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Fire and Brimstone, Cretaceous Style

29 April 2008 (All day)
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Jim Zuckerman/CORBIS; (inset) Harvey et al., Geology, Geological Society of America (May 2008)

Hot days ahead?
Researchers found microscopic evidence of hydrocarbons (cenosphere, inset), possibly from oil fires ignited by the impact that ended the reign of dinosaurs.

Talk about an oil crisis. An international team examining sediments from the end of the dinosaur age has discovered microscopic carbon spheres that can be produced only from burning fossil fuels. If confirmed, the finding means that the dinosaurs might have been wiped out at least partly by an oil-fueled conflagration.

Two major lines of evidence implicate an extraterrestrial object as the dinosaurs' killer. A thin layer of the mineral iridium--an element that is very rare on Earth's surface but more common in asteroids--in sediments deposited about 65 million years ago points to a catastrophic impact. Scientists have also located the probable site of the strike: the Chicxulub crater on the Yucatan Peninsula in Mexico (ScienceNOW, 13 May 2004).

But the impact might not have been big enough to exterminate the dinosaurs--something else was needed. Further research by several groups provided a possibility: Massive forest fires ignited by the heat of the blast could have pumped enough carbon dioxide into the atmosphere to cause a period of runaway global warming, cooking the dinosaurs (ScienceNOW, 6 June 2003). Or the fires might have spewed enough soot to block out the sun and kill off the plants on which herbivorous dinos fed, an effect that would have been felt all the way up the dino food chain. The problem with the fire scenario is that, although scientists have found soot at the Cretaceous-Tertiary boundary--also called the K-T boundary--from the time of the dinosaurs' demise, they have only unearthed a few traces of charred plant remains.

The new potential explanation derives from another component of the soot in the K-T sediments: distinctive carbon globs known as cenospheres. The microscopic spheres can form only when fossil hydrocarbons such as coal and crude oil burn. The team reports in the May issue of Geology that it found cenospheres at eight of 13 sites it examined around the world, and the objects were present only at the K-T boundary, not above or below it. The researchers suspect the Chicxulub object plowed into a huge oil reservoir in the Gulf of Mexico, like the ones feeding offshore platforms there today. The impact first vaporized the oil and then ignited it in the atmosphere, causing an enormous, spreading fireball probably hundreds of kilometers wide. Whether the conflagration was enough to do in the dinos--via fire, soot, or global warming--remains unknown, but it would have spared a variety of critters, including the ancestors of today's mammals.

The paper is an "eye opener," says paleobotanist Peter Wilf of Pennsylvania State University in State College. It makes "a strong case for the true source of the mysterious soot" in the K-T layer and casts doubt "on the venerable global wildfire hypothesis," he says. The results should help researchers "better understand environmental conditions during the disaster and are thus very important for interpreting the mass extinction and life's subsequent recovery."

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