Photograph by John Cancalosi, National Geographic
Published November 23, 2011
Long before the dinosaurs, a bleak environment of widespread fires and oxygen-poor coastal seawater killed off some 90 percent of all Earth's living species. The whole process took less than 200,000 years, according to a new study of the planet's most catastrophic mass-extinction event.
The end-Permian extinction probably isn't as well known as the Cretaceous extinction that wiped out the dinosaurs some 65 million years ago. But the end-Permian collapse nearly spelled the end of life on Earth.
Now scientists have painted a picture of just how fast the "Great Dying" unfolded 252 million years ago (prehistoric time line).
While the causes of the Permian extinction remain a mystery, from here on out, any theory must be compatible with a 200,000-year time frame centered around 252.28 million years ago, the authors assert. This time span is span indicated by analysis of fossils and chemical evidence of changes in Earth's carbon cycle in rocks from southern China to Tibet.
The 200-millennia period is far shorter than the time span that has been widely accepted, according to study co-author Shu-zhong Shen, a paleontologist at the China's Nanjing Institute of Geology and Palaeontology.
The rock and fossil data show that the nightmare scenario for living species unfolded on land and sea simultaneously, said co-author Sam Bowring of the Massachusetts Institute of Technology (MIT).
As of now, Bowring said, "whatever mechanism you invoke [as the cause of the mass extinction] can't be purely marine or purely continental."
Danger From Above, and Below
Wherever a life-form might have been during the Great Dying, the picture would have been anything but sunny, the researchers say.
"Widespread wildfires played an important role in the rapid deforestation at the end of Permian," said Nanjing's Shen, citing plentiful charcoal-rich and soot-bearing beds found in sedimentary rock of the period.
Rocks also provide hard evidence, via carbon isotope signatures, of oceans under stress as the carbon cycle was reorganized and oxygen levels fell during the Great Dying.
"Marine-organism fossils during the extinction period show a shift from normal species to those who could better tolerate low-oxygen waters," Shen said.
Also, "survivors of the extinction were smaller than the species that came before them—a phenomenon known as the Lilliput effect," after an island home to tiny humans in Gulliver's Travels. This shrinkage is partly attributable to the low oxygen levels, according to Shen.
MIT's Bowring added, "The beauty of those rocks is that we can see what's happening before the extinction event, during the extinction event, and after the extinction—and calibrate it all in absolute time."
Could Asteroid Explain Findings?
But what caused the extinction event to happen in the first place?
The paper's authors lean towards massive releases of carbon dioxide and/or methane from volcanic activities at the end-Permian, including in the Siberian Traps—a vast region of volcanic rock in north-central Russia—and in South China.
(For a contrary view, see "World's Greatest Extinction Not Caused By Toxic Gases.")
But it's not entirely certain whether the eruption and extinction events coincide perfectly in historical time, and other theories remain possible, Bowring concedes.
An asteroid or comet impact, for example would help explain the evidence for simultaneous catastrophe both on land and at sea.
"An impact would simultaneously and quickly affect marine and terrestrial ecosystems," Bowring said. "We have no evidence for an impact, but I am sure some people may believe that it simply remains to be found."
Point of No Return
The researchers say it's tough to tell yet how the Great Dying's extinction rates compare with current rates, which scientists struggle to agree on but which some believe to be among the highest on record. (See "One in Four Mammals at Risk of Extinction.")
But the past may still hold lessons.
"Our story tells us that environmental deterioration could be a long process," Shen said. "However, once it is beyond the threshold or tolerance of the ecosystems, the ecosystems will collapse in a rapid way."
The mass-extinction research was published online November 17 by ScienceXpress.
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