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"Nanodiamond" Find Supports Comet Extinction Theory

John Roach
for National Geographic News
January 5, 2009
 
The discovery of tiny "nanodiamonds" supports a controversial theory that a catastrophic bombardment of space rocks altered the course of Earth history.

About 12,900 years ago, Earth was escaping the grips of an ice age when something triggered a sudden refreeze.

The resulting 1,300-year cold spell coincides with the extinction of a host of massive mammals such as woolly mammoths, dire wolves, and sabertooths.

The Clovis culture, among the earliest humans in North America, may have suffered as well, according to Douglas Kennett, an archaeologist at the University of Oregon in Eugene, whose research appears in the current issue of the journal Science.

Kennett and colleagues found the layer of microscopic diamonds in sediments across North America that date to the start of the abrupt cooling.

The gems, the researchers argue, are consistent with a theory that Earth crossed a swarm of comets or carbon-rich meteorites that triggered cooling and die-offs.

(Related: Did Comets Cause Ancient American Extinctions? [May 6, 2008].)

Disastrous Changes

According to the theory, the impacts sparked catastrophic wildfires and destabilized ice sheets in North America, allowing a giant lake to drain into the North Atlantic.

The influx of fresh water temporarily shut down an ocean current that shuttles warm water—and a warmer climate—north from the tropics.

The inferno and sudden environmental changes apparently proved disastrous for at least 16 North American mammals.

No remains of mammals such as woolly mammoths are known from above the soil layer containing the newfound diamonds.

The details of what happened after the impact remain to be worked out. But the latest research clearly points to "some sort of cosmic impact," Kennett said.

A swarm of meteorites or comet fragments, he explained, would have generated the elevated temperatures and pressures needed to form diamonds.

"We're talking about nanodiamond [data] across North America that needs to be explained," he said. "And we think it is explainable through a major cosmic impact."

Unconvinced

David Kring, an expert on cosmic impacts at the Lunar and Planetary Institute in Houston, Texas, is skeptical the new findings are suggestive of an impact.

For example, he said the diamonds lack crystalline structures called stacking faults, which are well documented in nanodiamonds from known impact sites.

"In addition, these nanodiamonds occur in carbon spherules [small spheres], which is a context that they have not been described in in terrestrial impact settings," Kring added.

(Related: "World's Oldest Diamonds Discovered in Australia" [August 22, 2007].)

Dominique Schryvers of the University of Antwerp in Belgium is an expert on the use of electron microscopes to characterize atomic structures and microstructures.

He has detected nanodiamonds in carbon spherules from European soils that are consistent with formation under high temperature and pressure—though, he said, they are not necessarily proof of an impact.

"We can't conclude where the material is coming from," he wrote in an email.

The planetary institute's Kring noted "there was a very interesting change in the climate and megafauna of Earth about 13,000 years ago which we also need to explain."

"Whether an impact event has anything to do with either of them is still to be shown."
 

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