Purdue professor helps provide additional evidence that asteroid impact caused dinosaur extinction

March 5, 2010

WEST LAFAYETTE, Ind. - A Purdue University researcher found further evidence an asteroid that crashed into the Earth 65 million years ago caused the mass extinction that ended the age of the dinosaurs.

Jay Melosh

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Jay Melosh, a distinguished professor of earth and atmospheric sciences, co-authored a paper that will appear in Friday's (March 5) issue of the journal Science that refutes recent alternative hypotheses to the dinosaur extinction and offers new data to support the asteroid impact theory.

Melosh is among a panel of 41 experts from Europe, the United States, Mexico, Canada and Japan that evaluated new core samples from ocean and land sites and re-analyzed the relevant work within the field. The National Science Foundation funded this project.

"We find that alternative hypotheses are inadequate to explain the abrupt mass extinction and that the Chicxulub impact hypothesis has grown stronger than ever," Melosh said. "The impact hypothesis has been widely accepted by the scientific community, but there has still been some debate, and we continue to examine the evidence."

About 20 years ago an impact crater more than 200 kilometers wide was discovered in Yucatan, Mexico, that is widely accepted to be the impact that caused the mass extinction event 65 million years ago. However, some scientists suggest that this event, called the Chicxulub (Chick-shuh-loob) impact, happened 300,000 years earlier and could not have been the cause. These scientists suggest the Deccan Traps, unusually active volcanoes in modern-day India, led to global cooling and acid rain that caused the mass extinction.

"These scientists examined deposits at sites around the Gulf of Mexico with a layer of tiny glasslike blobs of melted impact material that they interpret to be deposited about 300,000 years before the mass extinction," Melosh said. "Our research found that although the material near the site appears to be a series of layers neatly laid down over 300,000 years, it actually was violently churned and then dumped in a thick pile."

Models suggest that the impact at Chicxulub was a million times more powerful than the largest nuclear bomb ever tested and would have ejected material at high velocity around the world, producing a thick and complex sequence of deposits, he said.

"Near the impact site is where the sedimentary deposits would be most disturbed," Melosh said. "It is perhaps the last place in the world we should look to unravel this mystery. However, as you go farther from the impact site, the amount of ejected material decreases and eventually becomes one layer that can be found globally exactly at the time of the mass extinction."

In addition, the panel determined that the material found in this layer is compositionally linked to the specific sediments and crystalline rocks at Chicxulub.

The researchers also found weaknesses in the Deccan hypothesis.

"Evidence suggests these volcanoes were active hundreds of thousands of years before the mass extinction event and caused little change to the marine and terrestrial ecosystems," he said. "We also reviewed models of atmospheric chemistry and found that the environmental effects of sulfur emissions from volcanic eruptions are short-lived. An asteroid impact is more likely to cause extreme environmental disturbances such as darkening and cooling."

Peter Schulte, lead author of the paper and assistant professor at the University of Erlangen in Germany, said, "Combining all available data from different science disciplines led us to conclude that a large asteroid impact 65 million years ago in modern-day Mexico was the major cause of the mass extinctions."

Writer: Elizabeth Gardner, 765-494-2081, ekgardner@purdue.edu 

Source: Jay Melosh, 520-243-9761, jmelosh@purdue.edu

Related Web sites:
NSF webcast for reporters

Related NSF news release:
A broad look at the evidence for a dinosaur-killing impact