Scientists call it "the Great Dying," a 250 million-year-old catastrophe that wiped out 90 percent of ocean species and 70 percent of land species in the biggest mass extinction in Earth's geologic history.
The cause of this cataclysm is a matter of great dispute among paleontologists, but research released yesterday offers new evidence that global warming caused by massive and prolonged volcanic activity may have been the chief culprit.
Huge amounts of carbon dioxide were released into the air from open volcanic fissures known to geologists as the "Siberian Traps," researchers said, triggering a greenhouse effect that warmed the earth and depleted oxygen from the atmosphere, causing environmental deterioration and finally collapse.
A second set of findings suggested that the warming also crippled the oceans' ability to refresh their oxygen supply, causing the seas to go sterile, destroying marine life and allowing anaerobic bacteria (which do not require oxygen) to release poisonous hydrogen sulfide "swamp" gas into the air.
The two reports, prepared independently, both cast doubt on another theory -- that the Great Dying was caused by the impact of an asteroid or comet such as the one that triggered the extinction of the dinosaurs 65 million years ago. Both studies were published yesterday by Science Express, the online version of the journal Science.
"This is not a world that is happy and then goes 'Bang!' " said University of Washington paleontologist Peter D. Ward, leader of one of the two new studies. "This is a world that's in trouble for a long time, and then it gets in even worse trouble."
Ward led a team of scientists in a seven-year project to chronicle 126 fossil skulls in a 1,000-foot-thick deposit of sedimentary rock in southeastern South Africa's Karoo Basin. He said in a telephone interview that the samples included reptiles and some amphibians, ranging from dog-size animals to predatory gorgonopsians, which he described as "a hideous cross between a lion and a particularly nasty lizard."
Ward said the team's excavations showed a steady decline in the number of species over 10 million years, followed by a sudden plunge 250 million years ago at the boundary between the Permian and Triassic periods of geologic time. The interval corresponds to a period of prolonged volcanic activity over one-third of modern-day Siberia.
Temperatures climbed globally as carbon dioxide poured into the atmosphere and oxygen levels fell, forcing gasping animals to gather at sea level, he said. "And the plants are not dealing well with the heat" either, he added. "Eventually the imbalance reaches a critical point, and everything dies."
The warming also meant that polar oceans were not cooled as much as they are today, and the convection cycle that circulates cold, oxygen- and nutrient-rich water between the poles and the tropics was slowed and even stopped, according to the second paper by a team of researchers led by Kliti Grice of the Curtin University of Technology in Perth, Australia.
"This has devastating effects on the marine organisms that rely on oxygen and nutrients to survive," the team said in an e-mail. "In the worst-case . . . a major part of the water column above the sea floor is devoid of oxygen."
Analyzing sulfur and carbon isotopes from core samples taken from the ocean bed off the coast of northwestern Australia, the team detected molecular traces from green sulfur bacteria, known as Chlorobiaceae, at the time of the Great Dying.
"The beauty of these [bacteria] is that they require sunlight and an anoxic [oxygen-free] environment," said team member Steven Turgeon, an Oak Ridge National Laboratory geochemist. "Because they live so close to the surface, we're pretty sure that what's beneath is anoxic."
This combination of factors, which has also been detected in waters off southern China, indicates that large swatches of ocean below a depth of 300 feet -- the deepest that significant light can penetrate -- became sterile, and that the entire ocean may have been oxygen-free.
Just as important, the bacteria derive energy from sulfate compounds in seawater and vent poisonous hydrogen sulfide gas into the air, Turgeon noted in a telephone interview.
The Grice team did not address the cause of the lethal warming, but Ward said his team found no evidence of the residue that would have fallen after a comet or asteroid impact threw tons of dust into the air to trigger a sudden and catastrophic greenhouse effect.
Still, University of Rochester earth scientist Robert Poreda, a proponent of the impact theory, noted that the "absence of evidence" at Karoo Basin "does not constitute evidence of absence."
"We propose there was preexisting volcanism" that became much worse because of the seismic energy released by the asteroid or comet impact, he said. "Some people have thought it feasible, while others have been adamantly opposed."