via ScienceDaily, again.
A new theory has been proposed regarding the Lesser Dryas climate event occuring about 13000 years ago. The Lesser Dryas was a cooling trend that briefly interrupted the warming climatic trend that ended the last ice age about 20000 years ago (which we are still experiencing). The Lesser Dryas is held responsible for a major extinction event that occurred about the same time that eliminated a number of the large mammal species in North America.
The theory? An impact event in the area of the Great Lakes, possibly a comet, that struck the ice sheet covering the area north of the lakes in present day Canada. The evidence? A large concentration of carbon rich soils pointing to such an impact event found in over 50 Clovis culture archaeological sites throughout North America, with the highest concentrations centered in the Lakes region.
"The researchers propose that a known reversal in the world's ocean currents and associated rapid global cooling, which some scientists blame for the extinction of multiple species of animals and the end of the Clovis Period, was itself the result of a bigger event. While generally accepted theory says glacial melting from the North American interior caused the shift in currents, the new proposal points to a large extraterrestrial object exploding above or even into the Laurentide Ice Sheet north of the Great Lakes."
Interesting. The one trouble with the theory is no impact crater, but that is easily explained by the ice sheet absorbing the impact. The results of such an impact would have been pretty devastating to both animal and human populations, and might explain the concentration of late era Clovis sites in the American Southwest.