Fossil Leaves Suggest Asteroid Killed Dinosaurs

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These controlled experiments have resulted in what scientists term the stomotal index, which shows an inverse relationship between the amount of carbon dioxide in the atmosphere and the number of breathing pores on the leaves.

The researchers compared the fossilized fern and gingko leaves with a stomotal index derived from the closest living relatives of the fossil plants, which allowed them to reconstruct past levels of atmospheric carbon dioxide for analysis.

The analysis indicates a sudden and dramatic increase in carbon dioxide levels equivalent to injecting 6,400 billion metric tons of carbon into the atmosphere, which is enough carbon to warm the Earth by 12 degrees Fahrenheit (7.5 degrees Celsius).

"6,400 billion metric tons of carbon is, by at least a factor of five, more than the entire carbon pool of either modern or latest Cretaceous vegetation," said Upchurch. "If our calculations are correct, a significant quantity of the carbon had to come from the vaporization of limestone rock by the asteroid impact on the Yucatan Peninsula," he said.

Another Theory

According to the analysis of the leaf fossils, the increase in carbon occurred over a period of 10,000 to 20,000 years, too short of a time period to lay the blame on volcanism at Deccan Traps, which scientists have said lasted from 500,000 to several million years.

Dewey McLean, emeritus professor of geology at Virginia Polytechnic University in Blacksburg, Virginia, who proposed the Deccan Traps volcano theory in 1981, said the fossil leaf database that Upchurch and colleagues used for their analysis is too small to accurately depict the timing of the K-T boundary record.

"At one collecting site, they have one leaf," he said. "And for each of the other sites only a few leaves."

According to McLean's analysis of the Deccan Traps, 70 to 90 percent of the entire lava pile from the eruptions at the Deccan Traps began 65 million years ago, right at the K-T boundary.

The Proceedings of the National Academy of Sciences paper does not change Dewey's belief that the Deccan Traps volcanism is the major culprit behind the global warming that led to the mass extinctions at the K-T boundary.

"I believe that a number of factors combined to trigger a major K-T transition greenhouse, of which Deccan Traps was the primary contributing factor," he said.

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