Hydrogen Sulfide, Not Carbon Dioxide, May Have Caused Largest Mass Extinction
Hydrogen sulfide, not carbon dioxide, may have caused largest mass extinction
November 3, 2003
Seattle, Wash. -- While most scientists agree that a meteor strike killed the
dinosaurs, the cause of the largest mass extinction in Earth's history, 251
million years ago, is still unknown, according to geologists.
"During the end-Permian extinction 95 percent of all species on Earth became
extinct, compared to only 75 percent during the KT when the dinosaurs
disappeared," says Lee R. Kump, professor of geosciences. "The end-Permian is
puzzling. There is no convincing smoking gun, no compelling evidence of an
Researchers have shown that the deep oceans were anoxic, lacking oxygen, in
the late Permian and research shows that the continental shelf areas in the
end-Permian were also anoxic. One explanation is that sea level rose so that
the anoxic deep water was covering the shelf. Another possibility is that the
surface ocean and deep ocean mixed, bringing anoxic waters to the surface.
Decomposition of organisms in the deep ocean could have caused an
overabundance of carbon dioxide, which is lethal to many oceanic organisms and
"However, we find mass extinction on land to be an unlikely consequence of
carbon dioxide levels of only seven times the preindustrial level," Kump told
attendees today (Nov. 3) at the annual meeting of the Geological Society of
America in Seattle. "Plants, in general, love carbon dioxide, so it is
difficult to think of carbon dioxide as a good kill mechanism."
On the other hand, hydrogen sulfide gas, produced in the oceans through
sulfate decomposition by sulfur bacteria, can easily kill both terrestrial and
oceanic plants and animals.
Humans can smell hydrogen sulfide gas, the smell of rotten cabbage, in the
parts per trillion range. In the deeps of the Black Sea today, hydrogen
sulfide exists at about 34 parts per million. This is a toxic brew in which
any aerobic, oxygen-needing, organism would die. For the Black Sea, the
hydrogen sulfide stays in the depths because our rich oxygen atmosphere mixes
in the top layer of water and controls the diffusion of hydrogen sulfide
In the end-Permian, as the levels of atmospheric oxygen fell and the levels of
hydrogen sulfide and carbon dioxide rose, the upper levels of the oceans could
have become rich in hydrogen sulfide catastrophically. This would kill most of
the oceanic plants and animals. The hydrogen sulfide dispersing in the
atmosphere would kill most terrestrial life.
Kump and colleagues, Alexander Pavlov, University of Colorado; Michael Arthur,
professor of geosciences, Penn State; Anthony Riccardi, graduate student, Penn
State; and Yashuhiro Kato, University of Tokyo, are looking at sediments from
the end-Permian found in Japan.
"We are looking for biomarkers, indications of photosynthetic sulfur
bacteria," says Kump. "These photo autotrophic organisms live in places where
there is no oxygen, but still some sunlight. They would have been in their hay
day in the end-Permian." Finding biomarkers of green sulfur bacteria would
provide evidence for hydrogen sulfide as the cause of the mass extinctions.
So, what of the 5 percent of the species on Earth that survived? Kump suggests
that the mixing of the deep ocean layers and the upper layer was not uniform
and that refugia, places where oxygen still existed, remained, both in the
oceans and on land.
Andrea Messer Vicki Fong
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