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Global Warming Preserved "Mass Kill" Fossils, Study Says

James Owen
for National Geographic News
October 18, 2005
 
Prehistoric global warming periodically wiped out much of the planet's wildlife and created exceptionally well-preserved fossils of the remains, according to a new study.

The enormous death toll inflicted by episodes of lethal atmospheric changes may have prevented life on Earth from being wiped out altogether, the study's author adds.

Greg Retallack, geologist at the University of Oregon, presented his study this week at the Geological Society of America's annual meeting in Salt Lake City, Utah.

Retallack says huge releases of carbon dioxide and methane starved the Earth of oxygen, causing mass extinctions over the last 500 million years.

He adds that the resulting buildup of billions of dead prehistoric creatures may have acted as a carbon sink, reducing CO2 in the atmosphere and preventing the Earth from becoming as hot and lifeless as Venus.

"It was the widespread death and burial of animals and their carbon that created fossil bonanzas, the likes of which may have saved us from the heat sterilization experienced by our sister planet," Retallack said.

He bases his theory on a compilation of unusually well preserved fossils found around the world, including fish, crustaceans, insects, and other ancient life forms.

The fossil record indicates about 40 episodes of exceptional preservation, he says—episodes that coincide with periods of global warming, when the Earth was low in oxygen.

"Lowered levels of oxygen can kill fish and other creatures … and also preserve their carcasses from dismemberment and decay," Retallack said.

Greenhouse Gases

In some mass extinctions, the scientist says, greenhouse gases "ramped up … to intolerable levels of more than ten times the modern level of atmospheric carbon dioxide."

Volcanic activity and outbursts of undersea gases are the prime suspects for these periods of lethal pollution.

"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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