[meteorite-list] Researchers: Meteorite Caused Extinction 251 Millions Years Ago

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu Apr 22 10:27:51 2004
Message-ID: <200311201849.KAA23847_at_zagami.jpl.nasa.gov>

http://cnews.canoe.ca/CNEWS/Science/2003/11/20/263478-ap.html

Researchers: Meteorite caused extinction
By PAUL RECER
Associated Press
November 20, 2003

WASHINGTON (AP) -- Researchers studying rocks from Antarctica have found
chemical evidence that a huge meteorite smashed the Earth 251 million years
ago and caused the greatest extinction event in the planet's history, killing
about 90 percent of all life.

The extinction, which scientists call the Permian-Triassic event, came some
185 million years before a similar meteorite collision with the planet killed
off the dinosaurs.

"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 R. 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 Basu, the first author of the study.

That supports the theory that space rock the size of a mountain streaked in from outer
space and smashed into the Earth. The violence of the impact would have caused a huge
fireball and sent billions of tons of dust into the atmosphere, enough to darken the sun for
months. It also would have laid down a layer of dust bearing the same chemical
composition as the meteorite.

Scientists believe the six-mile-wide asteroid that killed the dinosaurs left a thin layer of
a metal called iridium all over the globe. Basu said that element was not found in the
fragments recovered from the Antarctica, suggesting the earlier space rock had a different
composition than the dinosaur-killer rock.

Basu said specimens recovered from Permian-Triassic rocks 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, Basu said.

"The impact had to have been in the southern continents someplace," he said. "We think
it is near what is now western Australia."

Life on Earth at the time was far different from what it is now and what it was when
dinosaurs lived.

"There were no large animals then, but there were lots of species living on the land and
in the sea, and there were plants," said Basu. The most dominant plant, which is found
commonly in fossil beds from the Permian-Triassic, was a giant fern called glossopteris.
In the geological layers following the impact, that fern is absent from the fossil record.

"That was the last blooming of that plant," said Basu. "After that, it was gone forever
from the planet."

Around the same time of both the Permian-Triassic and the dinosaur extinctions, there
also were massive outflows of a lava called flood basalt. It continued for thousands of
years and thickly covered hundreds of miles. Basu said it is possible that asteroid
impacts triggered both eruptions of lava, but the connection has yet to be proven.

Some experts are skeptical that Basu and his co-authors have found
251-million-year-old meteorite metals, although nobody questions that the material did
come from outer space. The surprise is that the specimens survived the weathering on
Earth for so long.

"It's astonishing, it's incredible, it's unbelievable," Jeffrey Grossman of the U.S.
Geological Survey said in Science.

Birger Schmitz of the University of Goteborg in Sweden said in Science that it "would
really be remarkable" to find unaltered meteoritic fragments such as those claimed by
Basu and his colleagues.

"I get a gut feeling it's wrong," he said.

Basu, however, said, "nobody in the world can question our chemical findings."

The question, he said, is what were the unique circumstances that allowed the material
to survive virtually unchanged chemically for 251 million years.

"We have to work that out," said Basu.
Received on Thu 20 Nov 2003 01:49:04 PM PST