Department of Geology and Geophysics, Yale University, New Haven,
CT 06520-8109
Contributed by Robert A. Berner, February 15, 2002
The biological extinction that occurred at the Permian-Triassic
boundary represents the most extensive loss of species of any known
event of the past 550 million years. There have been a wide
variety of explanations offered for this extinction. In the present
paper, a number of the more popular recent hypotheses are evaluated in
terms of predictions that they make, or that they imply, concerning the
global carbon cycle. For this purpose, a mass balance model is used
that calculates atmospheric CO2 and oceanic
Geology / Evolution
Examination of hypotheses for the Permo-Triassic boundary
extinction by carbon cycle modeling
13C as a function of time. Hypotheses considered
include: (i) the release of massive amounts of
CO2 from the ocean to the atmosphere resulting in mass
poisoning; (ii) the release of large amounts of
CO2 from volcanic degassing; (iii) the
release of methane stored in methane hydrates; (iv) the
decomposition and oxidation of dead organisms to CO2 after
sudden mass mortality; and (v) the long-term reorganization of the global carbon cycle. The modeling
indicates that measured short-term changes in
13C at the
boundary are best explained by methane release with mass mortality and
volcanic degassing contributing in secondary roles. None of the
processes result in excessively high levels of atmospheric CO2 if they occurred on time scales of more than about
1,000 years. The idea of poisoning by high levels of atmospheric
CO2 depends on the absence of subthermocline calcium
carbonate deposition during the latest Permian. The most far-reaching
effect was found to be reorganization of the carbon cycle with major
sedimentary burial of organic matter shifting from the land to the sea,
resulting in less burial overall, decreased atmospheric O2,
and higher atmospheric CO2 for the entire Triassic Period.
*
E-mail: robert.berner{at}yale.edu.
www.pnas.org/cgi/doi/10.1073/pnas.032095199
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