Crater Yields Clues on Extinctions Did a Collision 65 Million Years Ago Help Do in the Dinosaurs?

Article excerpt

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. …