Dinosaur extinction followed asteroid impact 65 million years ago, panel says

By Thomas H. Maugh II
Tuesday, March 9, 2010

It's official: The extinction of the dinosaurs and a host of other species 65.5 million years ago was caused by a massive asteroid that crashed into the Gulf of Mexico, creating worldwide havoc, according to an international team of researchers.

The 7.5-mile-wide asteroid was traveling at about 10 times the speed of a rifle bullet when it hit, releasing a billion times more energy than the Hiroshima atomic bomb. The impact blew dirt and rock around the world, set massive wildfires, knocked down forests worldwide, and triggered massive tsunamis and earthquakes of magnitude 11 or larger, wiping out more than half of all species on Earth.

All of this may sound familiar. In fact, the idea was proposed 30 years ago by physicist Luis Alvarez and his son Walter after they found abnormally high concentrations of the element iridium in sediments from what was then known as the K-T boundary. This 65.5-million-year-old layer of Earth separates fossils of the Cretaceous period from those of the Tertiary period.

Iridium is rare on Earth but common in space, and the Alvarezes proposed that a giant asteroid had hit the Earth, producing the sudden decline in species.

Then, in 1991, researchers discovered a 120-mile-wide, 1.5-mile-deep crater called Chicxulub in Mexico of the same age as the K-T boundary. Most considered it the smoking gun for the extinction. In recent years, however, some scientists speculated about alternative causes, arguing that the extinction could have resulted from multiple asteroid impacts or, more likely, massive volcanic eruptions in India.

To settle the question, researchers assembled what Kirk R. Johnson of the Denver Museum of Nature & Science called "a K-T boundary dream team," 45 renowned scientists from an array of disciplines, to study possible causes of the mass extinction. Funding came from the National Science Foundation and from similar groups outside the United States. Their conclusions appear in the current issue of the journal Science.

"The answer is quite simple," Johnson, a co-author and spokesman for the group, said in a telephone news conference. "The crater really is the culprit."

Proponents of a multiple-impact theory had pointed to several distinct layers of comet debris near the Chicxulub crater as well as evidence that many species survived the initial impact, only to go extinct later. But the team concluded that those anomalies were created by jumbling of strata when debris flowed back into the crater after the impact.

And if volcanoes in India had caused the extinction, "we would expect to see events in the biological world associated with it, but we don't," Johnson said.

-- Los Angeles Times

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