Comet Impacts Triggered Ice Age Extinctions?

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The hexagonal diamonds were found in ancient sediment layers on Santa Rosa mixed in with other types of nanodiamonds and large amounts of charcoal from wildfires, Kennett and colleagues report this week in the journal Proceedings of the National Academy of Sciences.

"There was basically an inferno as far as we're concerned at this location at this time on this island," Kennett said.

Also, around the same time that large mammals vanished from the mainland, the island's fossil record suggests that pygmy mammoths disappeared and vegetation shifted to grasslands and oak trees.

All the evidence, Kennett said, points to sudden climate change caused by "some sort of cosmic impact."

Just Coincidence?

Kennett's team presents a compelling case for a comet impact, said Eric Steig, an earth scientist at the University of Washington in Seattle who also studies past climate change.

But such an event is unnecessary to explain the well-studied cooling 12,900 years ago.

"It doesn't prove that the impact was necessary, just coincident perhaps," Steig commented via email.

"And it doesn't explain all the similar climate events in the past. One would want to see evidence for at least a few more of them, not just one."

A commonly accepted explanation for such cooling events is a series of periodic, rapid shifts in the way ocean currents shuttle heat.

For Kennett, however, this explanation doesn't fully account for what he and his colleagues see in the archaeological and geological records associated with the ice age extinctions.

"If there had not been a cosmic impact 12.9 [thousand years ago], as this hypothesis proposes," he said, "we would argue that there would not have been a significant climatic reversal during the [ice age thaw], as, of course, occurred."

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