A Double Whammy?

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Dinosaurs have been extinct for 65 million years, but the controversy over what killed them goes on, having apparently developed a life of its own. Last week the Great Dinosaur Debate was in the news again, as scientists sparred over two of the newer theories about the prehistoric doomsday.

While both proposals acknowledge that the demise of the fabled beasts -- and of many other species -- was triggered by the impact of a huge comet or asteroid, they present different views of the blast's calamitous aftermath. One holds that clouds of sulfuric acid, formed from the debris thrown skyward by the impact, played the major role in an earth-enveloping shroud that destroyed much of life below. The other suggests that the cosmic collision also caused intense volcanic activity on the opposite side of the earth, creating a double whammy that made the extinctions inevitable.

Most scientists now generally agree with the brilliant theory of Luis Alvarez, the late Nobel laureate who in 1980 blamed a giant celestial intruder for the dinosaurs' downfall. The clue that inspired Alvarez was found in a thin layer of clay that forms the so-called K-T boundary between the fossil- rich rock of the Cretaceous period, which ended 65 million years ago with the extinctions, and the overlying, younger and sparsely fossiled rock of the Tertiary period. When analysis of the clay revealed that it had a far higher content of the rare element iridium thatn ordinarily found in the earth's crust, Alvarez proposed that the element might be of extraterrestrial origin. Both comets and asteroids, he knew, are rich in iridium.

From that evidence, Alvarez constructed this scenario: some 65 million years ago, a comet or asteroid at least five miles wide struck the earth and blasted out a tremendous crater. The cosmic interloper was completely vaporized, and a great fireball rose into the stratosphere, carrying with it vast amounts of pulverized debris.

These finer partricles remained suspended and were circulated by air currents until they enshrouded the earth, blocking sunlight for months. In the ensuing cold and dark, plants and animals periished. When the dust shroud -- including the iridium-rich remnants of the comet or asteroid -- eventually settled back to earth, it formed the telltale worldiwde layer of clay found at the K-T boundary.

Many scientists, particularly paleontologists, scoffed at the Alvarez theory. They argued that gradual climatic change, perphaps brought on by heightened volcanic activity, had caused the worldwide extinctions. But the discovery in 1990 of a buried crater 112 miles in diameter, centered below the town of Chicxulub on the northern tip of Mexico's Yucatan peninsula, gave the doubters pause. And the subsequent confirmation fo the crater's age -- 65 million years -- has led most scientists to jump aboard the Alvarez bandwagon.

Still, problems remained. NASA scientists, for example, have suggested that most of the airbone dust from the impact explosion and soot from fires ignited in forests would have settled back to the ground within six months. In that short a time, they say, the earth could not have cooled enough to cause the extinctions.