Big Clue in Comet Theory Reported : Science: Researchers think they have found the site where a giant projectile struck Earth and led to conditions that helped kill off dinosaurs.

Arizona scientists believe they have located the impact site of a massive comet that struck the Earth 65 million years ago, contributing to the extinction of the dinosaurs.

Their discovery of thick layers of mud and debris on Cuba and Haiti indicates that a six-mile-wide comet struck somewhere in the Colombian basin between North and South America, they report in today’s Science magazine.

Moreover, using seismic imaging, planetary scientists Alan R. Hildebrand and William V. Boynton of the University of Arizona have identified a candidate crater, a 180-mile-wide undersea depression, that may be the impact site.

The theory that the impact from a comet or meteorite did in the dinosaurs at the end of the Cretaceous Period 65 million years ago has been controversial since it was first promoted in 1980 by a research team from UC Berkeley headed by the late physicist Luis Alvarez and his geologist son Walter.

Firm evidence of an impact site is “a smoking gun that people have been looking for for a long time,” said geochemist Frank Kyte of UCLA. “It’s exciting that someone may finally have found it.”

Hildebrand and Boynton say they have not yet proved the crater they have found is the actual impact site, but they are confident they have at least pinpointed the region where the comet crashed.

“Now that we have a small part of the world to focus on, given five years we’ll be able to map out the locations of other debris ejected by the impact and know where it (the impact) had to be,” Hildebrand said in a telephone interview.

Already they have begun studying other sites in Central America, and other researchers are sure to follow. “Just to have it narrowed down to the Caribbean-Gulf of Mexico region will excite a lot of people and stimulate research,” Kyte said.

In studying marine sediments laid down between the end of the Cretaceous and the beginning of the Tertiary Period--the so-called K/T boundary--they found unusually high concentrations of the metal iridium, which is rare on Earth but is present in high concentrations in comets and meteors. They postulated that an iridium-containing comet or meteorite must have struck the Earth, injecting large amounts of matter into the atmosphere.

According to the theory, that matter would have blocked sunlight and created an “impact winter” that would have directly or indirectly led to the death of the dinosaurs, such as from the cold or lack of food resulting from the death of plants. The phenomenon is similar to the “nuclear winter” that many scientists think would result from a nuclear war.

Researchers have subsequently found much other evidence supporting the meteorite impact, but some scientists remain unconvinced. Palynologist Dewey McLean of Virginia Polytechnic Institute and State University in Blacksburg, for example, argues that volcanic activity in the Deccan Traps in India--not a comet--was the source of the iridium, and he sees “absolutely no evidence” of an impact winter.

“I want to see an awful lot more evidence before I am willing to accept (Hildebrand and Boynton’s) conjectures,” he said Thursday.

Many different sites have been put forth for the proposed impact crater, but none have met all the necessary criteria. Examination of the K/T boundary layers indicated that the impact must have been near the United States because granules in the boundary layer are larger here than elsewhere in the world, and those large granules would not travel as far as smaller ones.

Evidence also indicated that the meteorite must have struck in the ocean or at the continental shelf. The new site fulfills those criteria.

Actually, much of the evidence used by Hildebrand and Boynton has been available for a long time; people simply did not recognize its significance, Hildebrand said. Samples from cores drilled in the Caribbean and previous geological studies in Cuba and Haiti show that the sediment layer at the K/T boundary varies from 1 1/2 feet to 1,300 feet thick, suggesting the impact was nearby. By contrast, the sediments at the K/T boundary in the rest of the world are only about 1/10th of an inch thick.

Hildebrand and Boyle’s primary contribution, in addition to assembling the data, was demonstrating that these layers contain “shocked minerals,” pieces of quartz that have etched features that most scientists agree could only have been produced by a meteorite impact.

Based on the findings, geologist Bruce F. Bohor of the U.S. Geological Survey in Denver has gone back to early geological surveys of Cuba, which show the existence of a “big boulder bed” at the K/T boundary there. The huge stones and boulders there, some as big as 36 feet in diameter, must have been ejected from the impact site when the meteorite collided with Earth, Bohor said.

Geologists would love to go back and look at the K/T boundary in Cuba, Kyte said, but, “unfortunately, being in Cuba, it’s rather inaccessible” for political reasons. Others agree that more research is essential.

“I would say that the evidence is looking pretty good for an impact in the Caribbean,” Walter Alvarez said, “but it needs to be checked out one way or another.”