Ground zero, dinosaur time: Caribbean Sea.

Ground zero, dinosaur time: Caribbean Sea 

At the close of the Cretaceous period, according to a leading theory, a fiery asteroid or comet crashed into Earth, triggering a series of environmental disasters that killed off a significant fraction of life on the planet, including the last surviving species of dinosaurs. Now two geologists contend that a growing body of evidence points to the Caribbean as the 65-million-year-old impact site.

"I think the evidence is now compelling that the impact or impacts had to be in this region," says Alan R. Hildebrand of the University of Arizona in Tucson.

The debate over the mass extinctions at the boundary of the Cretaceous and Tertiary (K-T) periods has raged since 1980, when a team of scientists proposed the impact hypothesis. While many researchers now accept that theory, a vocal minority maintains that volcanic eruptions or slow climate change caused the widespread death.

Hildebrand and Arizona colleague William V. Boynton report in the May 18 SCIENCE that Haitian rock deposits bear several signs indicating a nearby impact. They have found "shocked" quartz grains in a thick layer of clay deposited at the end of the Cretaceous period, when Haiti was part of the seafloor. Shocked mineral grains, which scientists often view as telltale impact evidence, appear in many K-T sites around the world, particularly in North America.

The K-T clay layer in Haiti is also unusually thick -- about 25 times the size of any other such deposit. The Arizona team thinks the clay formed from tiny rock particles that fell to Earth's surface after they were thrown into the air by the impact. The thickness of Haiti's clay layer suggests the collision occurred within about 1,000 kilometers, Hildebrand says.

In addition, the researchers have re-examined some deep-sea sediments collected in the early 1970s from sites in the Colombian basin, to the north of South America. At the level of the K-T boundary, the sediments contain evidence that a huge wave scoured the seafloor, they report.

These deposits and others in Cuba and Texas suggest a K-T crash somewhere between North and South America. Hildebrand and Boynton believe the comet or asteroid landed on oceanic crust, and they propose that a 300-km-diameter circular depression in the Colombian basin may be the impact crater.

Other scientists contend the object crashed into continental crust. Last month, Hildebrand and Boynton's work led one team to suggest Cuba as the site for such a crash (SN: 4/28/90, p.268).