The report in the journal Science focuses on tiny "nanodiamonds," crystals tied to past comet impacts, at six sites across the continent in a soil layer dated to the start of a 1,300-year-long ice age.

Geologists and archaeologists have long argued about what caused the extinction of dozens of large North American "megafauna" species, such as saber-toothed cats and mammoths.

"What we're reporting is consistent with a major cosmic impact that had major consequences for the environment and Earth's climate," says study leader Douglas Kennett of the University of Oregon in Eugene.

"A swarm of comets" or carbon-rich meteorites either delivered or created the nanodiamonds in a fiery impact, the study suggests. The report relies on photomicrograph analyses of soil samples from Arizona, Minnesota, Oklahoma, South Carolina and two Canadian sites. Photomicrography captures images seen through a microscope.

"This is the 'smoking gun' evidence for a massive impact event 12,900 years ago that triggered the (ice age) and the extinction of the megafauna," says nuclear scientist Richard Firestone of the Lawrence Berkeley (Calif.) National Laboratory, who was not part of the study.

If true, the impact date coincides with the abrupt halting of deposits of "Clovis" Native American artifacts, distinctively fluted tools and arrowheads. Dozens of large animal species vanished then in North America. Kennett and other impact researchers have suggested a continent-wide wildfire may have contributed to the extinction of large North American creatures. In Europe, there were disruptions to the prehistoric culture and the demise there of species such as the cave bear and Irish elk.

Some scientists urge caution.

"We simply do not have conclusive evidence that nanodiamond materials aren't everywhere at many times," says geologist Nicholas Pinter of Southern Illinois University in Carbondale. "Tons of meteorite dust falls to Earth every year, after all."

Although more than 30 North American species died out about 12,900 years ago, about 50 large species died out a few centuries later in South America, and on some unpeopled Caribbean islands, species such as sloths survived an additional 6,000 years, says archaeologist Stuart Fiedel, author of Prehistory of the Americas. "Humans, not extraterrestrial objects," best explain the staggered extinctions in the New World, he says.

Kennett says future studies will show evidence of the nanodiamonds from Europe and further afield 12,900 years ago. Impact shock waves, debris and wildfires sparked by comets breaking apart in the atmosphere would have hit North America hardest, he says, but the effects would have been felt worldwide.

 http://www.usatoday.com/tech/science/2009-01-01-mammothimpact_N.htm?csp=15