[meteorite-list] Scientists Clash Over Origin of 'the Great Dying'

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Mon Dec 13 15:42:16 2004
Message-ID: <200412132041.MAA18044_at_zagami.jpl.nasa.gov>

http://www.sfgate.com/cgi-bin/article.cgi?file=/c/a/2004/12/13/MNGPRAB3QC1.DTL

Scientists clash over origin of 'the Great Dying'

Volcanic, celestial theories on extinction 250 million years ago take
stage in S.F.

David Perlman
San Francisco Chronicle
December 13, 2004

A cataclysm 250 million years ago wiped out nearly all life in the
Earth's oceans, and nearly three-quarters of the plants and animals on
land vanished too. It was the greatest catastrophe the Earth has ever
experienced - - but scientists who study such events are in sharp
disagreement over what caused it.

Was it the crash of a giant asteroid or meteorite that killed off so
much life? Or was a violent surge of volcanism from deep within the
Earth the deadly factor?

The argument over sketchy evidence from the long-ago geologic time
called the "End Permian" persists, and the contentious scientific debate
will continue this week when the American Geophysical Union meets at
Moscone Center, with nearly 11,000 scientists in attendance.

Scientists do agree that the mass extinction was sudden: The Earth is
known to be at least 4 billion years old, and "the Great Dying," as
paleontologists call it, may have lasted less than 200,000 years from
start to finish, a mere moment in geologic time.

In the most recent stage of the controversy, teams of researchers have
squared off in support of two opposing theories to explain what
triggered the disaster.

One international research group, led by Christian Koeberl of the
University of Vienna and Kenneth Farley of the California Institute of
Technology, is arguing that there is no hard evidence at all to support
the impact theory.

Newly discovered chemical signs in the Austrian Alps and the Italian
Dolomites, where elements typical of asteroids or meteorites are almost
nonexistent, is one key to their argument.

Koeberl and Farley, as well as many other geochemists, say the
extinction was more likely due to an immense outpouring of lava in the
northern part of a once huge super-continent known as Pangaea. The
remains of that event can be found today in a vast surface region of
basaltic rock in northern Russia known as the Siberian Traps.

The volcanic violence would have induced abrupt global heating, throwing
up a dark pall of hot ash, toxic gases and carbon dioxide that virtually
no living plants or animals could survive, according to this theory. The
darkness of the skies then would have caused a major period of global cold.

Farley, Koeberl and their colleagues have just published their arguments
in the December issue of the journal Geology.

"Our findings support the view that evidence for an extraterrestrial
impact event is weak and inconsistent," Koeberl commented by e-mail. "At
the same time, they suggest that widespread volcanic activity may have
been the 'smoking gun,' quite literally, that wiped out much of life on
Earth."

But another team, including Luann Becker of UC Santa Barbara and Asish
Basu of the University of Rochester in New York, insists that ancient
meteorite fragments discovered in Antarctica -- and traces of the unique
isotope of helium in Antarctic rocks -- clearly show that when Pangaea
was forming, a gigantic meteorite must have crashed into the Southern
Hemisphere.

That impact created an enormous crater hundreds of miles wide that may
be located now in the seabed beneath the Indian Ocean off Australia's
northwestern coast, on the edge of a seismically active desert region
called the Canning Basin. Becker believes that a submerged mountain
there called the Bedout High (pronounced Be-doo) could be the central
peak of the long-vanished crater.

The crater-forming blast could have darkened the skies, raised a life-
choking cloud of dust, rock fragments and gases, and created what the
late Carl Sagan and his colleagues 20 years ago termed a "nuclear
winter," Basu and Becker argue.

They and their colleagues published their view of the mass extinction's
causes in the journal Science last year, and pinpointed the Bedout High
as the best evidence for impact in another Science report last May.

The time when all life succumbed is known as the boundary between the
end of the Permian period -- an epoch that lasted from 286 to 245
million years ago -- and the start of the Triassic period, when life
flourished anew and the dinosaurs evolved, ultimately to rule the Earth.

The dinosaurs disappeared during the second greatest mass extinction
known in Earth's history. This event took place some 65 million years
ago when -- as almost all paleontologists and geologists agree -- an
asteroid impact extinguished 70 percent of Earth's life and created a
wide crater called Chixchilub that now lies under the sea just off the
Yucatan Peninsula.

One of the main pieces of evidence supporting an extraterrestrial impact
for the dinosaur-killing event is a thin layer of the element iridium in
rock formations around the world, discovered almost 25 years ago by
Walter Alvarez of UC Berkeley. Iridium is rare on Earth but common in
asteroids. Other evidence is the widespread presence of tiny grains of
shocked quartz -- quartz that was subjected to some violent impact --
and the presence of an isotope of helium -- unique in nature to
extraterrestrial objects like meteorites, asteroids and the moon -- in
the layers of iridium and other rocks.

According to Basu and Becker, iridium was rare in the rock samples
Becker found in Antarctica -- but there was plenty of shocked quartz and
many fragments of what could only be extremely ancient meteorites, they
reported. And in many formations from the end of the Permian period,
they said, they found clear signs of the crucial evidence: the presence
of the unique extraterrestrial isotope of helium, known as helium-3.

Farley, however, is an expert on the so-called noble gases, including
helium, and in all his laboratory tests he insists he has been unable to
find any trace of the extraterrestrial gases in the very same formations
that Becker and Basu have studied.

"There's absolutely no evidence of an extraterrestrial impact in any of
the formations that date from the end of the Permian period," Farley said.

To which Basu responded by telephone: "As the late Carl Sagan always
said, 'The absence of evidence is not evidence of absence.' " In other
words, just because Farley can't find helium-3 does not prove it isn't
there.

As a kind of neutral observer, Paul R. Renne, who directs the Berkeley
Geochronology Center, weighed in the other day. Renne's center is where
he and his fellow scientists determine the ages of the ancient rocks and
fossils that scientists find all over the world, and they have examined
the evidence on both sides.

"The debate is a real mess scientifically," Renne told The Chronicle,
"but there's massive doubt about the work in Antarctica, and there's no
really consistent evidence to support it."

The argument over what in fact wiped out so much of life on Earth some
250 million years ago is bound to continue for a long, long time, and
only a month ago in letters to the Science, no fewer than eight
scientists who study the issue called the report by Becker and Basu a
sensationalistic claim.

To which Becker replied, in an angry statement to the journal Nature,
"this is science by intimidation."
Received on Mon 13 Dec 2004 03:41:57 PM PST