.
9. 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.
10. Gulliksen, S., et al. (1998), A calendar age estimate of the Younger Dryas-Holocene boundary at Krakenes, western Norway, Holocene 8, 3, 249-259.
11. Hughen, K.A, et al. (2000), Synchronous radiocarbon and climate shifts during the last deglaciation, Science 290, 5498, 1951-1954. -- http://www.ngdc.noaa.gov/paleo/pubs/hughen2000/hughen2000.html
12. 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

External links

• http://faculty.washington.edu/wcalvin/teaching/Broecker99.html

fi:Nuorempi Dryas