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If "Deccan Traps Theory: Maybe an Asteroid Didn't Kill the Dinosaurs" is not shown property. Visit the source link above.
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Maybe an Asteroid Didn't Kill the
Dinosaurs |
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When a scientific principle is common knowledge
even in grammar school, you know it has long since crossed the line
from theory to established fact. That's the case with dinosaur
extinction. Some 65 million years ago — as we've all come to know —
an asteroid struck the earth, sending up a cloud that blocked the
sun and cooled the planet. That, in turn, wiped out the dinosaurs
and made way for the rise of mammals. The suddenness with which so
many species vanished after that time always suggested a single
cataclysmic event, and the 1978 discovery of a 112-mile,
65-million-year-old crater off the Yucatán Peninsula near the town
of Chicxulub seemed to seal the deal. |
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Now, however, a
study in the Journal of the Geological Society throws all
that into question. The asteroid impact and dinosaur extinction,
say the authors, may not have been simultaneous, instead occurring
300,000 years apart. That's an eyeblink in geologic time, but it's
a relevant eyeblink all the same — one that occurred at just the
right moment in ancient history to send the extinction theory
entirely awry. |
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The
controversial paper was written by geoscientists Gerta Keller of
Princeton University and Thierry Addate of the University of
Lausanne, in Switzerland. Both researchers knew that challenging
the impact doctrine would not be easy. The asteroid charged with
killing the dinosaurs, after all, left more than the Chicxulub
crater as its calling card. At the same 65-million-year depth, the
geologic record reveals that a thin layer of iridium was deposited
pretty much everywhere in the world. Iridium is an element that's
rare on Earth but common in asteroids, and a fine global dusting of
the stuff is precisely what you'd expect to find if an asteroid
struck the ground, vaporized on impact and eventually rained its
remains back down. Below that iridium layer, the fossil record
shows that a riot of species was thriving; above it, 65% of them
went suddenly missing. |
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But
Keller and Addate worried that we were misreading both the geologic
and fossil records. They conducted surveys at numerous sites in
Mexico, including a spot called El Peñón, near the impact crater.
They were especially interested in a 30-ft. layer of sediment just
above the iridium layer. That sediment, they calculate, was laid
down at a rate of about 0.8 in. to 1.2 in. per thousand years,
meaning that all 30 feet took 300,000 years to settle into
place. |
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Analyzing the
fossils at this small site, they counted 52 distinct species just
below the iridium layer. Then they counted the species above it.
The result: the same 52. It wasn't until they sampled 30 feet
higher — and 300,000 years later — that they saw the
die-offs. |
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"The mass extinction level can be seen above this interval,"
Keller says. "Not a single species went extinct as a result of the
Chicxulub impact."
Keller's and Addate's species samplings are not, of
course, conclusive, and plenty of other surveys since 1978 do tie
the extinctions closely to the asteroid. But since the new digs
were so close to ground zero, the immediate species loss ought to
be have been — if anything — greater there than anywhere else in
the world. Instead, the animals seemed to escape unharmed. Other
paleontologists, however, believe that the very proximity of El
Peñón to the impact site makes the results even less reliable.
Earthquakes and tsunamis that resulted from the collision could
have wrought havoc on the sedimentary record, causing discrete
strata to swirl together and completely scrambling time lines.
Keller disagrees, pointing out that the slow accretion of sediment
that she and Addate recorded is completely inconsistent with a
sudden event like a tsunami. |
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So if the Chicxulub
asteroid didn't kill the dinosaurs, what did? Paleontologists have
advanced all manner of other theories over the years, including the
appearance of land bridges that allowed different species to
migrate to different continents, bringing with them diseases to
which native species hadn't developed immunity. Keller and Addate
do not see any reason to stray so far from the prevailing model.
Some kind of atmospheric haze might indeed have blocked the sun,
making the planet too cold for the dinosaurs — it just didn't have
to have come from an asteroid. Rather, they say, the source might
have been massive volcanoes, like the ones that blew in the Deccan
Traps in what is now India at just the right point in
history. |
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