Bob Park writes, on Sept 3, 2010, in his weekly What’s New newsletter:
Earth went through a relatively brief cold spell about 13,000 years ago called the Younger Dryas climate episode. It coincided with the disappearance of the North American Clovis culture, and killed the mammoths and other Ice Age mega fauna. The accepted explanation was that ocean currents in the North Atlantic had been disrupted by a large body of freshwater emptying into the ocean. Three years ago, however, a story in Nature by Rex Dalton, reported an alternative explanation involving the impact of a comet. The key comet evidence was micro-diamonds found in sedimentary layers from that period. Now comes another group that goes to the same site but what they find is not diamonds but carbonized arthropod feces. The first group disputes this evidence and is preparing another paper. It’s not pretty, but it’s science.