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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 Peter Ward,
UW professor of earth and
space sciences, disputes that notion.
Ward and his colleagues believe that the cause of “the Great
Dying” 250 million years ago might have been atmospheric warming
due to greenhouse gases triggered by erupting volcanoes.
The extinction, considered the biggest catastrophe in the history
of life on Earth, occurred at a time when all land was concentrated
in a supercontinent called Pangea. Ninety percent of all marine
life and nearly three-quarters of land-based plant and animal life
became extinct.
“Based on the geochemical evidence we found, animals and plants
both on land and in the sea were dying at the same time,”
says Ward, “and apparently from the same causes—too
much heat and too little oxygen.”
The researchers were able to use chemical, biological, and magnetic
evidence to correlate sedimentary layers in the Karoo Basin of South
Africa to similar layers in China that have been tied to a marine
extinction during the same period.
Evidence from the marine extinction is “eerily similar”
to what the researchers found in the Karoo Basin, Ward says. Over
seven years, the team 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 ten million years leading
up to the boundary between the Permian and Triassic periods, and
the other showing a sharp increase in extinction rate at the boundary
that then lasted another five million years.
The scientists 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. Evidence from the Karoo, they
say, 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 says.
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. 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 also,”
Ward says. He notes 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 until they reached a critical point
and everything died,” Ward says. “It was a double-whammy
of warmer temperatures and low oxygen, and most life couldn’t
deal with it."
[Winter-Spring 2005 - Table of Contents]
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