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Ocean's stardust tells of asteroid impacts

作者:庾诂    发布时间:2019-02-28 03:06:05    

By Jeff Hecht SEDIMENTS from the ocean bed contain a record going back many tens of millions of years of the amount of interplanetary dust raining down on the Earth, according to an American geologist. Kenneth Farley of the California Institute of Technology in Pasadena has found that marine sediments laid down more than 70 million years ago contain large amounts of helium-3, a tell-tale sign of interplanetary dust. His results also help explain a mass extinction which occurred 38 million years ago. Helium-3 was formed in the big bang. But most of the Earth’s primordial helium escaped into space long ago, and today the vast majority of helium on our planet is helium-4, formed by the decay of radioactive nuclei. Dust falling from space, however, contains 200 times more helium-3 than the Earth’s atmosphere. In deep ocean sediments, this dust does not get mixed with large quantities of other debris, making the helium-3 easier to detect. In theory, deep-sea sediments could provide a record of the interplanetary dust that has fallen on the Earth in the past. However, experiments had indicated that all the helium-3 should have leaked out of the sediments after about a million years. Farley studied clays from the bed of the central Pacific Ocean, and to his surprise found that helium-3 levels remained high even in the oldest sediments he studied. By studying how levels of helium-3 varied with the sediments’ age, Farley was able to document variations in the amount of interplanetary dust raining down on the Earth. In this week’s issue of Nature (vol 376, p 153), he reports a strong peak of helium-3 about 38 million years ago, at the end of the Eocene period. Other researchers have found evidence of the impact of at least two asteroids or comets at about this time, which may have triggered a mass extinction of mammals and marine invertebrates. Farley believes his discovery that there were high levels of incoming dust at this time suggests the Earth was hit by a dusty shower of comets or asteroids,

 

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