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Here's an interesting bit from NGS last year.
> "Leaf fossils can indicate the amount of carbon dioxide in the atmosphere
> because of the
> relationship between the frequency of breathing pores on the leaves?termed
> stomata?and > levels of atmospheric carbon dioxide," said Upchurch.
> When there is more carbon dioxide in the atmosphere, leaves need fewer
> breathing pores to
> extract carbon dioxide from the atmosphere for photosynthesis. "This has been
> documented > in a number of modern plants grown under controlled conditions
> at different levels of > atmospheric CO2," said Upchurch.
> 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
> 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
> 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 > > 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.