Editing Younger Dryas impact hypothesis (section)

== Background ==
{{Main|Younger Dryas#Causes}}A number of theories have been put forward about the cause of the [[Younger Dryas]] climate event. The most widely accepted is that it began because of a significant reduction or [[Shutdown of thermohaline circulation|shutdown of the North Atlantic "Conveyor"]] – which circulates warm tropical waters northward – as the consequence of [[deglaciation]] in North America. Geological evidence for such an event is not fully secure,<ref name="Broecker2">{{cite journal |last=Broecker |first=Wallace S. |year=2006 |title=Was the Younger Dryas triggered by a flood? |url=https://semanticscholar.org/paper/42288c83cd71bc7051793716f2e8f3c58650ceac |journal=Science |volume=312 |issue=5777 |pages=1146–1148 |doi=10.1126/science.1123253 |pmid=16728622 |s2cid=39544213}}</ref> but recent work has identified a pathway along the [[Mackenzie River]] that would have spilled fresh water from [[Lake Agassiz]] into the Arctic and thence into the Atlantic.<ref>{{cite journal |last1=Murton |first1=Julian B. |last2=Bateman |first2=Mark D. |last3=Dallimore |first3=Scott R. |last4=Teller |first4=James T. |last5=Yang |first5=Zhirong |date=2010 |title=Identification of Younger Dryas outburst flood path from Lake Agassiz to the Arctic Ocean |journal=Nature |language=en |volume=464 |issue=7289 |pages=740–743 |bibcode=2010Natur.464..740M |doi=10.1038/nature08954 |issn=0028-0836 |pmid=20360738 |s2cid=4425933}}</ref><ref>{{cite journal |last1=Keigwin |first1=L.D. |last2=Klotsko |first2=S. |last3=Zhao |first3=N. |last4=Reilly |first4=B. |last5=Giosan |first5=L. |last6=Driscoll |first6=N.W. |date=2018 |title=Deglacial floods in the Beaufort Sea preceded Younger Dryas cooling |journal=Nature Geoscience |language=en |volume=11 |issue=8 |pages=599–604 |bibcode=2018NatGe..11..599K |doi=10.1038/s41561-018-0169-6 |issn=1752-0894 |hdl-access=free |hdl=1912/10543 |s2cid=133852610}}</ref> The global climate would then have become locked into the new state until freezing removed the fresh water "lid" from the North Atlantic.

An alternative theory suggests instead that the [[Jet stream|jet stream]] shifted northward in response to the melting of the North American ice sheet, which brought more rain to the North Atlantic, which freshened the ocean surface enough to slow the [[thermohaline circulation]]. Another proposed cause has been volcanic activity.However, this has been challenged recently due to improved dating of the most likely suspect, the [[Laacher See]] volcano. In 2021, research by Frederick Reinig et al. precisely dated the eruption to 200 ± 21 years before the onset of the Younger Dryas, therefore ruling it out as a culprit. firmly date the [Laacher See eruption] to 13,006 ± 9 calibrated years before present (BP; taken as AD 1950), which is more than a century earlier than previously accepted. ...thereby dating the onset of the Younger Dryas to 12,807 ± 12 calibrated years BP, which is around 130 years earlier than thought.  The same study also concluded that the onset took place synchronously over the entire North Atlantic and Central European region. A press release from the [[University of Mainz]] stated, "Due to the new dating, the European archives now have to be temporally adapted. At the same time, a previously existing temporal difference to the data from the Greenland ice cores was closed."That is 126 years earlier than the generally accepted dating based on sediments in the Meerfelder Maar from the Eifel region in Germany. ... This difference has far-reaching consequences for the synchronization of European climate archives and the understanding of North Atlantic and European climate history. ... This means that the [onset of the Younger Dryas] also occurred in Central Europe 130 years earlier, around 12,870 years ago respectively. This is in line with the onset of the cooling in the North Atlantic region identified in ice cores from Greenland. ... 'This strong cooling did not take place time transgressively, as previously thought, but rather synchronously over the entire North Atlantic and Central European region,' said Frederick Reinig.

In contrast, proponents of the impact hypothesis posit that fragments of a large disintegrating [[asteroid]] or [[comet]] struck the earth around 12,850 years ago, causing the [[Younger Dryas]] cooling event. They also hypothesize that the impact event triggered extensive [[biomass]] burning, a brief [[impact winter]], and an [[Younger Dryas#Abrupt climate change|abrupt climate change]]{{sfnp|Pino|Abarzúa|Astorga|Martel-Cea|2019}} which, they contend, directly brought about the extinction of many species of North American [[Pleistocene megafauna]]{{sfnp|Pino|Abarzúa|Astorga|Martel-Cea|2019}} including [[camel]]s, [[mammoth]]s, and the [[Arctodus|giant short-faced bear]].<ref name="Haynes b">{{Cite journal |vauthors=Haynes G |date=5 November 2010 |title=The catastrophic extinction of North American mammoths and mastodonts |journal=[[World Archaeology]] |volume=33 |issue=3 |pages=391–416 |doi=10.1080/00438240120107440 |s2cid=26671638}}</ref> They also say the event contributed to the transition from [[Clovis culture]] to subsequent traditions.<ref name="Carrasco">{{Cite journal |vauthors=Carrasco MA, Barnosky AD, Graham RW |date=December 2009 |title=Quantifying the extent of North American mammal extinction relative to the pre-anthropogenic baseline |journal=[[PLOS ONE]] |volume=4 |issue=12 |pages=e8331 |bibcode=2009PLoSO...4.8331C |doi=10.1371/journal.pone.0008331 |pmc=2789409 |pmid=20016820 |doi-access=free}}</ref>