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."
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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