MOST mass extinctions were caused by gradual climate change rather than catastrophic asteroid impacts. That's the controversial view of one palaeontologist, who says it could mean we are in the midst of a mass extinction now.
Other palaeontologists disagree, and the dispute is turning into a full-scale academic brawl. "It's a shoot-out at the OK Corral," says Peter Ward of the University of Washington in Seattle, who aired his climate change theory at NASA's Astrobiology Science Conference (AbSciCon) in Washington DC last week.
Five major extinctions have occurred in the past 500 million years: the Ordovician, the Devonian, the Permian, the Triassic and the Cretaceous. There is widespread agreement that the Cretaceous extinction, which wiped out the dinosaurs 65 million years ago, was triggered by an asteroid impact. "It's such a simple idea that for 20 years we just assumed the same was true for all extinctions," says Ward.
However, there is mounting evidence that the Permian extinction around 250 million years ago was caused by huge volcanic eruptions in Siberia, which led to catastrophic climate change (New Scientist, 10 December 2005, p 23). Ward thinks that such "greenhouse extinctions" are the rule and asteroid impacts the exception.
He and his colleagues studied carbon isotopes in rocks dating from the Triassic extinction event some 200 million years ago. These indicated that the amount of carbon dioxide in the atmosphere was up to 100 times what it is today, and that the levels fluctuated wildly over tens of thousands of years. In contrast, the climate recovered relatively rapidly after the Cretaceous event. "The Triassic event isn't something that happened overnight," says Ward.
Not everyone agrees with his interpretation. "On geological timescales, tens of thousands of years are still just an instant," says Luann Becker, a geophysicist at the University of California, Santa Barbara, who also spoke at the AbSciCon meeting. She supports the impact theory and points out that while large-scale volcanic eruptions happened throughout geological history, they didn't always cause mass extinctions.
Becker has an idea for breaking the deadlock. She and her colleagues are studying the more recent Younger-Dryas event some 13,000 years ago, when glacial conditions wiped out the woolly mammoths. As yet there is no evidence of volcanic activity in the period, she says. If she can find signs of an asteroid impact dating from around this time, it will help silence the doubters.
The two sides may have to agree to disagree. There is a large margin of error in dating older impact events, so it is almost impossible to determine whether they were the cause of the earliest mass extinctions.
For Ward there's a lot riding on the debate. "We are heading down the same road, but we've traded volcanoes as the agents of destruction for SUVs."
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THIS ARTICLE APPEARS IN NEW SCIENTIST MAGAZINE ISSUE: 1 APRIL 2006
Author: ZEEYA MERALI
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