Report offers proof of comet's deadly impact

CHICAGO — First an explosion as powerful as thousands of megatons of TNT rained meteorites down on North America. Then forest fires erupted across the continent, sending up a thick layer of soot and dust that blocked the sun. A sudden ice age ensued, and some of the Earth's largest animals went extinct in a blink of geological time.

It's well-known that a meteorite colliding with Earth is considered the most likely reason dinosaurs died off 65 million years ago. Now a team of scientists says it has found new evidence that a comet triggered a similar extinction much more recently: just 13,000 years ago, when humans were around to witness the event and suffer its consequences.

The researchers also think that when the comet exploded above the planet's surface, it killed off mammoths, saber-toothed tigers and other large mammals.

The scientists, led by University of Oregon anthropologist Douglas Kennett, said their report in today's issue of Science offers a "smoking bullet": proof it was a comet that set off the sudden, thousand-year freeze and wiped out the big animals of the era.

Working at multiple sites across the continent, researchers found nanodiamonds — microscopic particles thought to be found on comets — in a 13,000-year-old layer of rich sedimentary soil called a "black mat." Beneath the layer with the nanodiamonds, fossils of the animals are abundant. After that layer, they disappear, said Allen West, an Arizona geophysicist and one of the paper's co-authors.

"It's extraordinary that tens of millions of animals disappeared synchronously at exactly the time when the diamonds and carbon layer are laid down across the continent," he said.

Arrowheads and other artifacts from the Clovis culture of humans — an early hunter-gatherer society — also vanish after the black mat was laid down 13,000 years ago.

In 2007, West and a team of scientists published an analysis of black mats from several regions that found heavy metals, soot and charcoal suggestive of meteorite impacts and subsequent fires. The new report says the discovery of nanodiamonds in the same material is more evidence of a cosmic strike.

Archaeologists have long speculated about whether climate change or overhunting drove the mammoths, tigers and other "megafauna" to extinction and led to the decline of the Clovis culture.

Many scientists remain skeptical of the comet theory and think there may be better explanations for what happened, said Daniel Amick, an associate professor of anthropology at Loyola University.

The authors have much to prove before their theory is accepted, Amick said, such as pinpointing the date of the event and ruling out other potential causes.

In response to one common criticism of the comet theory — no craters have been found from an impact — West said the comet may not have reached Earth but exploded into fragments above the surface.

Where that might have happened is a mystery, but high concentrations of nanodiamonds at a site in Eastern Michigan suggest the Great Lakes as a possibility.

NASA space scientist David Morrison, an expert in impacts, said he doubts a comet could have broken up in the manner proposed by the Kennett group.

"They talk rather blithely about a comet disintegrating in the atmosphere," Morrison said. Of the nanodiamonds, he said: "They may have discovered something absolutely marvelous and unexplained. But the impact hypothesis just doesn't make sense."

Kennett acknowledged a lot of work is needed to firm up the claim: "It's a hypothesis ... Basically there's a suite of data that suggest that something like this occurred, but it still needs to be tested."

Information from The New York Times is included in this report.