U.S. scientists say they have strong evidence that a meteor the size of Mount Everest smashed into what is now northern Australia 251 million years ago, possibly triggering the most massive extinction of life the planet has ever known.
Most scientists agree that a meteor impact on Mexico's Yucatan Peninsula accompanied the extinction of the dinosaurs and many other life forms 65 million years ago, but evidence for the cause of an even greater mass extinction almost 200 million years earlier has been elusive.
A specialist on ancient biology at the Smithsonian Museum of Natural History in Washington, Douglas Erwin, says this so-called "Great Dying" involved the loss 90 to 95 percent of marine life and up to 75 percent of land life in an era when there was only one supercontinent and before mammals appeared.
?This is by far the largest of the six great mass extinctions that we have in the last 550 million years of Earth history,? he noted. ?It has been the most enigmatic. It's about twice as large in terms of the number of species that go extinct as any of the other events. So it's a fundamental transition in the history of life.?
Now, a team of geologists and geochemists has linked the extinction to a 200 kilometer wide crater buried three kilometers beneath the ocean floor off the northwest corner of Australia. They examined rock cores drilled from the site, known as Bedout, in the 1970s and 1980s by oil exploration companies.
Geochemist Robert Poreda of the University of Rochester, New York says the cores shows that a layer of the rocks the same age as the extinction was transformed by shock into a glassy material in a way only a massive impact could cause. Furthermore, some of the rock showed shock fractures in several directions, whereas he says extreme volcanic activity fractures it in only one direction.
?We were absolutely flabbergasted when we saw the material from Bedout and we realized that this was no ordinary volcanic rock,? he said. ?This is not something that ever occurs in volcanic debris. Believe me, when we saw this it was absolutely convincing proof of an impact formation.?
The evidence of the melted and fractured rock from the Bedout site is published in the journal Science and adds to a finding Mr. Poreda and colleagues reported last year, pieces of a meteor buried in layers of Antarctic rock also the same age as the extinction.
Other scientists are skeptical they have evidence of a meteor strike. Some argue that rock does not have to be shocked to exhibit the melt and fracture characteristics Mr. Poreda describes. Others are not convinced that the Bedout crater even exists. Douglas Erwin of the Smithsonian Institution says the researchers have more work to do to prove the case an impact caused the extinctions, but he is sympathetic to their evidence.
?In the past couple of years, the evidence for impact has been growing,? he said. ?We now have this paper which provides additional suggestive, I think, but perhaps not yet compelling evidence that we have an impact. I think the authors are to be congratulated on pursuing this, identifying a possible structure.?
Mr. Erwin said that a complicating factor is that a massive Siberian volcano occurred at the same time and injected tons of toxic gases into the atmosphere, gradually changing the Earth's climate. One leading theory is that this led to the mass extinction. He says no one knows if the volcano was independent of or the result of a meteor impact.
The researchers say they want more evidence from the purported crater itself, which is buried far below ocean floor sediments. One of them, Luann Becker of the University of California at Santa Barbara, suggests geological studies, such as taking magnetic readings of the feature from an airplane. ?This is where we would like to go and do a more sophisticated geophysical study, which would allow us to bring out that feature and peel away the sediments and get right down to the feature and do a much better job of trying to bring out what the crater looks like today,? she said.
Until the Bedout impact is clarified, it will not earn the status of the Mexican crater as a site that caused much of Earth's living species to be wiped out.