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Catastrophes and Prehistory


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Troy Exeter
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« Reply #30 on: March 19, 2007, 02:22:04 am »

Impact from the Deep
Strangling heat and gases emanating from the earth and sea, not asteroids, most likely caused several ancient mass extinctions. Could the same killer-greenhouse conditions build once again?
By Peter D. Ward


Philosopher and historian Thomas S. Kuhn has suggested that scientific disciplines act a lot like living organisms: instead of evolving slowly but continuously, they enjoy long stretches of stability punctuated by infrequent revolutions with the appearance of a new species--or in the case of science, a new theory. This description is particularly apt for my own area of study, the causes and consequences of mass extinctions--those periodic biological upheavals when a large proportion of the planet's living creatures died off and afterward nothing was ever the same again.
Since first recognizing these historical mass extinctions more than two centuries ago, paleontologists believed them to have been gradual events, caused by some combination of climate change and biological forces such as predation, competition and disease. But in 1980 the understanding of mass extinctions underwent a Kuhnian revolution when a team at the University of California, Berkeley, led by geologist Walter Alvarez proposed that the famous dinosaur-killing extinction 65 million years ago occurred swiftly, in the ecosystem catastrophe that followed an asteroid collision. Over the ensuing two decades, the idea that a bolide from space could smite a significant segment of life on the earth was widely embraced--and many researchers eventually came to believe that cosmic detritus probably caused at least three more of the five largest mass extinctions. Public acceptance of the notion crystallized with Hollywood blockbusters such as Deep Impact and Armageddon.



Now still another transformation in our thinking about life's punctuated past is brewing. New geochemical evidence is coming from the bands of stratified rock that delineate mass extinction events in the geologic record, including the exciting discovery of chemical residues, called organic biomarkers, produced by tiny life-forms that typically do not leave fossils. Together these data make it clear that cataclysmic impact as a cause of mass extinction was the exception, not the rule. In most cases, the earth itself appears to have become life's worst enemy in a previously unimagined way. And current human activities may be putting the biosphere at risk once again.

After Alvarez
To understand the general enthusiasm for the impact paradigm, it helps to review the evidence that fueled it. The scenario advanced by Alvarez, along with his father, physicist Luis W. Alvarez, and nuclear chemists Helen V. Michel and Frank Asaro, contained two separate hypotheses: first, that a fairly large asteroid--estimated to have been 10 kilometers in diameter--struck the earth 65 million years ago; second, that the environmental consequences of the impact snuffed out more than half of all species. They had found traces left by the blow in a thick layer of iridium--rare on the earth but common in extraterrestrial materials--that had dusted the globe.
Within a decade of this prodigious announcement the killer's thumbprint turned up, in the form of the Chicxulub crater hiding in plain sight on the Yucatán Peninsula of Mexico. Its discovery swept aside most lingering doubts about whether the reign of the dinosaurs had ended with a bang. At the same time, it raised new questions about other mass extinction events: If one was caused by impact, what about the rest? Five times in the past 500 million years most of the world's life-forms have simply ceased to exist. The first such event happened at the end of the Ordovician period, some 443 million years ago. The second, 374 million years ago, was near the close of the Devonian. The biggest of them all, the Great Dying, at the end of the Permian 251 million years ago, wiped out 90 percent of ocean dwellers and 70 percent of plants, animals, even insects, on land [see "The Mother of Mass Extinctions," by Douglas H. Erwin; Scientific American, July 1996]. Worldwide death happened again 201 million years ago, ending the Triassic period, and the last major extinction, 65 million years ago, concluded the Cretaceous with the aforementioned big bang.

http://www.sciam.com/article.cfm?articleId=00037A5D-A938-150E-A93883414B7F0000
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