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Did a Global Winter Kill Off the Dinosaurs?

Posted on: Thursday, 7 October 2004, 06:00 CDT

The demise of the dinosaurs remains one of science's great mysteries and a constant cause of speculation, controversy, and debate. While the mystery may never be solved unequivocally, a recent study using new methodology reinforces the theory that "global winter" wiped out the great creatures across the Cretaceous- Tertiary boundary.

The global winter theory states that meteor strikes on the area around the Gulf of Mexico's Yucatan Peninsula sent massive amounts of dust and debris into the atmosphere, shielding the Sun and sending temperatures on Earth plummeting. Research using oxygen isotopes to indicate paleoclimatic temperatures typically has yielded widely varying conclusions. So a team of scientists led by Simone Galeotti of the University of Urbino, Italy, used fossils left over from the extinction to represent temperature ranges from the period. Specifically, they looked at marine shales in northern Tunisia.

"The interesting twist is that they focus not on the extinction itself, but on the survivors, those who made it through the event," points out team member Elisabetta Pierazzo about the samples.

Analysis of the fossils in the shales indicated that organisms normally found in cold-ocean regions migrated to warmer waters after the presumed date of the theoretical meteoric event, which suggests that the Earth was cooling. The research also showed that the cold spell lasted around 2000 years, much longer then previous studies had theorized. This disparity could be attributed to the new study's focus on deep-water temperatures, whereas previous studies looked at surface temperatures.

"While after about a decade the solar radiation reaching the Earth's surface may have returned to its normal value, the ocean, because of its higher thermal inertia, will take longer to react," says Pierazzo.

While many researchers believe a combination of events-possibly including volcanic eruptions, devastating fires, greenhouse warming, acid rain, and destructive storms-is probably to blame for the dinosaurs' extinction, the new research, which appeared in the June issue of Geology, is the most convincing yet that a global winter may have played a role.

THOSE EVIL TREES

If you thought trees were upstanding citizens of nature, you may want to think again. Researchers say that trees may have contributed to harmful ground-level ozone concentrations during last summer's deadly heat wave in Great Britain-the first time this effect has been documented in that country. Measurements showed that at high temperatures, trees and some other plants discharged the chemical isoprene, which increases the efficiency of the reactions by which sunlight converts nitrogen oxide into ozone. The observations showed that temperatures above 95F caused deciduous trees to give off significantly larger amounts of isoprene than at lower temperatures. "At [86F], it's starting to become important. By [95F], the emission rate has gone up by maybe an order of magnitude (10 times)," says Alastair Lewis, of York University. His team's findings were discussed at a February meeting about the summer 2003 pollution, sponsored by Great Britain's National Environmental Research Council.

ECHOES

"I'm surprised we didn't lose the whole herd."

-NANCY SHAULL, a Brogue, Pennsylvania, farmer, on a lightning strike killing nine of her cows and eight calves. According to Shaull, because cattle bunch together during storms, they made an easy target for a single strike.

Copyright American Meteorological Society Sep 2004

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