Geologists may be closer to discovering the precise date and cause of the largest and most mysterious extinction event in Earth's history, thanks to volcanic rocks in China. The Permian mass extinction is marked by the loss of 90 percent of all species a quarter-billion years ago, but scientists are split about what happened.

"In order to make assumptions about any causes, you have to know when it happened and how fast it happened," said Roland Mundil of the Berkeley Geochronology Center in California.

Permian Boundary

Mundil and his colleagues' work appears in a paper in the Sept. 17 edition of Science.

To get a better idea of the when, Mundil's team employed a new method for measuring uranium, which predictably decays into lead over millions of years, inside zircon crystals from volcanic ash layers in rocks from Shangsi and Meishan, China.

The ash layers are from explosive volcanic eruptions and now serve as atomic clocks within layers of fossil-bearing rocks that recorded the sudden demise of most life at the end of the Permian period.

But because the zircon "clocks" can leak lead, giving a false younger date, or actually be zircons mixed into the ash from older rocks, it's been difficult to extract exact dates, Mundil said.

A Polished Zircon

By pre-treating the zircons and then measuring only the parts of the zircon crystals that are pristine and haven't leaked, Mundil's team believes they have finally overcome these problems. They now put the date of the mass extinction at 252,600,000 years ago, give or take 200,000 years. That puts the die off right in synch with the Siberian Flood Basalts, world's largest lava flow.

"The striking is that the volcanism all happened at the same time," said Mundil.

The gigantic flood of lava could have caused global climate changes that might have played a role in the extinction event, Mundil said. Other theories being hotly debated by geo-scientists include a meteor impact and a massive release of methane from the oceans.

In fact, it might have been a combination of catastrophes that lead to the Permian extinction event, said Luann Becker of the University of California at Santa Barbara. Becker is studying what appears to be a giant impact crater buried in Western Australia of end-Permian age.

"Both together caused a major, extreme event," said Becker. "I think this debate will go on for a while."