Worldwide Wipeout

Geophysicists Tie Volcanic Eruptions to a Mass Extinction Long Before the Time of the Dinosaurs

by Robert Sanders

Precise dating of a worldwide extinction 250 million years ago places it at the same time as a major series of volcanic eruptions in Siberia, lending strong support to the theory that the eruptions caused the global die-off.

The new date for the mass extinction at the boundary between the Permian and Triassic periods--the largest in Earth's history--is reported by geologists from the Berkeley Geochronology Center and the Berkeley campus in the Sept. 8 issue of Science.

Since 1976, when Berkeley researchers first blamed a meteor for killing off the dinosaurs 65 million years ago, scientists have preferred meteors over other causes--such as catastrophic volcanism--for the mass extinctions that have periodically plagued the Earth.

The impact theory of dinosaur extinction remains solid, but the close link between the Permian extinction and volcanism tips the balance away from meteors as the presumed cause of all extinctions.

"In extreme cases such as this one, volcanoes in and of themselves appear to be sufficient to wipe out life on the planet," concludes geologist Paul Renne, president and director of the Berkeley Geochronology Center and an adjunct associate professor of geology here.

According to co-author Mark Richards, professor of geophysics, the volcanic eruptions lasted for around a million years and churned out as much as a cubic mile of basalt lava each year. Richards and his colleagues proposed in the late 1980s that flood basalt lavas like the so-called Siberian Traps originated deep in the Earth, near the boundary between the mantle and the Earth's molten iron core.

This prediction was confirmed recently by Asish R. Basu, a geologist at the University of Rochester and a co-author of the current paper, and Renne. They reported in the Aug. 11 issue of Science that the Siberian basalts came from around 1,800 miles beneath the surface.

Significant evidence indicates that as the basalts broke through the surface they belched voluminous clouds of sulfur, probably sending much of it into the upper atmosphere.

According to standard scenarios, some of this sulfur would have fallen to Earth as acid rain, while some remained in the form of sulfate aerosols, blocking sunlight and rapidly cooling the Earth. Other volcanic gases such as carbon dioxide would eventually have reversed the cooling trend and caused temperatures to rise because of the greenhouse effect.

In combination, these climatic effects could have wiped out much of the life on Earth.

"A short-lived volcanic winter followed within several hundred thousand years by greenhouse conditions would fully explain the environmental extreme that caused the Permian-Triassic mass extinctions," the authors write.

If dinosaurs were the poster animals of the global extinction at the end of the Cretaceous period 65 million years ago, then the flat, segmented, insect-like creatures known as trilobites head the list of losers at the end of the Permian, a 30-40 million-year period that brought the Paleozoic era to an end. What paleontologist Stephen Jay Gould refers to as "the granddaddy of all extinctions" saw the demise of as much as 95 percent of all marine species and 70 percent of all families of land vertebrates, as well as much of terrestrial plant life.

Until now it has been hard to date the Permian-Triassic extinction because the Earth's surface has changed and eroded so much in the past 250 million years. A major drop in sea level--perhaps more than a hundred meters (300 feet)--occurred at the same time, interrupting a marine sedimentary record that would normally have provided a good date.

Several years ago, however, Chinese scientists recognized that sediment layers at two sites in the Meishan and Shangsi provinces of China represented complete sequences extending through the Permian-Triassic boundary. Obtaining samples of volcanic ash (tuff) from just below and just above the boundary layers from these two sites, Renne and his colleagues at the Berkeley Geochronology Center, an independent, non-profit scientific institute specializing in state-of-the-art geologic dating, dated them using the laser heating argon/argon isotope method. This is the same method that Renne used a year ago to date the oldest known human ancestor Ardipithecus ramidus at 4.4 million years.

The dates they attached to the two layers were indistinguishable, meaning that the boundary layer sandwiched between them is the same age: 250 million years, give or take 250,000 years.

Renne and crew dated anew the beginning of lava flows from the Siberian Traps, and found a date of 250 million years, give or take 300,000 years. "This really nails it down," Richards says. "They did happen at the same time, within the resolution of radiometric dating."

One important aspect of the new report is that it gives geologists a theory to test, since there should be features in Russia such as glacial till deposits that geologists can look for as confirmation.

"This is a very key observation about the timing of the Permian-Triassic boundary and volcanism," Richards says. "It will really stimulate people like me to look at what else was going on at the time."

The authors of the Science paper are Renne, Richards, Basu, Zhang Zichao, a Chinese geologist who supplied the rock samples for dating, and Michael T. Black of Duke University.


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