Massive volcanic eruptions more than 250 million years ago caused the greatest mass extinction of land and sea species |
By Richard A. Kerr, Science Now, November 17, 2011
The biggest volcanic eruptions of the past half eon had
seemed a likely culprit in the greatest mass extinction Earth has seen. Now the
closest look yet at events 252 million years ago is linking those eruptions
even more closely not only to the biotic cataclysm in the sea but also to the
mass extinction on land.
An international group of scientists led by
paleontologist Shu-zhong Shen of Nanjing Institute of Geology and Palaeontology
in China intensively sampled the fossil record, they report today in Science. They examined nine rock outcrops
across South China, not just the couple of sites most closely sampled in the
past. Each sampling site spanned the mass extinction 252 million years ago at
the end of the Permian period. The sites included records from the sea, where
fully 90% of species disappeared, as well as from the land.
Shen and his colleagues also used volcanic minerals to
gauge when and how fast things happened at each site. Occasional volcanic
eruptions had layered minerals throughout the outcrops. In order to date these
minerals, the group used the steady decay of radioactive uranium to lead. Even
though the eruptions happened a quarter-billion years ago, the method gave them
an error of only about 100,000 years. Improvements to the mass spectrometer
that counted uranium and lead atoms and to the sample preparation procedure had
reduced the dating error by a factor of four.
"What's striking is how fast the extinction
was," says paleontologist Douglas Erwin of the Smithsonian National Museum
of Natural History in Washington, D.C., a co-author on the paper. The event had
been seen as lasting half a million years, but the new dating limits it to no
more than 200,000 years and quite possibly less than 100,000 years, Erwin says.
"We're paleontologists studying events 250 million years ago," he
adds, so "a hundred thousand years sounds like overnight to us."
The dating also establishes that the extinction on land, apparently driven
by extreme drying and warming, happened simultaneously with the marine
extinction.
And the new age for the extinction of 252.28 million years puts it within a
mere few tens of thousands of years of the humongous lava outpourings that
formed large deposits of volcanic rock known as the Siberian Traps. "We
think the timing is consistent with the Siberian Traps eruptions being the
major cause of the extinctions," Erwin says.
"It's quality data," says Paul Wignall, a
paleontologist at the University of Leeds in the United Kingdom. "There
was something going on with the eruptions, though we still don't understand the
interaction" with living things. The new study may help answer that
question, too. It also refined the timing of geochemical changes, which may
hold clues to exactly how the eruptions trigged the biggest extinction. However
it happened, the eruptions' belching—which included greenhouse gases and
acid-generating sulfur—must have done in much of the life on land and in the
sea.
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