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C-T extinctions without the impact.

C-T extinctions without the impact

Ninety-two million years ago, the oceans of the earth suffered a mild mid-life crisis when various species of clams, plankton and nautilus-like creatures died out abruptly, an event that marks the boundary between the Cenomanian and Turonian ages in the geologic record. While experts have long puzzled over the cause of this die-off, geologists have found new evidence that suggests volcanic activity may have triggered the extinctions.

At 15 locations in western North America--all of which were at the bottom of an intercontinental sea 92 million years ago -- researchers from the Los Alamos (N.M.) National Laboratory (LANL) and the University of Colorado in Boulder have found a concentrated layer of the element iridium at the Cenomanian-Turonian (C-T) boundary, they report in the April GEOPHYSICAL RESEARCH LETTERS. Iridium, normally rare in the crust of the earth, is much more concentrated in meteorites and in the earth's interior.

Many scientists believe that an iridium layer at the Cretaceous-Tertiary (K-T) boundary, 65 million years ago, is a signal that one or several extraterrestrial bodies hit the earth and killed off a large fraction of earth life, including the dinosaurs. But Carl Orth of LANL and his colleagues are not looking skyward for an explanation of the C-T boundary iridium. "We think it's some terrestrial process," he says. "We can't completely rule out an impact cause, but it also looks like we're seeing enhanced levels of elements such as scandium and titanium that are normally pretty low in meteorites."

The elements concentrated at the C-T boundary are more characteristic of material in the upper mantle of the earth than of that in meteorites, Orth says. This leads him to suspect that at the end of the Cenomanian age, mantle material started erupting abruptly through a midocean rift or some other feature. The eruptions, he suggests, laid down a concentrated layer of these elements.

Gases from the eruptions could have proved toxic to the marine animals that died off at the same time, according to Orth. The extinctions may not be related to the layer of concentrated elements, "but it's a heck of a coincidence if they're not," he says.

The iridium layer seems strongest in the southwest United States and dwindles toward Manitoba, a distribution that suggests the eruptions were localized. Orth's group will next look for the element layer in Texas and Europe. If the layer does not appear elsewhere, he says, the eruption theory may not be able to explain why the extinctions affected the entire globe.
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Title Annotation:new evidence that volcanic activity caused extinctions between Cenomanian and Turonian ages
Author:Monastersky, Richard
Publication:Science News
Date:Apr 30, 1988
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