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Comet Impacts Triggered Ice Age Extinctions?

John Roach
for National Geographic News
July 21, 2009
 
Rare diamonds found buried on an island near southern California strengthen the controversial idea that comet impacts wiped out huge beasts and an early human culture in North America about 12,900 years ago.

Similar "nanodiamonds" found in sediments across North America were presented earlier this year as proof that space rocks colliding with Earth led to the ancient mass extinction.

According to the theory, a barrage of comet debris rained down on North America during the last ice age and sparked massive wildfires. That initial heat and pressure formed tiny diamonds in the soil.

But the heat also abruptly melted ice sheets, causing an influx of freshwater that shut down a key ocean current and reversed the region's thaw.

The sudden recooling killed off mammals such as saber-toothed cats, dire wolves, and mammoths and wiped out some of North America's earliest known human inhabitants, the Clovis culture.

Opponents of the theory have been skeptical of the evidence, saying that the previously found microscopic diamonds lacked crystalline structures associated with the "shock" of being struck by extraterrestrial objects.

The newfound diamonds, however, have a unique hexagonal structure that's only been found on Earth in places where known impacts have occurred, the study authors say.

"Therefore [the diamonds'] discovery is important for this hypothesis, because it is hard to explain these away," said study co-author James Kennett, an emeritus geologist at the University of California, Santa Barbara.

Island Inferno

Roughly 13,000 years ago California's Santa Rosa Island was part of a "mega island" that today is split into the Channel Islands, west of Los Angeles (see map).

The climate on this mega-island was much cooler than it is today, and the area was covered with juniper forests that sheltered pygmy mammoths and some of the first humans known in the Americas.

(Related: "Ancient Camels Butchered in Colorado, Stone Tools Show?")

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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