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Meteroic Evidence For Permian-Triassic Extinction



http://www.globeandmail.com/servlet/story/RTGAM.20031120.wmete1120/BNStory/International/

Washington  A massive asteroid may have collided with the Earth 251
million years ago and killed 90 per cent of all life, an extinction even
more severe than the meteorite impact that snuffed out the dinosaurs 66
million years ago.

A new study, based on meteorite fragments found in Antarctica, suggests
the Permian-Triassic event, the greatest extinction in the planet's
history, may have been triggered by a mountain-sized space rock that
smashed into a southern land mass.

"It appears to us that the two largest mass extinctions in Earth history
... were both caused by catastrophic collisions" with meteoroids, the
researchers say in their study appearing this week in the journal Science.

Asish Basu, a professor of Earth sciences at the University of Rochester,
said proof of a massive impact 251 million years ago is in the chemistry
found in rocky fragments recovered on Graphite Peak in Antarctica. He said
the fragments were found at a geological horizon, or layer, that was laid
down at the start of the Permian-Triassic extinction. Analysis shows the
fragments have chemical ratios that are unique to meteorites.

"The only place you would find the chemical composition that we found in
these fragments is in very primitive, 4.6-billion-year-old meteorites, as
old as our Earth," said Dr. Basu, the first author of the study.

Dr. Basu said the Permian-Triassic asteroid was probably bigger than the
almost-10-kilometre-wide space rock that is thought to have killed the
dinosaurs
...
Dr. Basu said specimens recovered from Permian-Triassic rock formations
in China, however, have a chemistry that matches that of the meteorite
fragments found in Antarctica, a discovery that supports the impact
theory. Also, shocked quartz, a telltale sign of an asteroid impact, has
been found at both sites, he said.

At the time of the Permian-Triassic event, Africa, South America, India,
Australia and Antarctica were joined in a giant continent called Pangea.
Just where the asteroid hit in that land mass is uncertain, Dr. Basu said,
but it could have been near what is now western Australia.
...