Shock rock? Microscopic section (left) of minerals potentially altered by an asteroid impact. Right: hand-sized sample of same.

Biggest Killer Found--and Disputed

Dick writes about Earth and planetary science for Science magazine.

Geoscientists claim to have found the impact crater of a hefty asteroid or comet that could have triggered the largest mass extinction ever. The proposed Bedout (pronounced “Bedoo”) impact could have triggered the Permian-Triassic (P-T) extinction 250 million years ago, they say, the way the Chicxulub impact on the Yucatán Peninsula caused the death of the dinosaurs. But other researchers have their doubts.

This search for a P-T impact crater started when oil explorationist John Gorter, now at ENI Australia Ltd. in West Perth, proposed in 1996 that the submerged Bedout High is the central peak of a large impact crater formed at the end of the Permian. Geochemist Luann Becker of the University of California, Santa Barbara, and colleagues had been searching for an impact crater in the Southern Hemisphere to explain the apparent impact debris they were finding in Antarctica. A map of subtle gravity variations across the region reveals a ring reminiscent of Chicxulub's, they argue in ScienceExpress. And radiometric dating of a Bedout mineral grain recovered from the bottom of an oil exploration well pegs it at 250.1 ± 4.5 million years ago, about the age of the P-T.

Central to their argument are rocks retrieved from the two wells drilled into Bedout High, a sea-floor peak located just off the northwest coast of Australia. In one case, a plagioclase crystal encloses a melted core of the same chemical composition as the crystal. “It's nigh unto impossible to get that in a volcanic process,” says geochemist Robert Poreda of the University of Rochester in New York, who did much of the mineral analyses. “The only way you can do that is to shock melt it” with an impact.Not so fast, say some experts. “There's no convincing evidence for an impact origin” in the studied rocks, says impact petrographer Bevan French of the National Museum of Natural History in Washington, D.C. Impact geologist Richard Grieve of the Canadian Geological Survey in Ottawa would also expect to see signs of flow frozen into the once-molten material. Before Bedout makes the list of proven impact craters, say Grieve and the others, Becker and her colleagues need to examine the minerals with more powerful analytical tools, such as micro-Raman spectroscopy. Then perhaps Bedout can join the club.

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Posted in Paleontology, Asia