Washington, Jan 2 : A nine-member scientific team has found abundant tiny particles of diamond dust in sediments dating to 12,900 years ago at six North American sites, adding strong evidence for Earth's impact with a rare swarm of carbon-and-water-rich comets.

Nanodiamonds can be produced on Earth, but only through high-explosive detonations or chemical vaporization.

These nanodiamonds, which are produced under high-temperature, high-pressure conditions created by cosmic impacts and have been found in meteorites, are concentrated in similarly aged sediments at Murray Springs, Arizona, Bull Creek, Oklahoma, Gainey, Michigan, and Topper, S.C., as well as Lake Hind, Manitoba, and Chobot, Alberta, in Canada.

Last year, a 26-member team from 16 institutions proposed that a cosmic impact event, possibly by multiple airbursts of comets, set off a 1,300-year-long cold spell known as the Younger Dryas, fragmented the prehistoric Clovis culture and led to the extinction of a large range of animals, including mammoths, across North America.

Now, a team led by the University of Oregon's Douglas J. Kennett, a member of the original research team, has reported finding billions of nanometer-sized diamonds concentrated in sediments - weighing from about 10 to 2,700 parts per billion - in the six locations during digs funded by the National Science Foundation.

"The nanodiamonds that we found at all six locations exist only in sediments associated with the Younger Dryas Boundary layers, not above it or below it," said Kennett, a UO (University of Oregon) archaeologist.

"These discoveries provide strong evidence for a cosmic impact event at approximately 12,900 years ago that would have had enormous environmental consequences for plants, animals and humans across North America," he added.