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Evidence Indicates Biggest Extinction Wasn't Caused by Asteroid or Comet

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  • Pat Neuman
    ... wrote: Source: University of Washington Released: Mon 17-Jan-2005, 14:00 ET Evidence Indicates Biggest Extinction Wasn t Caused by
    Message 1 of 1 , Jan 20, 2005
      --- In Paleontology_and_Climate_Articles@yahoogroups.com, Sonya
      <msredsonya@e...> wrote:
      Source: University of Washington Released: Mon 17-Jan-2005, 14:00 ET
      Evidence Indicates Biggest Extinction Wasn't Caused by Asteroid or Comet

      Newswise — For the last three years evidence has been building that the
      impact of a comet or asteroid triggered the biggest mass extinction in
      Earth history, but new research from a team headed by a University of
      Washington scientist disputes that notion.

      In a paper published Jan. 20 by Science Express, the online version of
      the journal Science, the researchers say they have found no evidence for
      an impact at the time of "the Great Dying" 250 million years ago.
      Instead, their research indicates the culprit might have been
      atmospheric warming because of greenhouse gases triggered by erupting
      volcanoes. The extinction occurred at the boundary between the Permian
      and Triassic periods at a time when all land was concentrated in a
      supercontinent called Pangea. The Great Dying is considered the biggest
      catastrophe in the history of life on Earth, with 90 percent of all
      marine life and nearly three-quarters of land-based plant and animal
      life going extinct.

      "The marine extinction and the land extinction appear to be
      simultaneous, based on the geochemical evidence we found," said UW
      paleontologist Peter Ward, lead author of the paper. "Animals and plants
      both on land and in the sea were dying at the same time, and apparently
      from the same causes – too much heat and too little oxygen." The paper
      is to be published in the print edition of Science in a few weeks.
      Co-authors are Roger Buick and Geoffrey Garrison of the UW; Jennifer
      Botha and Roger Smith of the South African Museum; Joseph Kirschvink of
      the California Institute of Technology; Michael De Kock of Rand
      Afrikaans University in South Africa; and Douglas Erwin of the
      Smithsonian Institution.

      The Karoo Basin of South Africa has provided the most intensively
      studied record of Permian-Triassic vertebrate fossils. In their work,
      the researchers were able to use chemical, biological and magnetic
      evidence to correlate sedimentary layers in the Karoo to similar layers
      in China that previous research has tied to the marine extinction at the
      end of the Permian period.

      Evidence from the marine extinction is "eerily similar" to what the
      researchers found in the Karoo Basin, Ward said. Over seven years, they
      collected 126 reptile or amphibian skulls from a nearly 1,000-foot thick
      section of exposed Karoo sediment deposits from the time of the
      extinction. They found two patterns, one showing gradual extinction over
      about 10 million years leading up to the boundary between the Permian
      and Triassic periods, and the other for a sharp increase in extinction
      rate at the boundary that then lasted another 5 million years.

      The scientists said they found nothing in the Karoo that would indicate
      a body such as an asteroid hit around the time of the extinction, though
      they looked specifically for impact clays or material ejected from a
      crater left by such an impact. They contend that if there was a comet or
      asteroid impact, it was a minor element of the Permian extinction.
      Evidence from the Karoo, they said, is consistent with a mass extinction
      resulting from catastrophic ecosystem changes over a long time scale,
      not sudden changes associated with an impact.

      The work, funded by the National Aeronautics and Space Administration's
      Astrobiology Institute, the National Science Foundation and the National
      Research Foundation of South Africa, provides a glimpse of what can
      happen with long-term climate warming, Ward said. In this case, there is
      ample evidence that the world got much warmer over a long period because
      of continuous volcanic eruptions in an area known as the Siberian Traps.
      As volcanism warmed the planet, large stores of methane gas frozen on
      the ocean floor might have been released to trigger runaway greenhouse
      warming, Ward said. But evidence suggests that species began dying out
      gradually as the planet warmed until conditions reached a critical
      threshold beyond which most species could not survive.

      "It appears that atmospheric oxygen levels were dropping at this point
      also," he said. "If that's true, then high and intermediate elevations
      would have become uninhabitable. More than half the world would have
      been unlivable, life could only exist at the lowest elevations." He
      noted that the normal atmospheric oxygen level is around 21 percent, but
      evidence indicates that at the time of the Great Dying it dropped to
      about 16 percent – the equivalent of trying to breathe at the top of a
      14,000-foot mountain. "I think temperatures rose to a critical point. It
      got hotter and hotter until it reached a critical point and everything
      died," Ward said. "It was a double-whammy of warmer temperatures and low
      oxygen, and most life couldn't deal with it."


      Sonya PLoS Medicine
      The open-access general medical journal from the Public Library of
      Inaugural issue: Autumn 2004 Share your discoveries with the world.
      --- End forwarded message ---
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