Scientists say comet impact caused mass extinction in North America 12,900 years ago

Ancient history may be rewritten as some scientists argue in a recent article that they have uncovered evidence of a cataclysmic event that occurred 12,900 years ago and wiped out a considerable portion of the flora and fauna of modern-day North America. Their hypothesis of a comet impact is in contrast to the more widely accepted belief gradual extinction due to adverse climatic conditions.

Scientists have long blamed climate change for the extinctions, for it was 12,900 years ago that the planet's emergence from the Ice Age came to a halt, reverting to glacial conditions for 1,500 years, an epoch known as the Younger Dryas.

In just the last few years, there has arisen a controversial scientific hypothesis to explain this chain of events, and it involves an extraterrestrial calamity: a comet, broken into fragments, turning the sky ablaze, sending a shock wave across the landscape and scorching forests, creatures, people and anything exposed to the heavenly fire.

Now the proponents of this apocalyptic scenario say they have found a new line of evidence  - nanodiamonds. They say they have found these tiny structures across North America in sediments from 12,900 years ago, and they argue that the diamonds had to have been formed by a high-temperature, high-pressure event, such as a cometary impact.

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 the father-son duo 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.

Absence of impact crater
But there are several criticisms of this theory. One is the absence of a corresponding impact crater. Nor are there signs of deformation in rock debris that is a signature of the massive impact that, 65 million years ago, apparently wiped out the dinosaurs. However, proponents of this new hypothesis, published today in Science, explain the absence of visible impact signs by proposing that the comet broke up into several smaller fragments.

"Imagine these fireballs exploding in the air. A Clovis hunter standing and looking at these things would have seen a canopy of fire as these things came in and exploded," said Allen West, a geophysicist and one of the paper's co-authors. "There would have been no sound. There would have been massive explosions. Brilliant light, brighter than the sun. There would have been radiant heat - it would have been capable, at the very least, of giving him serious burns and, at the maximum, of incinerating him."

But Kennett and his colleagues say that they have found these diamonds at the layer of sediment that marks the start of the Younger Dryas. They are not found above or below that layer. "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 Douglas Kennett. "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."

The study was conducted by a group of eight archaeologists and geologists from the universities of Oregon and California, Northern Arizona University, Oklahoma University and DePaul University. The scientists, studying layers of sediment dated to 12,900 years ago at six North American locations, including one directly on top of a Clovis site in Murray Springs, Arizona.

Each layer was rich in nanodiamonds, which are produced under high-temperature, high-pressure conditions created by cosmic impacts, the report said. The other sites studied were in Bull Creek, Oklahoma; Gainey, Michigan and Topper, South Carolina, as well as Lake Hind, Manitoba; and Chobot, in the Canadian province of Alberta.