for National Geographic News
A comet exploded over North America about 13,000 years ago, causing a long bout of climate cooling, according to a controversial new theory presented today.
The extraterrestrial impact may help explain massive mammal die-offs and the demise of one of the earliest American cultures.
It would also be the first known extraterrestrial impact to affect modern humans.
Evidence for the impact comes from a thin layer of sediment found throughout North America, said James Kennett, a geologist at the University of California in Santa Barbara.
"There are materials with particular chemistries in that layer that collectively provide very strong evidence that the layer was produced by this extraterrestrial impact," he said in a telephone interview.
Kennett said the layer contains tiny spheres of carbon and metals, bits of diamonds, and extraterrestrial concentrations of helium 3 and the element iridium.
The layer dates to 12,900 years ago, he added.
At about the same time, according to the researchers, Earth's climate cooled; mammals like mammoths, mastodons, and saber-toothed cats went extinct; and one of the first American cultures disappeared.
Kennett and colleagues outlined evidence of the impact and its repercussions at a meeting of the American Geophysical Union this week in Acapulco, Mexico.
The timing of the impact, according to Kennett, coincides with an era of climate cooling known as the Younger Dryas.
"We are suggesting for the first time that this very abrupt and large cooling that occurred at 12.9 thousand years ago was triggered by the extraterrestrial impact," he said.
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