WHEN LIFE NEARLY DIED

By J. MADELEINE NASH/WASHINGTON

Ancient volcanoes begin spewing smoke and lava across the plains of Siberia and don't stop for a million years. The sky all around the earth becomes dusty and foul. Eventually the planet is rendered so wretchedly uninhabitable that life itself is nearly snuffed out.

Science fiction? The apocalyptic vision of a doomsday cult? No, this disastrous scenario came last week from the sober pages of the journal Science. A team of U.S. and Chinese researchers studying the remains of volcanoes that began erupting 250 million years ago reported that according to radioactive dating, the eruptions coincided in geologic time with one of the great unexplained cataclysms in earth's history--a mass extinction at the end of the Permian period that wiped out up to 95% of all ocean-dwelling species and at least 70% of land-dwelling vertebrates.

The Great Dying, paleontologists call it, and with good reason. No event in the fossil record--not even the catastrophe that would kill off the dinosaurs 185 million years later--was more devastating or left a greater mark on the history of life. Not just a few laggard species, but entire communities of plants and animals, even hardy insects, suddenly vanished. Among the casualties: coral reefs and all their inhabitants, dense forests of fernlike trees, giant amphibians and pred atory reptiles, and the last of the trilobites, those hard-shelled marine invertebrates with complex eyes that once dominated the prehistoric oceans.

What could have caused such indiscriminate carnage? Marauding comets, exploding stars, greenhouse warming, ice-age cooling, sea-level drops, sea-level rises, ocean stagnation, oxygen depletion--every calamity imaginable has been invoked to explain the Permian extinction. But none of these agents of doom, argues geologist Paul Renne, director of the Berkeley Geochron ol ogy Center in California (and lead author of the Science article), comes as close to explaining what happened at the end of the Permian as the rampant, prolonged volcanism that created the terrace-like formations known as the Siberian Traps.

According to Renne, these traps (from trappa, the Swedish word for stairs) are composed mainly of glassy basalt, laid down by huge rivers of flowing lava. But amid the basalt, which extends across an area of a million square miles, scientists have also found telltale pieces of tuff, a type of rock indicative of powerful explosions. What this means, says Renne, is that the volcanoes could have easily hurled sulfur dioxide and other gases high enough into the atmosphere to block sunlight and cause substantial cooling. And if the earth cooled enough--locking up more and more water in polar ice--the sea levels would have plummeted.

But the Siberian eruptions could have killed off plant and animal life in half a dozen different ways. An atmospheric mist of sulfur dioxide, for example, could have stoked lethal storms of acid rain. Carbon dioxide, injected into the atmosphere by erupting volcanoes, could have trapped solar heat, disrupting climate through global warming. Even the physical force exerted by the rising plume of molten magma could have contributed to the extinction by uplifting a substantial section of the earth's crust. Since temperatures fall with elevation, says Renne, snow and ice would have quickly accumulated, wrecking ecosystems at higher elevations and contributing to the drop in sea level.

The problem with these theories, observed scientists attending a workshop on the Permian extinction held by the Smithsonian Institution last week, is that they are still highly speculative. For instance, the geological record clearly shows that a staggering drop in sea level-perhaps as much as 300 ft.--did in fact occur during the Permian. But there is no evidence that global cooling triggered by volcanism was the cause. Similarly, new analyses of late Permian soils suggest that a substantial surge of acid rain accompanied the extinction. Acid rain, however, does not require a volcanic source. It could also have been caused by changes in atmospheric chemistry after the impact of a big comet or meteorite.

As Smithsonian paleobiologist Douglas Erwin warned his colleagues last week, it is dangerous to try to explain a complex calamity like the Permian extinction in simplistic terms. The Great Dying, Erwin believes, was produced by an interplay of many forces--"a tangled web rather than a single mechanism"--and if paleontologists and geologists want to sort out the puzzle, they must spend long hours in the field searching for further clues. Even after scientists reach a consensus about what caused the extinction, observes Renne, a central mystery will remain. What is it about life, he marvels, that enables it to rebound with such extraordinary resilience from the brink of so catastrophic a collapse?