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Did asteroid cause the ‘Great Dying’?

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Study provides evidence that impact sparked die-off 251 million years ago

A longstanding mystery over what caused five great mass extinctions, including one that destroyed the dinosaurs, has grown with the publication of two studies in Friday’s issue of the journal Science. In one study, researchers make the bold claim that an asteroid is responsible for the death of most life on Earth in a catastrophic extinction 251 million years ago. Other scientists are not ready to accept the claim.

MANY EXPERTS have become convinced over the past two decades that the dinosaurs were exterminated 65 million years ago by an asteroid impact. Some findings suggest other mass extinctions, such as the one 251 million years ago, might also have been caused by rocks from space.

But the evidence is scant. Volcanic activity remains a suspect in the extinction cases, and a growing scientific minority is skeptical of the whole death-by-space-rock scenario.

The new study uncovered 40 extraterrestrial mineral fragments in the Antarctic, indicating that there was an asteroid impact 251 million years ago. The timing coincides with the well-documented Permian-Triassic mass extinction, the worst of five major events scientists have identified through fossil records. About 90 percent of all species disappeared, by some accounts.

Scientists generally agree that the newfound tiny grains, called chondritic meteorite fragments, are indeed from space. But agreement stops there.

TOO GOOD TO BE TRUE?

Study leader Asish Basu, a geochemist at the University of Rochester, and his colleagues are puzzled by their own discovery but have arrived at a conclusion nonetheless.

“It appears to us that the two largest mass extinctions in Earth history [65 million and 251 million years ago] were both caused by catastrophic collisions with chondritic meteoroids,” the researchers write.

The pristine state of the fragments, however, does not make sense to other researchers. They should have long ago become indistinguishable soil, conventional wisdom holds. The fragments were collected from a layer dated to the Permian-Triassic boundary in time. They were embedded in rock 4 to 8 inches (10 to 20 centimeters) beneath the surface.

In a related analysis in Science by science writer Richard Kerr, other scientists say they are stunned that the fragments survived for a quarter-billion years.

“I get the gut feeling it’s wrong,” said geochemist Birger Schmitz of the University of Goteborg in Sweden.

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What if it was a Polar shift?

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