http://www.bbc.co.uk/science/horizon/2002/dayearthdied.shtmlCarbon copious
The Greenland rock told a very different story to that Michael Rampino had found in the Alps. Instead of a rapid event of under 10,000 years, the extinction beds Wignall examined lasted 80,000 years and showed three distinctive phases in the plant and animal fossils they contained. The extinction appeared to kill land and marine life selectively at different times. Such a long process contradicted the catastrophic meteorite theory but Wignall couldn't explain what had come close to killing all life on Earth. His best clue was the carbon isotope balance in the rock, which showed an increase in carbon-12 over time. The standard explanation - rotting vegetation - could not have caused such a marked effect. Wignall was curious what this could mean.
An answer came from geologist Gerry Dickens, who knew just how to get large amounts of carbon-12 rapidly, thanks to his work with offshore drilling companies in the USA. He had spent time helping them try to tap reserves of frozen methane hydrate from the seabed of the Gulf of Mexico. He knew methane hydrate is found around many of the world's coasts. Dickens wondered how large a rise in sea temperature was necessary to cause the solid chemical to gasify and ascend to the atmosphere. Experiments suggested a rise of 5°C would be sufficient. And he was amazed to see how much gas came from pieces of solid methane hydrate that were placed in water.
"The south of England would turn into the Sahara Desert"
Michael Benton, University of Bristol
When Paul Wignall learned of Dickens' findings, he used his carbon-12 data to estimate how much methane hydrate would have to be released to affect the isotope balance. Methane is one of the most potent greenhouse gases and he deduced that unlocking frozen methane hydrate would have caused a temperature rise of 4-5°C over time. Not enough to kill off 95% of life on Earth but he realised this was a compounded effect. A rise of about 5°C must already have occurred to prompt the frozen methane to melt. The combined temperature rise of 10°C is generally accepted as a figure able to cause truly mass extinction.
So it seems likely there were two Permian killers. The Siberian Traps did erupt, contributing first to a nuclear winter cooling effect (caused by dust) and and then to global warming (due to greenhouse gases). Over 40,000 years, some land animals gradually died out while life in the seas lived relatively calmly on, as the water temperature gently rose. Then the seas gave up their frozen methane. In just 5,000 years, there was massive loss of species from the world's oceans. In a third and final phase of the extinction, the Permian killer returned to stalk the land for another 35,000 years. By the end of that process, 95% of the Earth's species were extinct.