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SCIENCE / PALEONTOLOGY : Meteor Link to Dinosaur Deaths Bolstered

September 17, 1993|MARK A. STEIN | TIMES SCIENCE WRITER

A chunk of rock sizzled out of space and into the Gulf of Mexico off the Yucatan Peninsula around the time the dinosaurs died off 65 million years ago. Geologists have long blamed the meteorite for the animal's extinction, but paleontologists argued that the rock was too small to do the job.

A refined analysis of the suspected impact site near the Mexican village of Chicxulub indicates that the suspect meteorite indeed may have been big enough--perhaps twice as big and eight times as powerful as once believed--to cause the extinction, American and Mexican geologists report in today's issue of the journal Science.

Ranging from five to 10 miles in diameter, the meteorite--whether comet or asteroid--packed an almost unfathomable amount of energy, said Virgil L. Sharpton of the Lunar and Planetary Institute in Houston. He estimated that it was equal to 200 million megatons of TNT, or 10,000 times the combined power of every nuclear bomb ever made.

At the same time, however, fossil evidence unearthed by UC Berkeley researchers indicates that at least some dinosaur species thrived in cold weather, and thus were unlikely to have died after a few months of false winter caused by all the dust kicked up by a meteorite.

Together, the two findings suggest the impact-extinction theory is more likely and probably more complex than initially thought. In addition to creating false winter, the meteorite would have had to produce some equally deadly secondary effects--from widespread firestorms to global acid rainstorms--if the theory is to be believed, scientists said.

"How (a meteorite impact) affected the biosphere remains to be seen, and we will probably spend the next decade sorting it out," said Sharpton, who led a team of 10 scientists from Universidad Nacional Autonoma de Mexico, the University of Houston and Petroleos Mexicanos, the Mexican national oil company.

But, he added, "there is no question in most reasonable people's mind that this meteorite is related to the Cretaceous-Tertiary extinction event."

Richard Pike, who specializes in studying meteorite craters for the U.S. Geological Survey in Menlo Park, Calif., said Sharpton's new evidence is "very exciting" and agreed that it appears to support the impact-extinction theory.

Sharpton and his colleagues used sensitive gravity meters to map out concentric "impact rings" formed by the meteorite when it struck Earth. The rings are made of dense rock pushed to the surface from as far as 15 miles underground by the force of the meteorite. Between the rings are rock fragments that exert slightly less gravity.

Even though the entire crater has been covered by about two miles of sediment, these gravity differences are measurable at the surface, Sharpton said.

By mapping the gravity changes, the scientists found that the crater apparently is 185 miles wide--about the size of Connecticut--and not 110 miles wide as earlier believed. This would make the crater the largest in the inner solar system, beating the 175-mile-wide Mead basin on Venus.

Ever since the Chicxulub crater was discovered by Mexican oil drillers in 1981, scientists have been refining the theory that dinosaurs were made extinct when a giant meteorite impact near Mexico threw vast amounts of dust in the air, blocking out the sun and dropping global temperatures for several months.

But the theory has always had a major sticking point: Many other animals, including such temperature-sensitive species as turtles and plankton, apparently survived the hypothesized catastrophe unscathed.

Recently, a second sticking point was added when UC Berkeley paleontologists William A. Clemens and L. Gayle Nelms discovered that several dinosaur species evidently enjoyed cold weather, and thus were unlikely to have expired after only a few months of a dust-induced false winter.

They based their conclusions, published in June in the journal Geology, on dinosaur fossils in Alaska, which indicate that a five- to six-foot-tall dinosaur species lived year-round in a climate as chilly as modern Anchorage. More recently, a cousin was discovered in southern Australia, which at the time of dinosaurs was still connected to Antarctica.

Believers in the impact theory of extinction say these findings indicate only that the theory is more complicated than earlier suspected: The false winter could have lasted five or 10 years instead of a few months, they suggest, or it could have triggered global wildfires or persistent acid rains that could have done in the dinosaurs without necessarily wiping out other creatures.

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