Volcanic suspect in global murder mystery.

The greatest extinction in Earth's history -- a profound biological crisis that set the stage for the world of the dinosaurs -- may trace to a series of massive volcanic eruptions, according to new geologic evidence.

Life on Earth has passed through several mass extinctions, including those that wiped out the dinosaurs at the end of the Cretaceous period. But the widespread die-offs at the boundary between the Permian and Triassic periods stand out above all others in the fossil record. By some accounts, 96 percent of the ocean species vanished altogether, and the dominant land animals suffered remarkably. Such drastic winnowing of Permian organisms allowed new forms to rise in prominence, ultimately leading to the ascendance of the dinosaurs. Indeed, paleontologists use the Permo-Triassic event as a dividing line to separate the Paleozoic ("old life") era from the Mesozoic ("middle life") era.

"This big extinction has long been of interest because it was so severe. But it has never been explained satisfactorily," says paleontologist Steven M. Stanley of the Johns Hopkins University of Baltimore.

Now, two scientists provide a possible answer: climate-disrupting outpourings from huge volcanic eruptions in Siberia.

Paul R. Renne of the Institute of Human Origins in Berkeley, Calif., and Asish R. Basu of the University of Rochester in New York used high-precision techniques to pin down the age of vast deposits of volcanic basalts, known as the Siberian Traps, the cover 340,000 square kilometers in the northern Soviet Union. In the July 12 SCIENCE, they report that the massive eruptions occurred within a very short geologic span some 248 million years ago -- coincident with the widespread Permo-Triassic extinctions.

In previous work, Soviet researchers had dated the eruptions to well after the extinctions, precluding any relationship between the two. But Renne and Basu used a more sophisticated process, called argon-40/argon-39 dating. This technique determines age by calculating how much radioactive potassium has decayed to argon over millions of years.

"We have really nailed it down very very precisely. It is Permo-Triassic age," says Basu. The finding reopens the possibility that the eruptions caused the extensive extinctions.

The Siberian Traps, which rank among the world's largest eruptions, spewed out roughly 1.5 million times as much rock as the Mount St. Helens blast of 1981. Gases lofted into the atmosphere by the Siberian outpourings could have devastated life at the time by altering the global climate, says Basu.

While the new work points to the Siberian Traps as a prime suspect, the biggest murder mystery in Earth's history will not yield to a solution overnight. Scientists continue to debate the age of the Siberian Traps. In the May GEOLOGY, Ajoy K. Baksi of the Louisiana State University in Baton Rouge and a co-worker report using the argon technique to date the eruptions to 238 million years ago. If correct, that would rule out the Traps as the cause of the die-offs.

However, says Baksi, "the geologic pedigree of my samples is a little suspect," because he received them third-hand, years after Soviet researchers collected them. He is now checking his results for any analytical errors. Researchers with the U.S. Geological Survey are currently using the argon method to date yet another set of samples from the Traps.

The topic of eruptions and extinctions heated up this spring when scientists reported new evidence linking India's Deccan Traps with the infamous Cretaceous extinctions. Geologists have spent the last decade arguing whether that die-off resulted from a meteorite impact or from volcanic eruptions, but the latest findings suggest that both types of disaster may have hit at the same time in a double whammy of bad luck.

At the spring meeting of the American Geophysical Union in Baltimore, researchers from the USGS and Oregon State University presented argon-dating results indicating that the Deccan Traps erupted over a very short geologic span about 65 million years ago, emitting a substantial burst at roughly the same time as the purported meteorite impact.