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WASHINGTON, May 14: A recently discovered layer of glassy fragments could only have been made by a meteor slamming into Earth and helps prove such an impact nearly wiped out life on the planet 250 million years ago, scientists said on Thursday.
They outlined what they believe is an ancient crater left by the impact off the coast of what is now Australia.
"What we found is a melt sheet which is defining the top of this very large feature out in the ocean," geologist Luann Becker of the University of California Santa Barbara told a news conference.
"It is buried under a deep pile of sediment," she added. "This is very likely the impact site we've been looking for," added Robert Poreda, a professor of earth and environmental sciences at the University of Rochester in New York.
"For years we've been observing evidence that a meteor or comet hit the Southern Hemisphere 251 million years ago, and this structure matches everything we've been expecting."
The findings, published in Friday's issue of the journal Science, are controversial and the scientists say they need much more evidence. But they believe they have found the smoking gun that shows a meteor sparked the "Great Dying", much as an asteroid wiped out the dinosaurs 65 million years ago forming what is now the Chicxulub crater off Mexico's Yucatan Peninsula.
During the Permian extinction, 90 per cent of life in the seas and up to 70 per cent of the species on land died off very quickly. No one has been able to show what caused it.
Poreda, Becker and colleagues have been gathering evidence an asteroid, comet or meteor was responsible.
DUST AND LAVA: Such an impact would have blown up a pall of dust and rock that would dim the sun's light, and leave telltale evidence globally. It would also have set off volcanic eruptions.
Evidence of those have been found. Layers of basalt dating to 250 million or so years ago have been found in Siberia.
In 2001, Poreda and Becker found specific isotopes, or varieties, of helium and argon trapped inside a 250 million-year-old layer of carbon shells called buckyballs, which could only have come from space.
They have also found larger pieces of what they believe could be the meteorite scattered in Antarctica. They include "shocked" quartz that looks like it was splattered by a powerful force - something called impact breccia.
Now they describe a 200-km-wide crater called Bedout off the northwestern coast of Australia.
Becker had heard oil companies drilled two cores into the Bedout structure thinking it looked like an oil dome in the 1970s and 1980s. All they found was volcanic debris and they abandoned the site.
Her team looked at the cores.
"Bob and I were absolutely flabbergasted at this core because it does look just exactly like impact brecchia," Becker told a news conference. In the cores, Becker's team found evidence for a layer of melted glass formed by an impact.
"We realized that this was no ordinary volcanic rock," Poreda added.
Critics disagree. Bevan French of the National Museum of Natural History in Washington told Science it could be explained by volcanic activity.
"I see nothing that would convince me there was an impact," agreed Christian Kolberl of the University of Vienna in Austria.
Becker and colleagues say they have to do more work to prove their theory. They say it will be much more difficult to prove Bedout is a crater than it was for Chicxulub, because it is older and underwater.
Bedout and Chicxulub look to be about the same size, suggesting they were formed by similar events, the researchers said.-Reuters
