Early meteor impact linked to extinction in Earth's seas

Just as a meteor impact is believed to have brought about the extinction of the dinosaurs 65 million years ago, scientists say a similar event may have killed off many fish and other creatures of an earlier era, about 380 million years ago.

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 at a time when most life was still contained in the oceans.

That time, in the middle of Devonian geological period, is often called the "age of fishes." The extinction, while global in scale, was less severe than the half dozen known major extinctions in earth's history.

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

The research adds a new point of contention to the debate over the influence of cataclysms from outer space on the shape of life on earth. 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 elimination 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 previously reported tantalizing clues of meteor impacts coinciding with other major extinctions.

Last year, 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. 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 -- like the formation of the Manicougan crater in Quebec, 214 million years ago - - when large meteors slammed into the planet but had little, if any, effect on plant and animal life.

In the new research, Ellwood and his colleagues examined samples of rocks from Morocco. In a 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 melted when a meteor struck. As further evidence, they found 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 indicated rapid, widespread deaths of organisms.

"All of those things together are very strong evidence that this is an impact layer," Ellwood said.

It is not known where the meteor hit, and Ellwood said he also had no estimate of its size.

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.

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.

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

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