[meteorite-list] Meteor Impact Theory Takes a Hit

From: Gerald Flaherty <grf2_at_meteoritecentral.com>
Date: Thu Jan 20 18:08:25 2005
Message-ID: <037d01c4ff44$eefccb20$6401a8c0_at_Dell>

These guys won't be happy until one smacks them in the head!
----- Original Message -----
From: "Ron Baalke" <baalke_at_zagami.jpl.nasa.gov>
To: "Meteorite Mailing List" <meteorite-list_at_meteoritecentral.com>
Sent: Thursday, January 20, 2005 5:59 PM
Subject: [meteorite-list] Meteor Impact Theory Takes a Hit


>
>
> 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."
>
>
>
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Received on Thu 20 Jan 2005 06:08:19 PM PST