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Comet impact theory faces repeat analysis

Study starts to investigate cause of mass extinction 250 million years ago.

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Luann Becker's claims that a comet caused mass extinctions 250 million years ago are disputed. Credit: K. SAUBER/UNIV. WASHINGTON

NASA is backing attempts to duplicate a published claim that a comet impact caused a mass extinction of species about 250 million years ago.

The space agency sent three scientists to China earlier this month to collect geological samples in an attempt to repeat the results of Luann Becker, a geochemist at the University of California, Santa Barbara, and her colleagues. Becker, whose research is funded by NASA, accompanied the group. The collected samples will be sent to ten laboratories for analysis early next year.

Deciphering the geochemical and seismic profiles of impacts millions of years ago that could have caused abrupt major changes in conditions on Earth is a highly contentious field. Inferences are drawn from geological samples taken around the globe, as researchers seek to identify impact craters that might be associated with a particular set of extinctions.

In 2001, Becker and her colleagues published an article reporting that the ratios of noble gases found in sediments in southern China were consistent with the theory that they originated in a comet or asteroid that hit Earth about 250 million years ago, at the boundary between the Permian and Triassic periods1. It is known that up to 90% of Earth's species were wiped out at around that time. But no one has yet duplicated Becker's results, which have now been disputed in correspondence to Science.

In 2003, Becker's group published another article2, which argued that pieces of meteorite found in Antarctica proved that an asteroid impact had caused the Permian/Triassic extinction. Other researchers have disputed that result, contending that the meteorite fragments in question are not as weathered as they should be for that age.

And last week Science published two letters3,4 and a technical comment5 from Earth scientists criticizing a more recent article by Becker and her colleagues. That article located the asteroid's crater at a site off northwestern Australia6.

Jay Melosh, a geophysicist at the University of Arizona in Tucson and one of eight signatories to one of the letters in Science4, says that the Becker group “have deeply muddied the waters about what is going on at the Permian/Triassic boundary”.

But Becker says that she is looking forward to the new sample analyses for NASA, some of which she will conduct herself. She says her critics are being unreasonably aggressive. “This is science by intimidation,” she says.

The critics say that they are driven by the lack of data backing up the original Becker papers. “They presented insufficient evidence of an impact crater or an age ascribed to it,” says Paul Renne, a geochronologist at the University of California, Berkeley, and a signatory to the same letter to Science. “The latest Science paper undermines their credibility,” says Renne, who argues that the data in the published paper do not support its conclusions. “A lot of researchers who were sceptical before are now sure Becker's group are wrong.”

Michael New, a biophysicist who manages NASA's exobiology research programme, says that the agency learned earlier this year that some scientists were planning to do blinded studies that would repeat Becker's analysis of the Chinese samples, and decided to back it. The review is expected to cost about US$100,000, with the results being published in mid-2005. “I thought it was a good idea to put together a consortium to figure out a consensus answer,” he says.


  1. 1

    Becker, L., Poreda, R. J., Hunt, A. G., Bunch, T. E. & Rampino, M. Science 291, 1530–1533 (2001).

  2. 2

    Basu, A. R., Petaev, M. I., Poreda, R. J., Jacobsen, S. B. & Becker, L. Science 302, 1388–1392 (2003).

  3. 3

    Wignall, P., Thomas, B., Willink, R. & Watling, J. Science 306, 609 (2004).

  4. 4

    Renne, P. R. et al. Science 306, 610–611 (2004).

  5. 5

    Glikson, A. Science 306, 613 (2004).

  6. 6

    Becker, L. et al. Science 304, 1469–1476 (2004).

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Permian-Triassic extinction

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