Did A Celestial Explosion Kill North American Mammals?

tags: bolide, mammals, mass extinction, North America, Younger Dryas

Why did huge numbers of large mammals in North America suddenly die out approximately 13,000 years ago? Over the years, there have been plenty of hypotheses proposed, ranging from overhunting and disease to death by freezing. However, another group of scientists have recently proposed a different hypothesis; a large space rock exploding over North America.

Collecting layers of sediment from more than 20 sites across North America, James Kennett, from the University of California in Santa Barbara (UCSB) and his colleagues, found that these sediments contained a variety of unusual materials: tiny spheres of glass and carbon, ultra-small specks of diamond -- known as nanodiamonds -- and the rare element iridium. Iridium especially, was present in amounts that were too high to have come from Earth.

The evidence points to an explosion of an extraterrestrial object of up to 5 kilometers in diameter. According to Kennett's team, this explosion occurred approximately 12,900 years ago. Unfortunately, no crater has been discovered, possibly because it was masked by the Laurentide Ice Sheet, which blanketed thousands of square kilometers of North America during the last Ice Age. Another possibility is that the bolide exploded before it hit the ground. Regardless of where it occurred, this explosion could be the reason for the extinction of several large North American mammals at the end of the last Ice Age.

"All the elephants, including the mastodon and the mammoth, all the ground sloths, including the giant ground sloth -- which, when standing on its hind legs, would have been as big as a mammoth," Kennett said. "All the horses went out, all the North American camels went out. There were large carnivores like the saber-toothed cat and an enormous bear called the short-faced bear."

In addition to killing large mammals, this explosion could also have impacted early humans. The Clovis culture was one of the earliest known human cultures on North America. These hunter-gatherers are known for the Clovis point; a distinctive thin, fluted spearhead, which is regarded as one of the most sophisticated stone tools ever developed. The Clovis culture vanished abruptly and mysteriously from the archaeological record, and was replaced by a variety of different local hunter-gatherer cultures.

The exploding bolide would have caused widespread melting of the North American ice sheet, according to this new hypothesis. The resulting meltwaters would have rushed into the Atlantic Ocean, disrupting its currents. This disruption could have caused the 1,000 year-long Younger Dryas cold spell, which also affected Asia and Europe.

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