Younger Dryas Impact?

The Younger Dryas was a cold period about 12,000 years ago that lasted about a thousand years. The previous ice age had seemingly ended, but then the Younger Dryas was a return to cold conditions. Many large animals became extinct around that time, possibly because of the rapidly changing climate.
There is a controversial hypothesis that the Younger Dryas was caused by an extraterrestrial impact. See Evidence for an extraterrestrial impact 12,900 years ago that contributed to the megafaunal extinctions and the Younger Dryas cooling for the original article in PNAS (2007). The controversy peaked with this highly skeptical article Mammoth-Killer Impact Flunks Out in Science (2010).

After a new study failed to find nanodiamonds, impact experts are flatly rejecting outsiders' claims that an impact 12,900 years ago devastated the megafauna.

However the proponents of the impact hypothesis have not surrendered! Evidence from central Mexico supporting the Younger Dryas extraterrestrial impact hypothesis in PNAS (2012) details new evidence from lake sediments in Mexico.