The top graphs show the major extinction events corresponding with average CO2 concentration. As you can see, the Permian is marked by a major volcanic event, which coincided with high atmospheric CO2. During this event very high atmospheric CO2 (3,000 ppm) would have created a runaway Greenhouse Effect.
This would have heated the Earth's oceans to the point where they could no longer contain dissolved oxygen. Warm water holds less dissolved gas than cold water. Therefore, it is postulated that the Permian oceans became extremely warmed and devoid of oxygen. This environment is perfect for sulphur creating anaerobic bacteria. Sulphur could combine with hydrogen creating hydrogen sulphide.
An ocean containing high levels of bacteria creating hydrogen sulphide would kill most of its inhabitants. Continued heating of the ocean could also degas hydrogen sulphide from the ocean into the atmosphere.
This theory is thought to cause the Permian Extinction that was responsible for killing 95% of all life on Earth at the time, both in the ocean and on land. This was by far, the largest extinction in the history of the Earth. However, more data needs to be collected and supported, world-wide to support this theory.
The Extinction marking the end of the Cretaceous 65 Million years ago, where Iridium is found world wide, provided conclusive evidence for an meteorite impact as the cause. Debate over the cause of the Largest Extinction in Earth History will continue until more conclusive world-wide evidence is found.
This may be difficult because the Earth's crust is convected down into the mantle every 200 Million years like a conveyor belt. As the Permian Extinction occurred about 250 Million years ago, most of the rocks would have reprocessed into the Mantle by now.
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