From Wikipedia, the free encyclopedia
It followed the Bølling-Allerød interstadial (warm period) at the end of the Pleistocene and preceded the preboreal of the early Holocene. It is named after an indicator genus, the alpine-tundra wildflower Dryas octopetala. In Ireland, the period has been known as the Nahanagan Stadial, while in the United Kingdom it has been called the Loch Lomond Stadial and most recently Greenland Stadial 1 (GS1).[3][4] The Younger Dryas (GS1) is also a Blytt-Sernander climate period detected from layers in north European bog peat.[citation needed]
The Dryas stadials were cold periods which interrupted the warming trend since the Last Glacial Maximum 20,000 years ago. The Older Dryas occurred approximately 1,000 years before the Younger Dryas and lasted about 300 years.[5] The Oldest Dryas is dated between approximately 18,000 and 15,000 BP.[citation needed]
Contents
Abrupt climate change
The Younger Dryas saw a rapid return to glacial conditions in the higher latitudes of the Northern Hemisphere between 12.9–11.5 ka BP,[6] in sharp contrast to the warming of the preceding interstadial deglaciation. It has been believed that the transitions each occurred over a period of a decade or so,[7] but the onset may have been faster.[8] Thermally fractionated nitrogen and argon isotope data from Greenland ice core GISP2 indicate that the summit of Greenland was approximately 15 °C (27 °F) colder during the Younger Dryas[7] than today. In the UK, coleopteran fossil evidence (from beetles) suggests that mean annual temperature dropped to approximately 5 °C (41 °F),[9] and periglacial conditions prevailed in lowland areas, while icefields and glaciers formed in upland areas.[10] Nothing of the size, extent, or rapidity of this period of abrupt climate change has been experienced since.[6]Global effects
In western Europe and Greenland, the Younger Dryas is a well-defined synchronous cool period.[11] 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 start or end; Huybers has argued that there is fair confidence in the absence of the Younger Dryas in Antarctica, New Zealand and parts of Oceania.[12] Timing of the tropical counterpart to the Younger Dryas – the Deglaciation Climate Reversal (DCR) – is difficult to establish as low latitude ice core records generally lack independent dating over this interval. An example of this is the Sajama ice core (Bolivia), for which the timing of the DCR has been pinned to that of the GISP2 ice core record (central Greenland). Climatic change in the central Andes during the DCR, however, was significant and characterized by a shift to much wetter, and likely colder, conditions.[13] The magnitude and abruptness of these changes would suggest that low latitude climate did not respond passively during the YD/DCR.
In western North America it is likely that the effects of the Younger Dryas were less intense than in Europe; however, evidence of glacial re-advance[14] indicates Younger Dryas cooling occurred in the Pacific Northwest.
Other features seen include:
- 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.
- Formation of solifluction layers and loess deposits in Northern Europe.
- More dust in the atmosphere, originating from deserts in Asia.
- Drought in the Levant, perhaps motivating the Natufian culture to develop agriculture.
- The Huelmo/Mascardi Cold Reversal in the Southern Hemisphere ended at the same time.
- Decline of the Clovis Culture and extinction of animal species in North America.
Causes
The prevailing theory is that the Younger Dryas was caused by significant reduction or shutdown of the North Atlantic "Conveyor", which circulates warm tropical waters northward, in response to a sudden influx of fresh water from Lake Agassiz and deglaciation in North America. Geological evidence for such an event is thus far lacking.[15] The global climate would then have become locked into the new state until freezing removed the fresh water "lid" from the north Atlantic Ocean. An alternative theory suggests instead that the jet stream shifted northward in response to the changing topographic forcing of the melting North American ice sheet, bringing more rain to the North Atlantic which freshened the ocean surface enough to slow the thermohaline circulation.[16] There is also some evidence that a solar flare may have been responsible for the megafaunal extinction, though it cannot explain the apparent variability in the extinction across all continents.[17]There is evidence that some previous glacial terminations had post glacial cooling periods similar to the Younger Dryas.[18]
Impact hypothesis
Main article: Younger Dryas impact hypothesis
A hypothesized Younger Dryas impact event,
presumed to have occurred in North America around 12.9 ka BP, has been
proposed as the mechanism to have initiated the Younger Dryas cooling.
Amongst other things findings of melt-glass material in sediments in
Pennsylvania, South Carolina, and Syria have been reported. These
researchers argue that this material, which dates back nearly 13,000
years, was formed at temperatures of 1,700 to 2,200 °C (3,092 to
3,992 °F) as the result of a bolide impact. They argue that these
findings support the controversial Younger Dryas Boundary (YDB)
hypothesis, that the bolide impact occurred at the onset of the Younger
Dryas.[19]
The hypothesis has been largely questioned by research that stated that
most of the conclusions cannot be repeated by other scientists,
misinterpretation of data, and the lack of confirmatory evidence.[20][21][22]Volcanoes
Although there may be several causes of the Younger Dryas, volcanic activity is considered one possibility.[1] The Laacher See volcano in Germany was of sufficient size, VEI 6, with over 10 km3 (2.4 cu mi) tephra ejected, to have caused significant temperature changes in the northern hemisphere. Laacher See tephra is found throughout the Younger Dryas boundary layer.[23][24][25] This possibility has been disputed by 14C analysis.[citation needed] In the view of Cambridge University volcanologist, Clive Oppenheimer, the magnitude of Laacher See was similar to the 1991 Mount Pinatubo eruption, and the effects were a year or two of northern hemisphere summer cooling and winter warming, and up to two decades of environmental disruption in Germany.[26]End of the climate period
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 about a 7 °C (12.6 °F) warming in just a few years.[6][7][27][28] Total warming in Greenland was 10 ± 4 °C (18 ± 7 °F).[29]The end of the Younger Dryas has been dated to around 11.55 ka BP, occurring at 10 ka BP (radiocarbon year), a "radiocarbon plateau" by a variety of methods, with mostly consistent results:
-
11.50 ± 0.05 ka BP — GRIP ice core, Greenland[30] 11.53 + 0.04
− 0.06ka BP — Kråkenes Lake, western Norway.[31] 11.57 ka BP — Cariaco Basin core, Venezuela[32] 11.57 ka BP — German oak/pine dendrochronology[33] 11.64 ± 0.28 ka BP — GISP2 ice core, Greenland[27]
No comments:
Post a Comment