科学美国人60秒:二叠纪末大灭绝-托福听力下载

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　　It was the largest of the five major mass extinctions in Earth's history—well before the dinosaur-killer 66 million years ago.

　　What's called the End Permian extinction, 252 million years ago, wiped out 96 percent of aquatic species and 70 percent of species on land.

　　Scientists have been trying to gauge the time frame of the extinction, in the hopes of determining its causes.

　　Now researchers say it's the fastest mass extinction known.

　　二叠纪末生物大灭绝.jpgUsing new tools and models—including a fresh analysis of rock formations in China—the researchers determined that the extinction took only about 60,000 years.

　　That's incredibly quick by geological standards, and is more than 10 times faster than previous estimates.

　　The report is in the Proceedings of the National Academy of Sciences.

　　Study author Samuel Bowring from M.I.T. says they can't yet compare the speed of the previous extinction to the extinction rates caused by human activities today.

　　But, he says, their research is starting to help reveal how past environmental changes that influenced extinctions—such as levels of carbon dioxide in the atmosphere—compare to the changes in those levels seen today.

　　In the midst of what many call the sixth extinction.

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