A discovery of microscopic diamonds a few feet beneath the surface of North America reveals that a comet caused a cataclysm of fire, flood and devastation nearly 13,000 years ago that extinguished mammoths and mastodons and dealt a blow to early civilization, scientists said Friday.
The nanodiamonds, so small that they are barely visible in an electron microscope, are thought to be remnants of that comet, which would have hit about 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 the forests and creating choking clouds of smoke.
Heat from the explosions and the massive fires melted substantial portions of the Laurentide glacier in Canada, sending waves of water down the Mississippi and into the Gulf of Mexico. That triggered changes in Atlantic Ocean currents, which ushered in 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, 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, 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 -- had earlier discovered the thin layer of black soil containing iridium and other debris that they thought indicated a massive comet or meteor impact. But critics suggested a variety of less dire explanations.
The discovery of the nanodiamonds, however, reported Friday in the journal Science, provides the most powerful support for the comet theory because the gems can only be created under the extreme temperatures and pressures of a massive 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 paleooceanographer.
Such an impact would be the most likely source of nanodiamonds, critics agreed. But many argued that the one-page paper in Science did 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."
Spectroscopist Peter Buseck of Arizona State University said that he "wouldn't question that they saw nanodiamonds," but added that for such a potentially important discovery, he "would like to have it well supported."
Archaeologist Douglas Kennett, lead author of the report and James Kennett's son, conceded that the restrictive format of the rapid publication limited the amount of data the team could incorporate into their paper.
But he said the presence of nanodiamonds had been confirmed in three separate laboratories. "There are going to be a lot of follow-up papers that will clearly demonstrate that these are diamonds," he said.
The findings may tie together a variety of hitherto mysterious events in North America that all occurred beginning about 12,900 years ago, the beginning of the Younger Dryas -- also known as the Big Freeze.
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 scattered around 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 different 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 have previously been found in the thin layer of soil that marks the boundary between the Cretaceous and Tertiary eras of Earth's history 65 million years ago, when the dinosaurs disappeared in the aftermath of what is widely believed to have been a meteorite impact.
In a series of 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.
Their fossils appear "right up to the base of the black mat and never go above it," James Kennett said.
The other species may have been hunted to extinction by humans, as has often been speculated, but only after their numbers were greatly reduced by the fires and climate change.
There was also a "major population crash" of humans, he added: In the Great Plains, all traces of humans disappeared for about 200 years. Southern California was even more severely affected, with evidence of human occupation disappearing for most of the Younger Dryas.
James Kennett is convinced there was a cometary impact.
"I've counted up 15 major things that occurred at 12,900 years ago," he said. "I'd like somebody to explain that to me in some other way."