Comet impact might have caused extinctions

BERKELEY, Calif., Sept. 25 (UPI) -- A U.S.-led team of scientists suggests a comet or meteorite exploding near Earth about 12,900 years ago was responsible for prehistoric extinctions.

The team said such an explosion could have produced abrupt and severe climate changes causing the disappearance of such animals as the wooly mammoth, giant sloths, the teratorn -- a condor with a 16-foot wingspan -- and other giant prehistoric beasts.

The scientists, led by Richard Firestone of the Lawrence Berkeley National Laboratory, believe evidence for such an extraterrestrial impact is hidden in a layer of dirt sometimes called a black mat that's found in more than 50 sites around North America and in Belgium.

Directly beneath the black mat, researchers found high concentrations of magnetic grains containing iridium, charcoal, soot, carbon spherules, glass-like carbon containing nanodiamonds and fullerenes packed with extraterrestrial helium.

Brown University planetary geologist Professor Peter Schultz said such nanodiamonds can only be formed by the kind of intense pressure generated by a comet or meteorite slamming into the Earth.

"We don't have a smoking gun for our theory, but we sure have a lot of shell casings," Schultz said.

The theory is detailed in the Proceedings of the National Academy of Sciences.