Crater Yields Clues on Extinctions

FOR scientists interested in what happened when the dinosaurs disappeared, 1992 was a banner year.

Research confirmed that, 65 million years ago, a comet or asteroid crashed into Earth at the northern tip of what now is the Yucatan peninsula. It may indeed have helped to do in the dinosaurs.

Whether it had a major role in that great extinction or was only a supporting player, geologists now know that it left a geological treasure, which they have only begun to appreciate.

As Virgil L. Sharpton of the Lunar and Planetary Institute in

Houston notes, the impact left a crater roughly 125 miles or 200 kilometers across. It is one of the largest and best preserved craters now known on our planet. Dr. Sharpton explains that this kind of large crater occurs only about once in 200 million to 300 million years. The few other such craters that have been identified are hard to reconstruct. They have been flattened by mountain building or distorted by erosion and other geological processes. But a slow buildup of sediments has protected the Yucatan

crater.

Sharpton says that geologists "can learn a lot" from what appears to be a uniquely well preserved feature. They can check out theories of crater formation. They can see how such a massive impact affected geology over a large area. Because of this, Sharpton says, the crater "is of fundamental importance" in its own right.

But does it have anything to do with the dinosaurs? Sharpton has been one of the skeptics on this point. Now research reported last year by a number of scientists, including Sharpton, dates the crater to the time of that event. Sharpton says he now is convinced that the connection between the great crater and the extinctions "is also a monumental thing to understand."

Thus, for scientists interested in a connection between those extinctions and asteroid or comet impacts, 1992 was the year when a probable cause was pinned down. Yet this identification hasn't resolved the extinction mystery.

Walter Alvarez of the University of California, Berkeley, notes, "What this year's [1992] results clearly show is that an impact is part of the story." In commenting on this in a telephone interview, however, he hesitated. Then he explained: "If you detect some puzzlement on my part, it's because the mystery is much deeper than we realized. We have one part of the puzzle. But there's probably more to the story than that."

Twelve years of debate and research among geologists and paleontologists has brought Dr. Alvarez to this humbling realization. Together with his father - the late Luis Alvarez - and Frank Asaro and Helen Michel, he opened up this research by suggesting the impact-extinction connection. They had found unusually high concentrations of the element iridium in the geological layer that corresponds to the extinction epoch. The iridium abundance was more in line with that found in asteroids or comets than what normally occurs on Earth. This iridium-rich layer has since been found at about 100 sites around the world.

The layer marks the boundary between the Cretaceous and Tertiary periods. Geologists call it the K-T boundary. (They use the letter "K" because "C" refers to the earlier Cambrian period.) According to the fossil record, as much as 75 percent of Cretaceous species, including dinosaurs, never made it across that boundary.

Impact proponents have noted that a large comet or asteroid - having a diameter of, say, six miles or 10 kilometers - would generate intense shock pressures and high temperatures where it struck. It would throw up a global veil of dust and sulfuric-acid droplets. This could cause mass extinctions by blocking sunlight and perhaps by rain-out of the acid. Grains of quartz evidencing shock pressure appear in the K-T boundary layer, adding to the plausibility of this scenario. But, until recently, there was no telltale crater to back up the speculation.

Actually, indications of such a crater had been locked up in the files of the Mexican oil company Petroleos Mexicanos (Pemex). Surveys of the local gravity field in the 1950s showed a circular pattern centered on the Yucatan coastal village of Puerto Chicxulub (CHEE-shoo-loob). A 1978 magnetic survey showed the same pattern. In a confidential report to Pemex, Glen T. Penfield, who supervised the magnetic survey, and Antonio Carmargo Z. suggested the pattern might be that of an impact crater. Pemex allowe d them to discuss this briefly at a 1981 meeting of the Society of Exploration Geologists.

But their suggestion went unnoticed until 1990, when Houston Chronicle science writer Carlos Byars mentioned it to Alan R. Hildebrand, a graduate student at the University of Arizona, Tucson.

Mr. Hildebrand and Prof. William V. Boynton had found 65-million-year-old impact-wave deposits along what at that time would have been the Caribbean and North American southern shore. A comet or asteroid hitting the sea would have created waves several kilometers high that left these deposits of ripped-up sea floor and land surface. Hildebrand and Boynton also found tektites in K-T boundary deposits in Haiti. These are glassy spherules that solidify from drops of melted rock thrown out by a meteorite imp act. The suspected Chicxulub crater looked like a good candidate for the related impact site.

Subsequent research by the Arizona geologists and by Sharpton, Penfield, and other scientists has confirmed that the crater exists. Buried under nearly a kilometer of sediment, it is centered on Puerto Chicxulub and underlies the cities of Progresso and Merida, and a large area offshore.

In 1991, Glen A. Izett with the United States Geological Survey in Denver and colleagues showed that the Haitian tektites are indeed 65 million years old. Last May, Carl C. Swisher III of the Institute of Human Origins in Berkeley, Calif., and colleagues showed in a paper in Science that the tektites also have the same age as melt rock from core samples that Pemex had drilled from the Chicxulub crater.

In October, Virgil Sharpton and several colleagues published results in Nature that confirm the Chicxulub crater dates to the time of the K-T extinctions.

Sharpton cautions that final proof waits on new drilling in the crater and results of further analysis of the old Pemex drilling samples, which he expects to have in a few months' time. He adds, however, that the crater does look "like it is a real K-T event."

But the comet or asteroid that gouged it out seems, at most, to have been only one cause of the massive K-T extinctions. Doubters have insisted that late Cretaceous volcanism in India and a long period of climatic change were more likely causes of the extinctions. Also, not all the lost species disappeared suddenly. Some underwent a long decline.

Noting such caveats, Walter Alvarez says he is rethinking his position on the extinctions. He now suggests that "the apparent opponents" in the impact-versus-volcanism debate "are just seeing different parts of the puzzle."

He adds: "If a single big impact and a single big extinction were the whole story, it would be clear by now. Instead we keep finding mysteries wrapped up in enigmas."