Comet swarm led to the demise of wolly mammoth

— -- A swarm of comets that smacked North America 12,900 years ago wiped out the wooly mammoth and early Native American cultures, according to a new soil study released Thursday.

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. In particular, the report relies on "gold standard" 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. "Nanodiamonds are only known to occur in extraterrestrial sources." Firestone suggests the Great Lakes may contain remnants of impact craters.

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 continental-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, cave lion and Irish elk.