Crater Studies Shed Light on Demise of Dinosaurs

The rock and dust kicked up by an asteroid impact 65 million years ago was not enough to kill the dinosaurs, according to researchers--but the debris might have sparked a deadly chemical reaction in the atmosphere.

New studies show that the Chicxulub impact crater on the coast of Mexico’s Yucatan Peninsula is smaller than once thought, making dinosaur extinction difficult to explain completely. Researchers presented those findings Sunday at the American Geophysical Union’s fall meeting.

“If you rely on little pieces of debris actually clobbering organisms, then you’re in trouble,” said Virgil “Buck” Sharpton of the Geophysical Institute at the University of Alaska in Fairbanks.

Since 1980, research on the dinosaurs’ disappearance has focused on the 125-mile crater and the 10-mile-wide asteroid believed to have created it. Dust from the impact was thought to have blocked out sunlight for years.

Now, however, drilling around the Yucatan crater indicates the presence of carbonates and sulfate rocks. The new theory is that these were vaporized by the asteroid impact, a process that would have released chemicals that produce sulfur and the greenhouse gas carbon dioxide.

The sulfur compounds would be especially toxic, Sharpton said.

“They do nasty things. They form little globules that persist in the atmosphere for some considerable amount of time--decades to a hundred years,” he said. “They also mix with water in the atmosphere and produce sulfuric acid.”

So besides old theories about a nuclear winter-type global cooling, researchers believe the giant oxygen-breathing reptiles also may have choked on carbon dioxide and suffered showers of caustic acid.

“How do you initiate the global crisis? It had to be atmospheric chemistry of some sort,” Sharpton said. “That’s the only way you can transport the effect globally of something that dumps the majority of its energy into a single spot on the Earth’s surface.”

Rock and dust alone from Chicxulub probably would not have been sufficient to snuff out life on the other side of the globe, Sharpton said. Even a small pocket of life would have repopulated the planet.

To test the theories further, Sharpton and colleagues plan to drill 1.5 miles into the crater and retrieve samples of the rock present in what was a shallow sea when the asteroid hit. The project, 50 miles south of Merida, will not begin before June.

Studies of Chicxulub have more value than explaining the dinosaur extinction, said Gail Christeson of the University of Texas Institute for Geophysics in Austin.

“We’re interested not just because it’s the point of impact but because of what we can learn about other asteroid impact craters,” she said.

Scientists hope to learn what might happen if an asteroid or comet crashes into Earth.

Other studies presented at the meeting compare Chicxulub with the much older Sudbury crater near Ontario. By comparing different levels of melted rock at the bottom of both craters, researchers are more confident that Sudbury was formed by a high-velocity comet and Chicxulub by a slower-moving asteroid.

Comets are chunks of dirty ice; asteroids are giant rocks.

Such large impacts are estimated to occur only once every 350 million years. That makes such craters--especially well-preserved ones such as Chicxulub--difficult to find on Earth.

“We’ve got an opportunity, a unique opportunity on the face of the Earth, to study a crater in three dimensions that has been preserved almost in pristine shape,” Sharpton said. “And that’s really what we want to do.”

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On the Net:

Asteroid impact theory: https://ucaswww.mcm.uc.edu/geology/huff/Chicxulub.html