(5 of 10)
Journeying to Denmark, another site where the Cretaceous geological record is complete, Walter Alvarez gathered corroborating samples of iridium and received still more from colleagues working at a third site on the other side of the world, in New Zealand. The evidence seemed overwhelming. In 1980 the Alvarez team finally published its results in the journal Science and stirred up some scientific debris of its own. Says Paleontologist Leo Hickey, director of the Peabody Museum of Natural History at Yale: "My first thought was this is one of Walter's practical jokes."
Other scientists, drawn into the fray by the Alvarez conjecture, have since suggested that a large comet might have similar consequences. Los Alamos Weapons Experts Stirling Colgate and Albert Petschek computed that a comet six miles in diameter hitting the earth would have an effect 100,000 to 1 million times as great as a large nuclear explosion, and would blow a 100-sq.-mi. hole in the atmosphere.
Several researchers rushed forth to deny any extraterrestrial origins for the iridium, attributing it first to a gradual process of sedimentation that concentrated the metal. Later an old favorite was proposed--volcanic eruptions, which might have forced iridium from the mantle to the surface. The most recent naysayers are Dartmouth Geologists Charles Officer and Charles Drake, who reported in Science on their studies of two other telltale elements in the clay boundary between the Cretaceous and Tertiary periods. They found that the levels of arsenic and antimony correspond to decidedly terrestrial, not cosmic, concentrations.
One of the Alvarezes' staunchest critics has been William Clemens, a paleontologist at the University of California, Berkeley. After systematically sampling parts of the eastern Montana area, he declared that the layer of iridium and the bones of the last surviving dinosaur were too far apart to share any meaningful connection. Besides, he asked, why should the mammals have survived any Cretaceous catastrophe? Says he: "If you're going to have a nuclear winter killing off the dinosaurs, why didn't it kill off everything else?"
Nonetheless, the evidence in favor of an impact was rapidly accumulating. Other geologists uncovered similar iridium deposits just above Cretaceous rock beneath the floor of the Atlantic Ocean and under the Raton basin in northeastern New Mexico. Additional analysis showed that the samples contained ratios of gold and platinum nearly identical to those found in meteorites. Furthermore, other sediment layers containing abnormally high amounts of iridium were discovered under both the Caribbean Sea and the Gulf of Mexico; these layers were deposited around the time of a smaller mass extinction that occurred more than 30 million years ago, at the end of the Eocene epoch. In addition to the iridium anomaly, the sediments harbored microscopic balls of glass called microtektites, which form in rock when something hits it with great force--further evidence of a major collision. Yet investigators continued to regard the findings from the two time periods as being no more related than, say, separate automobile accidents in Des Moines and Miami. Explains Walter Alvarez: "It seemed to everybody involved that extinctions and impacts should be random in time."