Comet hit North America 12,900 years ago

Neat. [Link]

According to the theory -- which has its critics -- as the comet broke apart, it rained fire over the entire continent, igniting the plains and the forests and creating choking clouds of smoke.

Heat from the explosions and the massive fires melted substantial portions of the Laurentide glacier in Canada, sending waves of water down the Mississippi and into the Gulf of Mexico. That triggered changes in Atlantic Ocean currents, which ushered in a 1,300-year ice age known as the Younger Dryas.

Battered by fire and ice, as many as 35 species of mammals, including American camels, the short-faced bear, the giant beaver, the dire wolf and the American lion, either immediately vanished or were so depleted in number that humans hunted them to extinction.

The humans, a Paleo-Indian grouping known as the Clovis culture for the distinctive spear points they employed, suffered a major population drop, disappearing in many areas for hundreds of years.

The researchers -- including James P. Kennett of UC Santa Barbara and Douglas J. Kennett of the University of Oregon -- had earlier discovered the thin layer of black soil containing iridium and other debris that they thought indicated a massive comet or meteor impact. But critics suggested a variety of less dire explanations.

The discovery of the nanodiamonds, however, reported Friday in the journal Science, provides the most powerful support for the comet theory because the gems can only be created under the extreme temperatures and pressures of a massive explosion, such as a comet striking the Earth's surface.

"There's no other way we can interpret the presence of these diamonds other than an extraterrestrial impact," said James Kennett, a paleooceanographer.