[meteorite-list] Meteor Impact Theory Takes a Hit

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu Jan 20 18:00:25 2005
Message-ID: <200501202259.OAA22888_at_zagami.jpl.nasa.gov>

http://www.wired.com/news/technology/0,1282,66345,00.html?tw=wn_tophead_4

Meteor Impact Theory Takes a Hit
By Amit Asaravala
Wired News
January 20, 2005

The catastrophe that killed off the majority of life on Earth 250
million years ago was not a meteorite impact, but a gradual rise in
global temperatures, according to a new study published Thursday on the
website of the journal Science.

The study is the second in two months to question the validity of the
meteorite impact theory, which suggests that a giant asteroid or comet
struck the Earth with such force that it led to a massive, global
extinction that scientists call the "Great Dying."

The impact would have been similar to the one that is widely believed to
have led to the extinction of the dinosaurs 65 million years ago. But to
date, evidence for the dinosaur's demise has far exceeded that for the
Great Dying.

"We all assumed in the scientific community that if one extinction could
be caused by an impact, they all could," said Peter Ward, a University
of Washington paleontologist and lead author of the new study. "I went
(to South Africa) specifically to prove that this was caused by an
impact and walked out of there thinking that, no, it wasn't."

Ward and his fellow researchers traveled to the Karoo Basin in South
Africa to examine fossils that have been traced back to the time of the
Great Dying, also known as the end-Permian period. Rather than finding
that a great number of animals and plants had all died at once, however,
the team detected signs of a gradual extinction over nearly 10 million
years. Then, a second extinction seems to have started and lasted
approximately 5 million years.

Such patterns suggest that long-term environmental changes, like global
warming and falling oxygen levels, are more to blame than a meteor
impact, said Ward. Continuous volcanic eruptions during the end-Permian
period could have contributed to these changes by triggering the release
of methane that had previously been frozen at the bottom of the ocean,
he suggested.

Ward added that the team did not find, in the sediment that it examined,
the sorts of minerals that are normally associated with meteorite
impacts. Those minerals include iridium, which hitches a ride to Earth
on asteroids, and "shocked" quartz, which takes on an altered appearance
after a massive impact.

The findings -- or lack thereof -- contradict a controversial study
published in June 2004 by Science. In that study,
University of California at
Santa Barbara geologist Luann Becker and several other scientists
claimed to have discovered evidence of a giant impact crater off the
coast of Australia. The crater could be dated back to the beginning of
the Great Dying, they wrote in the study, making it the likely cause of
the mass extinction.

However, a number of geologists have since questioned the evidence.

"They've been very broadly criticized," said Paul Renne, director of the
Berkeley Geochronology Center. "Many of their claims are completely
unsupportable."

The impact theory received another major blow in December when a team
led by geologist Christian Koeberl from the University of Vienna
published a paper in the journal Geology showing that samples of
end-Permian rock in Western Europe did not contain iridium and shocked
quartz.

University of Rochester geochemist Robert Poreda, who co-authored the
June impact paper with Becker, defended his team's study Wednesday and
said that he still supported the impact theory.

"A lot of things can explain why there was no evidence of shocked
quartz," he said. "For one, there's not a complete section (of sediment)
to analyze at Karoo."

In addition, an impact off the coast of Australia would not have struck
the appropriate rocks that would lead to the creation of mass quantities
of shocked quartz, he said. Plus, an impact by a comet -- not an
asteroid -- would probably not have carried iridium with it, he added.

Berkeley's Renne, who was not involved in any of the aforementioned
studies, agreed that Poreda's arguments are valid. However, he noted
that he and many of his colleagues were beginning to have less and less
faith in the impact theory. Indeed, Renne's own research supports the
idea that the extinction occurred gradually, he said.

"We've found that the atmosphere was changing, in terms of oxygen levels
and in carbon and so on -- all told, these things were probably going on
over a million years," he said. "And we're beginning to think that the
main pulse of extinction occurred over 100,000 years, which is pretty
fast in geologic time, but it's not an instant."

To resolve the argument, scientists are now turning their attention to
fullerenes, tiny balls of carbon that can lock up gases inside. If
fullerenes taken from sediment dated back to the beginning of the Great
Dying are found to contain gases more commonly found in space than on
Earth, the chances are good that a large meteorite struck the planet
around the same time.

But even this technique has its problems, warned Renne.

"A lot of things have to be done to establish a link between the gases
in the fullerenes and an impact," he said. "The timing (of any detected
impact) has to be perfect, and it has to be shown that this is an
anomalous concentration of gas."
Received on Thu 20 Jan 2005 05:59:49 PM PST