[meteorite-list] Fossils Point To Asteroid Causing Dinosaurs' Demise
From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu Apr 22 10:01:32 2004
Fossils point to asteroid causing dinosaurs' demise
June 11, 2002
A massive asteroid impact, not volcanic activity, caused the climate
change that wiped out the dinosaurs, new fossil evidence suggests.
David Beerling at the University of Sheffield and colleagues analysed
fossilised leaves from plants living before and after the extinction.
The work suggests the amount of carbon dioxide in the atmosphere increased
suddenly and dramatically 65 million years ago.
The increase was the equivalent of injecting at least 6,400 billion tonnes
of carbon into the atmosphere, they say - enough to warm the Earth by as
much as 7.5 degrees C and dramatically change the environment .
Other researchers have suggested that a series of volcanic eruptions at the
Deccan Traps in India, dated to about 65 million years ago, might have
prompted the climate changes that accompanied the extinction of 70 per cent
of life on the planet. But 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 about the same time - could have vapourised enough carbon-containing
material in the Earth's crust to account for this rapid carbon dioxide increase,
says Beerling's team.
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.
"We estimate that carbon dioxide levels were four or five times higher about
10,000 years after the impact," says Beerling. "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," he adds.
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," he
Journal reference: Proceedings of the National Academy of Sciences (vol 12, p 7836)
Received on Tue 11 Jun 2002 12:11:14 PM PDT