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NTimes article 3 (mass extinction)



 

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June 13, 2003
Meteor Impact Is Linked to an Extinction of Fish
By KENNETH CHANG

 
Just as dinosaurs died out 65 million years ago when a meteor struck the 
earth, many fish and other creatures of an earlier era â about 380 million 
years 
ago â may have been similarly killed off.

Writing in today's issue of the journal Science, geologists at Louisiana 
State University, the University of Texas at Arlington and the Scientific 
Institute in Morocco report several lines of evidence that point to a meteor 
impact 
that coincides with a mass extinction.

Most life was still contained in the oceans then, in the middle of the 
Devonian geological period that is often called the "age of fishes." The 
extinction, 
while global in scale, was less severe than the half-dozen major extinctions 
in the earth's history.

Still, "It was probably a fairly significant impact," said Dr. Brooks B. 
Ellwood, chairman of the geology and geophysics department at Louisiana State 
and 
the lead author of the Science paper.

The research adds a new point of contention to the debate about the influence 
of cataclysms from outer space on the shape of life on earth. Dr. Rex E. 
Crick, a professor of geology at the University of Texas at Arlington and 
another 
author of the Science paper, said that small extinctions caused by numerous 
small meteor impacts "could be one mechanism for driving evolution."

That is still far from clear. The end of the dinosaurs is the only one that 
geologists universally agree coincides with a meteor impact. Even there, some 
scientists believe that vast volcanic eruptions in India contributed more to 
the extinctions.

Scientists have also reported tantalizing clues of meteor impacts coinciding 
with major extinctions.

In 2001, Dr. Luann Becker, then a visiting professor at the University of 
Washington, reported finding extraterrestrial gases in the geologic layer 
deposited 250 million years ago. That coincides with the largest mass 
extinction in 
the planet's history, when it is believed that 90 percent of marine species and 
70 percent of backboned land animals died. That finding, though, has not been 
reproduced and is widely discounted by experts in the field.

Last year, Dr. Paul E. Olsen, a professor of earth and environmental sciences 
at Columbia University's Lamont-Doherty Earth Observatory in Palisades, N.Y., 
reported some signs of a meteor impact at 200 million years ago, the time of 
another mass extinction. Dr. Olsen's evidence included a modest spike in 
levels of iridium, an element more common in meteors than on earth.

But scientists also know of several instances â like the formation of the 
Manicougan crater in Quebec 214 million years ago â when meteors slammed into 
the 
planet but had little if any effect on plant and animal life.

In the new research, Dr. Ellwood and his colleagues examined samples of rocks 
from Morocco. In the layer of rock 380 million years old, corresponding to 
the extinction, they found grains of quartz with microscopic lines that form 
when it is hit with a tremendous impact. They also discovered spheres and 
crystals less than one-hundredth of an inch wide that may be droplets of rock 
that 
melted when a meteor struck.

As further evidence, they identified elevated levels of elements like nickel, 
chromium and cobalt, all associated with meteors.

The scientists did not find these indicators in rocks several yards above or 
below the extinction layer. They also reported chemical signs in carbon in the 
rocks that indicate rapid, widespread deaths of organisms. "All of those 
things together are very strong evidence that this is an impact layer," Dr. 
Ellwood said.

It is not known where the meteor hit or how large it was.

Dr. Olsen, who was not involved with the report from Morocco, said the new 
evidence was more convincing than what he had presented for the later 
extinction. "On a scale of 0 to 100, I would put it at 50 and probably put my 
own at a 
30," he said.

Dr. Ellwood said the researchers also found signs of a large iridium spike, 
but because of uncertainties about the testing method, they did not report that 
in the Science paper. 

Dr. Ellwood also said that the researchers had found shocked quartz of the 
same age in Spain, but that evidence has not been published. "I'm pretty sure 
we're going to find it elsewhere," he said.



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