Atmospheric changes related to volcanic eruptions, and not a giant asteroid, were probably the main factors behind Earth's biggest die-off about 250 million years ago, according to two separate teams of scientists working at sites around the globe.
The mass extinction, known as the “Great Dying,” extinguished 90 percent of sea life and nearly three-quarters of land-based plants and animals.
There has been recent evidence that a big asteroid or meteor hit the Earth and triggered the catastrophe, but researchers say they now have evidence that something much more long-term was the culprit.
Kliti Grice of Curtin University of Technology in Perth, Australia, and colleagues studied sediment cores drilled off the coasts of Australia and China and found evidence that the ocean was lacking oxygen and full of sulfur-loving bacteria at that time.
This finding would be consistent with an atmosphere low in oxygen and poisoned by hot, sulfurous, volcanic emissions, they wrote in a report published in the journal Science.
A second team, led by Peter Ward at the University of Washington, looked at fossil evidence in South Africa — they found little evidence of a sudden catastrophe but instead saw signs of a gradual die-off.
They examined 126 reptile and amphibian skulls from the Karoo Basin in South Africa, where there is an exposed piece of dried sediment from the end of the Permian Era and the beginning of the Triassic, 250 million years ago.
They found two patterns, one showing gradual extinction over about 10 million years leading up to the time of the extinction, and then a spike in extinction rates that lasted another 5 million years, Ward’s team reported.
“Animals and plants both on land and in the sea were dying at the same time, and apparently from the same causes — too much heat and too little oxygen,” Ward said in a statement.
Ward believes mass volcanic eruptions may have pumped greenhouse gases into the air, which would have trapped heat in the atmosphere and raised temperatures.
'Double-whammy' at work
“I think temperatures rose to a critical point. It got hotter and hotter until it reached a critical point and everything died,” Ward said. “It was a double-whammy of warmer temperatures and low oxygen, and most life couldn’t deal with it.”
But Ward also warned against drawing too close of a comparison between the Permian-Triassic climate changes and the current rise in average global temperatures, which some scientists attribute to increases in industrial greenhouse gases.
"What we're not having is this humongous sea-level drop at the end of the Permian Era," Ward told MSNBC.com. Lower sea levels, perhaps caused by geological shifts, exposed wide stretches of carbon-rich sea sediment to the air, and Ward said the resulting chemical reaction played a crucial role in changing Earth's atmosphere.
"When that stuff hit the air, it started sucking the oxygen out," Ward said. That "suck-down" of oxygen had a suffocating effect on land animals, he said.
What about an asteroid?
Last May, researchers said they found evidence of a giant asteroid striking Earth off the coast of what is now Australia 251 million years ago, and suggested that the impact may have played a role in the Permian-Triassic extinction. Others have disputed their conclusions, however.
One of the main researchers behind the asteroid study, Luann Becker of the University of California at Santa Barbara, said the causes of the Permian-Triassic extinction are the subject of an "ongoing discussion."
She said Ward’s research team “did a nice job of presenting the paleontological data and the stratigraphy, which seem to show some indication of an evolutionary change going on for a prolonged period of time.” However, she added, she doesn’t believe that addresses the subject of cause and effect.
The Permian-Triassic extinction opened the way for the rise of dinosaurs — which themselves fell victim to another mass die-off 65 million years ago. Most experts agree that the catastrophic impact of a massive asteroid or crater played the primary role in that extinction, at the boundary between the Cretaceous and Tertiary geologic periods. The blast is thought to have formed the Chicxulub crater off Mexico’s Yucatan Peninsula.
This report includes information from Reuters and The Associated Press as well as from MSNBC's Alan Boyle.