New Theory On Mass Extinction
Blames Sudden Drop In Oxygen

By Richard L. Hill
The Oregonian

Researchers taking the pulse of Earth's worst mass extinction 250 million years ago say they have diagnosed the ailment that doomed most animals: altitude sickness.
A drop in oxygen and a rise in carbon dioxide wiped out invertebrates that weren't biologically equipped to deal with the atmospheric change, says Gregory J. Retallack, a professor of geological sciences at the University of Oregon.
"They suffered from mountain sickness, which is a nasty way to go," said Retallack, who has experienced it himself in high altitudes. He said the animals at sea level found themselves with a shortage of oxygen comparable to that found at elevations of about 16,000 feet today.
The UO scientist is a leading researcher into the die-off, which annihilated about 90 percent of marine species and 70 percent of land vertebrates. Known as the Permian-Triassic extinction, it was a key event in Earth's history. It opened the door to the age of dinosaurs, which went extinct themselves 135 million years later.
Retallack, along with Roger M.H. Smith, curator of geology at the South African Museum in Capetown, South Africa, and Peter D. Ward, a paleontologist with the University of Washington, report their findings in the September issue of the Geological Society of America Bulletin.
Survivors' characteristics The scientists, who conducted their study in the rich fossil beds of South Africa's Karoo Basin, report that the mammal-like reptiles that survived had short snouts, barrel chests and other characteristics of animals that live in the rarified air of high altitudes. Some also have been found to be burrowing animals, Retallack said.
"In a burrowing environment, you're living in your own carbon dioxide quite a bit," he said, "so I think this burrowing habitat preadapted them to this atmospheric crisis that came along."
A specialist in fossil soils, Retallack took samples of ancient sediment found at the boundary between the Permian and Triassic periods from the Karoo Basin. Studies back at his UO lab indicated an abrupt, but not dramatic, ecosystem shift that changed the dry region to a semiarid one.
"In terms of Oregon, it would be like going from Burns to Sisters, from a sagebrush kind of environment to an open woodland," Retallack said.
Exactly what triggered the planet's life crises is a mystery. The extinction occurred when all Earth's land formed a supercontinent called Pangea. Most of the rocks that are 250 million years old have been recycled by the movement of crustal plates, making it challenging for scientists to study the causes of the mass extinction.
The lethal chain of environment-altering events could have been launched by an asteroid hitting the planet; massive flows of molten rock in Siberia that pumped huge amounts of carbon dioxide into the air; an enormous, explosive burp of methane released from the deep ocean; or a combination of events.
Release of methane Gregory Ryskin, a chemical engineer at Northwestern University, proposes in the September issue of the journal Geology that an explosive release of methane that had accumulated in stagnant ocean water could have doomed much of the planet's life. The eruption would have brought oxygen-free water to the surface that would have killed marine life, with extinctions on land caused by explosions and fires following the release of the methane into the air.
Retallack's study supports the concept that there was a huge release of methane from reservoirs of methane hydrates -- crystalline substances composed of ice and gas -- on continental shelves offshore or in permafrost. When methane is released into the atmosphere, it oxidizes into carbon dioxide.
He isn't sure what unleashed the methane, but "I think a small, well-placed asteroid or meteorite would do the trick quite nicely. You wouldn't need a huge impact to do it."
Luann Becker, a scientist at the University of California at Santa Barbara, who also specializes in studying the mass extinction, said the study presents "an interesting idea" that the rapid increase of a greenhouse gas such as methane would have left unacclimated animals at risk.
Becker said an asteroid impact may have been involved in initiating "multiple catastrophes" such as volcanism and climate changes that didn't allow life to recover fast enough to avoid extinction. She disagrees with Retallack about the asteroid's size. "I still favor a large impact, which would have released a lot of methane very quickly."
The idea of an asteroid striking Earth "would gain some strength if we could find the crater," she said.
Retallack and Becker are planning an expedition in November to Antarctica to search for evidence of such an impact in Permian-Triassic boundary sites. Richard L. Hill: 503-221-8238;




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