WASHINGTON - Fossil plankton dating from 65 million years ago helps confirm the theory that a dark winter lasting many thousands of years doomed the dinosaurs, researchers said Wednesday.

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Many experts believe that an asteroid struck Earth back then and kicked up dust that obscured the sun, and that the impact also set off volcanic eruptions that disrupted the climate for centuries.

These changes forced many species into extinction, including the dinosaurs. The ancestors of today’s mammals survived to later emerge and fill the empty niches left by their former rivals and predators.

Evidence of a giant crater dating back to around the right time has been found in what is now Mexico’s Yucatan Peninsula area.

Signs of sudden cooling
Writing in the journal Geology, a team of scientists, led by Simone Galeotti of the University of Urbino in Italy and Henk Brinkhuis of the University of Utrecht in the Netherlands, said a Tunisian site called El Kef had now yielded evidence of the sudden cooling that would have followed.

The evidence comes in the form of small, cold-loving ocean organisms called dinoflagellates and benthic formanifera. They seem to have appeared suddenly in an ancient sea that had previously been very warm, said Matthew Huber of Purdue University in Indiana, who worked on the study.

“The fossils indicate that something suddenly made the water cold enough to support these tiny critters,” Huber said in a statement.

“We theorize that the meteor strike produced huge quantities of sulfate particles, such as are often blown high into the atmosphere during a volcanic eruption, and these particles shielded the earth’s surface from sunlight. The decrease in solar energy ultimately caused a long cold spell, called an ‘impact winter,’ that persisted for years.”

Huber, an assistant professor of earth and atmospheric sciences, said this was the first time anyone had found fossil evidence of the cooling, although there has been geologic evidence.

Implications for climate change
The findings can also help experts understand today’s climate changes, said Huber.

“This discovery, which certainly has relevance to theories about dinosaur extinction, is also significant because it confirms our computer models of the earth’s climate — they predict that the climate would respond in this way under the circumstances.”

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