Last Sunday the New York Times had an interesting article about a comet that struck Earth somewhere in the upper mid-west about 12,900 years ago. The impact led to megafaunal extinctions, and the soot from extensive forest fires blotted out the sun. The reference below gives a detailed account of the evidence for the impact. Curiously, while the title mentions Younger Dryas (YD), the article says nothing about the connection to the YD other than that it appears to start at the time of the impact. The NYT article expands on this and mentions the role of Lake Agassiz, a huge expanse of glacial meltwater stretching across the northern tier and southern Canada. The comet impact shattered the ice dam holding back the lake causing the run-off to flood the North Atlantic Ocean with the resulting fresh water interrupting the flow of warm salty water into the northern North Atlantic where it normally cools and sinks, what is known as the Atlantic Meridional Overturning circulation (AMOC).
I didn’t know about any of this but apparently there has been quite a debate about whether there really was a comet, and what actually happened. Evidence for an explosion has been found in sedimentary records all around North America so there seems to be a good case for the comet impact (see reference below). The reason I bring this up is because I always thought the Younger Dryas event ended the last of the Dansgaard-Oeschger (DO) events that took place on a regular basis throughout the last glacial period: a sharp rise in temperature followed by a gradual decline and a final collapse before the cycle starts again. The Bølling-Allerød (BA) sudden and extraordinary warming at 14,690 BP as recorded in the Greenland ice record would have been the start of that last DO event. Then followed a roughly stepwise or gradual decrease in temperature over 1500+ years until it bottoms out for a roughly 1000-year period before the YD event suddenly ends with an incredibly sharp rise in temperature at 12,500 years BP. This sequence seemed to follow a template similar to earlier D-O events suggesting that the transition into the cold YD was well underway before the comet impact. Perhaps flooding from Lake Agassiz contributed to the final collapse of the AMOC, but the rapid sea level rise (~2 cm/year) immediately following the BA event always struck me as a good reason for both the gradual cooling that followed and the weakening of the AMOC. This suggests that the sequence of events leading to the collapse of the AMOC and the cold YD began a good 1500 years before the comet impact.
Firestone, R. B. et al., (2007). Evidence for an extraterrestrial impact 12,900 years ago that contributed to the megafaunal extinctions and the Younger Dryas. PNAS, 104 (41), 16016-16021
https://www.pnas.org/doi/full/10.1073/pnas.0706977104
https://www.nytimes.com/2024/03/05/magazine/younger-dryas-impact-hypothesis-comet.html