Gems show comet hit continent, scientists say Scientists say gems formed by impact; some are doubtful

Diamonds are generally considered a sign of love and devotion, but a thin layer of microscopic diamonds a couple of feet beneath the surface of North America represents a far more horrific tale - one of fire, flood and devastation that extinguished woolly mammoths and dealt a blow to early civilization, scientists reported today.

The nanodiamonds, so small that they are barely visible in an electron microscope, are thought to be remnants of a comet that hit North America 13,000 years ago - some 65 million years after the much larger collision that wiped out the dinosaurs.

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

Heat from the explosions and fires melted substantial portions of the Laurentide glacier in Canada, sending waves of water down the Mississippi and into the Gulf of Mexico. That caused changes in Atlantic Ocean currents, which started 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, mammoths, mastodons, 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 themselves, 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 - earlier had discovered the thin layer of black soil containing iridium and other debris that they thought indicated impact by a massive comet or meteor. Critics suggested a variety of less cataclysmic explanations.

However, the discovery of the nanodiamonds, reported today in the journal Science, provides the most powerful support for the comet theory because the gems can be created under only the extreme temperatures and pressures of a vast, 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 paleo-oceanographer.

Such an impact would be the most likely source of nanodiamonds, critics agreed. But many argued that the one-page paper in Science does not provide enough evidence to support the authors' claim.

"Nanodiamonds could be a good indicator of an impact event ... but after reading the paper, I wasn't convinced they found diamonds," said physicist Tyrone Daulton of Washington University in St. Louis. "Maybe they found diamonds, and maybe they didn't."

Archaeologist Douglas J. Kennett, lead author of the report and James Kennett's son, conceded that the publication's restrictive format limited the amount of data the team could incorporate.

But he said the presence of nanodiamonds had been confirmed in three laboratories. "There are going to be a lot of follow-up papers that will clearly demonstrate that these are diamonds," he said.

The Kennetts and their colleagues reported last year that they had found the black layer, radiocarbon-dated to 12,900 years ago, at 10 archaeological sites on the continent. In addition to charred remains from forests and other flora, the black mat contains iridium, carbon spherules and fullerenes containing helium-3, all characteristic of an extraterrestrial impact.

But critics said the evidence was insufficient to prove an impact, particularly in the absence of a demonstrable crater.

James Kennett and his colleagues went back to the mats they had collected and performed what he termed the "extremely labor-intensive" process of looking for the nanodiamonds. That involved using acids to dissolve everything else in the samples, then using a variety of techniques to identify the diamond residue.

They found a family of at least five forms of diamonds, including some that are formed only by impacts, they reported in the Science paper. Moreover, the nanodiamonds were found only at the bottom of the black mat - not in the soil either below or above it.

Such diamonds previously have been found in the soil boundary between the Cretaceous and Tertiary eras when the dinosaurs disappeared in the aftermath of what is widely believed to have been a meteorite impact.

In papers presented last month at a meeting of the American Geophysical Union in San Francisco, the team also reported that at least 15 of the 35 species that became extinct during the Younger Dryas disappeared within 100 years of the event.