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Signs of Crater Linked to Mass Extinction Said Found




DON FOLEY
The world was not a great place to be 250 million years ago. That¿s because some 90 percent of the planet¿s marine life and 80 percent of life on land had gone extinct at the end of the Permian period. Exactly what caused the mass extinction is a matter of debate, with the two leading theories positing massive volcanism in Siberia or a collision with a meteor much like the one that killed the dinosaurs 65 million years ago. New findings published online today by the journal Science bolster the impact hypothesis and argue that the resulting crater lies buried off the coast of northwest Australia.

Luann Becker of the University of California at Santa Barbara and her colleagues studied two cores drilled by oil companies in the 1970s and 1980s into a geologic structure off the Australian coast known as the Bedout High. "The moment we saw the cores we thought it looked like an impact breccia," Becker says. Specifically, the team found what they say is evidence of a telltale melt layer that formed when a meteor crashed into the earth and created the 125-mile-wide Bedout. Additional support for their contention that Bedout is an impact crater comes from the fact that material from the cores dates to 250 million years ago, give or take 4.5 million years. Together with earlier evidence that Becker and her team collected in Antarctica and Australia--including shocked quartz and molecules called fullerenes containing extraterrestrial helium and argon--the new results provide further evidence that a massive impact brought about the Great Dying, the scientists say. "We think that mass extinctions may be defined by catastrophes like impact and volcanism occurring synchronously in time," Becker remarks. "This is what happened 65 million years ago at Chicxulub but was largely dismissed by scientists as merely a coincidence. With the discovery of Bedout I don't think we can call such catastrophes occurring together a coincidence anymore."

The findings do not close the case of exactly what caused the Permian-Triassic (P-T) extinction, however. Some scientists remain unconvinced that Bedout is in fact an impact crater. In addition, although the date given in the new paper is consistent with the timing of the P-T dieout, it is not yet exact enough to be considered simultaneous with the extinction. Becker notes that the team plans to pursue more precise dating. ¿Evidence for an impact [at the end of the Permian era] has been growing over the last few years,¿ notes Doug Erwin of the Smithsonian Institution in Washington, D.C. ¿It¿s not yet a slam dunk, but [the new work] makes it a more plausible contender.¿

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