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At the end of the Permian, 250 Ma, 95 percent of all species on Earth became extinct, compared to only 75 percent during the KT extinction, 65 Ma, when the dinosaurs disappeared. The cause for this extinction is puzzling, as there is no smoking gun evidence of any event. But new research has found hydrogen sulphide from the deep ocean could have been abundant, producing lethal conditions for life at that time.
Previous work have shown that the deep oceans were anoxic, lacking oxygen, in the late Permian and 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 land-based animals. However this would only have been about seven times that of the pre-industrial level, which would stimulate plant growth and it would be hard to see carbon dioxide a killing mechanism.
Although hydrogen sulphide gas, produced in the oceans through sulphate decomposition by sulphur bacteria, can easily kill both terrestrial and oceanic plants and animals. Humans can smell hydrogen sulphide gas, the smell of rotten cabbage, in the parts per trillion range. In the deeps of the Black Sea today, hydrogen sulphide exists at about 34 parts per million. Within this toxic brew any aerobic, oxygen-needing, organism would die. For the Black Sea, the hydrogen sulphide stays in the depths because our rich oxygen atmosphere mixes in the top layer of water and controls the diffusion of hydrogen sulphide upwards.
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. This would kill most of the oceanic plants and animals. The hydrogen sulfide dispersing in the atmosphere would kill most terrestrial life.
More info: SpaceDaily.com
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