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Deccan produced global warming that was over before the K-T boundary



This seems not to have been mentioned onlist yet:

G. Ravizza & B. Peucker-Ehrenbrink: Chemostratigraphic evidence of Deccan
Volcanism from the Marine Osmium Isotope Record, Science 302, 1392 -- 1395
(21 November 2003)

Abstract:
"Continental flood basalt (CFB) volcanism is hypothesized to have played a
causative role in global climate change and mass extinctions. Uncertainties
associated with radiometric dating preclude a clear chronological assessment
of the environmental consequences of CFB volcanism. Our results document a
25% decline in the marine 187Os/188Os record that predates the
Cretaceous-Tertiary boundary (KTB) and coincides with late Maastrichtian
warming. We argue that this decline provides a chemostratigraphic marker of
Deccan volcanism and thus constitutes compelling evidence that the main
environmental consequence of Deccan volcanism was a transient global warming
event of 3° to 5°C that is fully resolved from the KTB mass extinction."

Normal weathering of continental crust puts more osmium-187 than osmium-188
into the oceans, while weathering of volcanic rocks and extraterrestrial
input have the opposite effect. Os stays in the ocean for a short time,
therefore complete changes in the oceanic Os isotope ratio can happen within
10,000 years. The decline mentioned above (which ends a steady state) starts
near the end of magnetic chron 30N, is gradual, and ends some 100,000 years
before the K-T boundary; superimposed on the new steady state a very narrow
and extremely high respectively deep spike occurs at the boundary. The spike
is ascribed to the meteorite*, the decline to the major episode of Deccan
volcanism. During the decline global temperatures are higher than before and
afterwards; temperatures return to normal when the decline ends and do not
change around the K-T boundary. (Resolution at most or all investigated
sites seems to be too low to detect the impact-induced temperature spike
upwards which is supposed to have lasted only some 1,000 years or less, not
to mention the spike downwards for which there is no direct fossil evidence
[AFAIK] and which is supposed to have lasted just a few months.)

Interesting is the date issue -- the authors** put the K-T boundary at 65.5
Ma ago instead of at 65.0. Is this just the usual difference between
argon-whatever and uranium-lead?

* At the same time a huge spike in the total amount of osmium and of course
iridium occurs. The mass extinction also happens at this time.
** Also those of the paper on the rainforest "1.4" Ma younger than the K-T
boundary -- according to them it is 64.1 Ma old. I've just read that paper,
too... interestingly it confirms my ignorant suspicion that no rainforests
whatsoever are known from the K.