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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."
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
"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.
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.