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- The greatest mass extinction in Earth history eliminated
85 percent to 90 percent of all marine and land vertebrate species 250
million years ago, at the end of the Permian Period and the beginning of
the Triassic. New evidence from researchers at the University of Washington
and the South African Museum shows the extinction was accompanied by a
massive loss of vegetation, causing major changes in river systems.
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- Probing sedimentary layers in the Karoo Basin of South
Africa, the scientists found evidence that, with the loss of deep-rooting
plants, meandering river systems changed rapidly to braided systems. Braided
streams run much straighter and faster and branch out for short distances
before merging back to the primary stream. They also cause much faster
sediment buildup because vegetation is not holding streamside soil in place
and it is easily swept away by the faster-moving water.
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- Using data from the Karoo and elsewhere, the scientists
attribute the drastic change in river character to a catastrophic global
die off of vegetation that likely resulted from the same cause as the mass
extinction among marine and land animals.
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- Peter Ward, a UW geological sciences professor, along
with David Montgomery, a UW associate geological sciences professor, and
Roger Smith, the South African Museum's curator of geology, publish their
findings in the Sept. 8 issue of the journal Science.
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- Sedimentary layers from the Permo-Triassic boundary were
examined at seven different sites scattered across 250 miles of the Karoo
Basin, and the researchers found striking similarities in the evidence
for a rapid shift from meandering to braided streams.
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- Major tectonic activity could change streams from meandering
to braided, Ward said. However, recent studies have shown there was no
major tectonic activity at the time of the Permo-Triassic extinction, which
occurred when the Earth's land was still locked in a supercontinent called
Pangea.
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- Braided streams were common until the Silurian Period
some 400 million years ago, but then gave way to meandering streams as
plant life evolved. Today it is rare to see a braided stream unless it
is in a place, like Mount St. Helens in Washington state, where the landscape
has been denuded by a catastrophic event such as a volcanic eruption.
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- "The thing we take so for granted now - meandering
rivers - is a very recent feature on Earth," Ward said. "This
didn't appear until the Silurian, when land plants started to take over
the Earth."
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- The sudden reappearance of braided streams, probably
on a global scale, 250 million years ago is strong evidence of a major
catastrophe that wiped out plant life as well as much of animal life, Ward
said. He noted that plant life emerged again relatively quickly at the
beginning of the Triassic Period.
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- A number of potential causes for the mass extinction
have been postulated, including the impact of an asteroid or comet, environmental
shifts, volcanism or the overturning of the oceans to release trapped gases
into the atmosphere. In their research, paid for by a National Science
Foundation grant, Ward, Montgomery and Smith do not speculate which, if
any, of the theories is correct. But they say that the way plant life disappeared
indicates events happened very quickly on a geological scale.
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- "Whenever you describe something as happening in
thousands, rather than millions, of years, that's very fast geologically
speaking," Ward said. "The new evidence helps us understand how
rapid this was, because the transition from meandering to braided streams
was quick. And I think the most important thing is that it tells us how
catastrophic this was. It was the most catastrophic event in Earth history,
or at least in the history of life." _____
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- For more information, contact:
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- Ward <argo@u.washington.edu (206) 543-2962 Montgomery
<dave@geology.washington.edu (206) 685-2560 Smith <rsmith@samuseum.ac.za
011-27-21-424-3330
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