Interplanetary securityScientists say comet killed off mammoths, saber-toothed tigers

It probably went like this: First there was an explosion as powerful as thousands of megatons of TNT showering North America with meteorites. Then forest fires broke out across the continent, sending up a heavy cloud of soot and dust that blocked out the sun. The result was an ice age, and some of the Earth’s largest animals went extinct. Scientists consider this scenario as the most likely reason dinosaurs died off sixty-five million years ago. USA Today’s Dan Vergano writes that now a team of scientists says it has found new evidence that a comet triggered a similar extinction much more recently: about 12,900 years ago, when humans were around to witness the event and suffer its terrible consequences.

The researchers also think that when the comet exploded above the planet’s surface — ultimately killing off mammoths, saber-toothed tigers, and other large mammals that roamed North America — Chicago (today’s Chicago, that is; it did not exists back then) was not far from ground zero. “If you’d been in Chicago back in that time, it would’ve been one very bad day,” said Allen West, an Arizona geophysicist and one of the authors of a paper appearing today in Science. The scientists, led by University of Oregon anthropologist Douglas Kennett, say their report offers up 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 several 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, West said. “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,” said West, whose co-authors include DePaul University chemist Wendy Wolbach.

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. Chicago Tribune’s Robert Mitchum writes that 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 over-hunting drove the mammoths, tigers, and other “megafauna” to extinction and led to the decline of the Clovis culture.

There scientists who urge caution. “We simply do not have conclusive evidence that nanodiamond materials aren’t everywhere at many times,” says geologist Nicholas Pinter of Southern Illinois University in Carbondale. “Tons of meteorite dust falls to Earth every year, after all.”

Skeptics such as archaeologist Stuart Fiedel, author of Prehistory of the Americas, note that although more than 30 North American species died out about 12,900 years ago, about 50 large species died out a few centuries later in South America, and on some unpeopled Caribbean islands, species such as sloths survived an additional 6,000 years. “Humans, not extraterrestrial objects,” best explain the staggered extinctions in the New World, he says.

Kennett says future studies will show evidence of the nanodiamonds from Europe and further afield 12,900 years ago. Impact shock waves, debris, and wildfires sparked by comets breaking apart in the atmosphere would have hit North America hardest, he says, but the effects would have been felt worldwide.

-read more in Richard A. Kerr, “Did the Mammoth Slayer Leave a Diamond Calling Card?” Science 323. no. 5910 (2 January 2009):
26 (DOI: 10.1126/science.323.5910.26) (sub. req.)