Global Warming Preserved "Mass Kill" Fossils, Study Says

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"The principal gas responsible is methane, either released from … ices in marine sediments or by volcanic intrusion," Retallack said.

Once in the atmosphere, methane combines with oxygen to form CO2 over a period of 10 to 20 years, depleting the Earth's stores of oxygen in the process.

If oxygen levels drop too low, not only will many animals die but they will be well preserved in death.

This is especially true for creatures in oxygen-poor marine environments, says Paul Wignall, paleontologist at the University of Leeds, England.

"Organic matter breaks down in the presence of oxygen," he said. "[Decay] still happens in the absence of oxygen, but it's a lot slower.

"If fish sink down to the bottom [of the ocean] and there's little oxygen around, then they'll fossilize very nicely indeed, because nothing happens to them once they're down there."

Evidence for prehistoric spikes in CO2 is found in fossilized plant leaves and fossils showing that heat-loving species migrated toward the polar regions in response to global warming.

"The number of [pores] on the leaves of plants is a direct reflection on how much CO2 is in the atmosphere," Wignall added. "These little holes are for absorbing CO2 into the plant, so if they're bathed in huge amounts of CO2 then they need very few holes."

Retallack's fossil evidence suggests these episodes of dramatic climate change coincide with all the "big five" mass extinction events, as well as a number of other lesser extinctions.

Wiped Out

The greatest mass extinction occurred some 245 million years ago, when an estimated 96 percent of all marine species were wiped out.

Around three-quarters of land species also went extinct.

Exceptional fossil records of huge fish kills from the period have been found in Greenland, Norway, and Madagascar.

"Generally the latter stages of global warming correspond with all our major extinction events," Wignall added.

This includes the mass extinction that killed off the dinosaurs around 66 million years ago.

Wignall says a major phase of global warming was already causing extinctions when a giant meteorite strike is thought to have sealed the dinosaurs' fate.

Retallack says fossils from such times took huge amounts of carbon out of circulation.

"Carbon burial lowers potential atmospheric carbon dioxide," he said. "This is the main effect, but also most of the fossils such as fish and trilobites are [breathing] animals, thus [they] are no longer contributing carbon dioxide."

"One of the main ways of drawing down greenhouse gases is to bury the carbon in stagnant, low-oxygen oceans," Wignall added. "It's a self-healing mechanism the planet has.

"[But] it takes hundreds of thousands of years for the planet to heal itself, which is why you get extinctions."

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