Global warming caused by widespread volcanic eruptions was most likely the cause of the largest extinction event in history, an epic disaster 250 million years ago in which 90% of marine life and 70% of species living on land died off, two teams of researchers reported Thursday.
Sulfur spewed out by volcanoes in the so-called Siberian Traps depleted oxygen in the air while creating powerful greenhouse gases that trapped sunlight and raised the Earth’s temperature sharply, producing the event known as the “Great Dying.”
“It got hotter and hotter until it reached a critical point and everything died,” said paleontologist Peter D. Ward of the University of Washington, lead author of one of the two papers published online by the journal Science.
“It was a double-whammy of warmer temperatures and low oxygen, and most life couldn’t deal with it.”
Global temperatures probably rose about 18 degrees Fahrenheit, Ward said, killing off many of the relatively primitive plants that served as a food base for land animals.
At the same time, the proportion of oxygen in the atmosphere dropped from the normal level of about 21% to as low as 16%, a change that is equivalent to climbing to the top of a 14,000-foot mountain.
“High and intermediate elevations would have become uninhabitable,” he said. “More than half the world would have been unlivable. Life could only exist at the lowest elevations.”
Ward and his colleagues studied 1,000-foot-thick sediment in the Karoo Basin of South Africa and off the coast of China, while a second team headed by paleontologist Kliti Grice of the Curtin University of Technology in Perth, Australia, studied sediment from the Perth Basin. Sediment in both areas provides a cross-section of materials deposited over millions of years around the end of the Permian period and the beginning of the Triassic, when the extinction event occurred.
Grice’s team found that the ocean was low in oxygen and full of sulfur-loving bacteria during the time of the event, strong evidence that the atmosphere had unusually low oxygen levels.
Ward’s team studied fossils of a variety of species and concluded that there was a slow die-off over a period of about 10 million years, followed by a much higher rate of dying over the next 5 million years.
Such a pattern is consistent with damage produced by global warming but not with an abrupt extinction event that might have been caused by an asteroid impact, such as that which led to the demise of the dinosaurs some 65 millions years ago.
Both teams reported that they could find no evidence supporting such an impact at the end of the Permian period.
If one did occur, they concluded, it would have made only a minor contribution to the Great Dying.
Instead, massive volcanic flows in Siberia released millions of tons of corrosive sulfur into the atmosphere over a long period.
At the same time, shifts in the Earth’s tectonic plates lowered the oceans’ average level, exposing the seafloor and releasing methane and other gases trapped in sediment.