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Re: extinction

Response to Jim Cunningham's Sat, 10 Jan 2004 11:38:18 post (below the dashed line).

Dear Jim, I see that you found my review of the Beerling et al. paper titled "An atmospheric pCO2 reconstruction across the Cretaceous-Tertiary boundary from leaf megafossils" (PNAS, 2000, v. 99), that I provided for the New Scientist.

Actually, I thought the paper was imaginative, and had fun studying it. I had taught paleobotany for nearly 20 years, was well aware of the fossils and methodology used by the authors, and had no problem whatsoever in understanding how the authors were attempting to use fossil leaves to interpret causation in K-T climate change.

However, I did have problems with the tiny leaf database, and the broad geographic sampling extent, from which the authors based their conclusions. Here they are:

Site 1: North Dakota, 1 leaf
Site 2: North Dakota, 2 leaves
Site 3: North Dakota, 31 leaves
Site 4: North Dakota, 2 leaves
Site 5: Raton Basin, 13 leaves
Site 6: Montana, 5 leaves
Site 7: Wyoming, 15 leaves
Site 8: Colorado, 5 leaves
Site 9: Spitsbergen, 8 leaves

Had the authors based their study on closely spaced vertical samples containing large number of leaves taken from a proven _complete_ K-T stratigraphic section, their sweeping conclusions might be more plausible.

Here is a quotation from my review:

"The paper is so loosely constrained by existing K-T transition geobiological data--and is based on so tiny a Ginkgo leaf data base--that it cannot differentiate between a short-duration impact event and a long-duration Deccan Traps release of CO2. Most certainly, the data do not support the authors' claim that the post K-T geobiological record indicates instantaneous transfer of C into the atmosphere via impact by a large extraterrestrial bolide impact."

Dewey McLean


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 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

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.