(Report from New Scientist.com)
Fossilised leaves analysed from plants living before and after the extinction suggests the amount of carbon dioxide in the atmosphere increased suddenly and dramatically 65 million years ago, so says David Beerling and colleagues at the University of Sheffield in June 2002. The Earth was probably warmed to as much as 7.5º C changing the environment dramatically, the equivalent of injecting at least 6,400 billion tonnes of carbon into the atmosphere.
Other research suggests that maybe a series of volcanic eruptions, such as at the Deccan Traps in India, dated to about 65 million years ago, could have prompted the climate changes that resulted in the extinction of 70 per cent of life on the Earth. This is now put back into shadow as the new evidence reveals that only a massive asteroid impact, such as the one that left a 145 to 180-kilometre wide crater in the Yucatan peninsula in Mexico at around the time, could have vapourised enough carbon-containing material in the Earth's crust to account for this rapid carbon dioxide increase. So claims David Beerling and his colleagues.
As carbon dioxide levels in the atmosphere go up, plant's leaves have fewer pores, because it becomes easier for them to extract carbon dioxide from the air for photosynthesis. So by counting the number of pores on leaves from about 70 plants that were alive between 64 and 65.9 million years ago, Beerling's team could work out the concentration of carbon dioxide in the atmosphere at that time.
David Beerling said...
"We estimate that carbon dioxide levels were four or five times higher about 10,000 years after the impact. For this much carbon dioxide to come from the Deccan Traps, all the eruptions would have had to come in that time window - but we know they took more like two million,"
But Dewey McLean of Virginia Polytechnic Institute who proposed the volcano theory in 1981 is not convinced that Beerling can be so precise about the timing...
"Their leaf database is so tiny as to be almost meaningless."