2.5亿年前流星撞地球导致生物大灭绝

Scientists claim 'Great Dying' crater

Meteoric evidence discovered off Australia may hold a key to explanation of 'Great Dying,' the mass extinction about 250 million years ago

导读：2.5亿年前一颗巨型流星撞击地球在地层中残留了大量陨石碎片，这一证据也有力的佐证了这一撞击几乎消灭了当时地球上的所有生物。这一撞击残留的古老陨石坑位于现在澳大利亚海岸线附近的海洋中。

在澳大利亚确定陨石坑的存在对于证明2.5亿年前的大撞击提供了更加系统的证据。也有批评者认为，所发现的碎片层也有可能是火山活动的结果。拜克及其同事表示，他们将继续工作寻找更多的理论以证明自己的理论。 

Millions of years before the dinosaurs vanished, an even bigger mass extinction wiped out more than 90 percent of the species on Earth. Now scientists think they may have evidence of an impact crater(陨石坑) that contributed to the "Great Dying."

The Permian(二叠纪)-Triassic(三叠纪) Extinction took place about 250 million years ago; 90 percent of life in the seas and up to 70 percent of the species on land died off very quickly. Scientists have debated its cause for years.

The end of the dinosaurs came 185 million years later, only 65 million years ago, and is widely thought to have been caused by a meteor(陨石) impact off the Yucatan Peninsula in Mexico.

A team led by Luann Becker of the University of California, Santa Barbara, reported in today's issue of the journal Science that a crater off the northwest coast of Australia shows evidence of a large meteor impact at the time of the early extinction. 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(火山爆发).

They call the site the Bedout Crater (pronounced Beh-doo).

Becker said her team was "flabbergasted([口]使大吃一惊)" when they looked at never-before studied core samples that had been drilled in the region in the search for oil. The samples contained meteorite fragments, "shocked" quartz and other impact evidence.

In addition, quartz and other minerals blasted out by the impact have been found in Australia, Antarctica and possibly India, said Kevin Pope of Geo Eco Arc Research, a private geological research company in Aquasco, Md.

The impact occurred at the right time, so it is a good candidate for the cause of the extinction, said Robert Poreda of the University of Rochester in New York.

The new find provides "suggestive . . . but perhaps not yet compelling evidence" that an impact was involved, said Douglas H. Erwin, a senior paleobiologist at the Smithsonian's National Museum of Natural History. This mass extinction was a fundamental transition in the history of life on Earth, Erwin said. He said further study will be done to try to confirm the new theory.

One difficulty, he said, is that there was a complex set of events occurring at the same time, including the eruptions in Siberia.

Other scientists are skeptical.

"It's not yet persuasive that it's even a crater," said Peter D. Ward, a professor of geological sciences at the University of Washington in Seattle(西雅图<美国一港市>) who has long studied impact craters and mass extinctions.

Intensive study is required to join the list of the world's proven impact craters. Most have been eroded by rain, wind and earthquakes over millions of years. This possible new site is poorly preserved and deeply buried.

Becker's team was funded by the National Aeronautics and Space Administration and the National Science Foundation.