Extinctions Due To Endogenous Sources

The warming caused by volcanoes through carbon dioxide emissions would not be large enough to cause mass extinctions by itself. That warming, however, could set off a series of events that may have led to mass extinction. During the P-T extinction 95

percent of all species on Earth became extinct, compared to only 75 percent during the K-T when a large asteroid apparently caused the dinosaurs to disappear.

Volcanic carbon dioxide would cause atmospheric warming that would, in turn, warm surface-ocean water. Normally, the deep ocean gets its oxygen from the atmosphere at the poles. Cold water there soaks up oxygen from the air and because cold water is dense, it sinks and slowly moves equator-ward, taking oxygen with it. The warmer the water, the less oxygen can dissolve and the slower the water sinks and moves toward the equator (Kump et al., 2005).

The constant rain of organic debris produced by marine plants and animals, needs oxygen to decompose. With less oxygen, fewer organics are aerobically consumed. In the Permian, if the warming from the volcanic carbon dioxide decreased oceanic oxygen, especially if atmospheric oxygen levels were lower, the oceans would be depleted of oxygen. Once the oxygen is gone, the oceans become the realm of bacteria that obtain their oxygen from sulphur oxide compounds. These bacteria strip oxygen from the compounds and produce hydrogen sulphide. Hydrogen sulphide kills aerobic organisms.

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 200 parts per million. This is a toxic brew in which 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. At the end-Permian, as the levels of atmospheric oxygen fell and the levels of hydrogen sulphide and carbon dioxide rose, the upper levels of the oceans could have become rich in hydrogen sulphide catastrophically. This would kill most the oceanic plants and animals. The hydrogen sulphide dispersing in the atmosphere would kill most terrestrial life. Another aspect of this extinction is that hydrogen sulphide gas destroys the ozone layer. Once this process has started, methane produced in the ample swamps of this time period has little in the atmosphere to destroy it. The atmosphere becomes one of hydrogen sulphide, methane and ultra violet radiation.

Biomarkers of photosynthetic sulphur bacteria in deep-sea sediments were recently reported in shallow water sediments of an age comparable to the P-T transition (Grice et al., 2005). These bacteria live in places where no oxygen exists, but there is some sunlight, as it may have happened at the end of the Permian. Confirming the evidence for these microorganisms would suggest hydrogen sulphide to have been the cause of the mass extinctions. The question remains however whether the extinction may have been the effect of another underlying process.

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