Posted: Tuesday, November 21, 2006 5:41 PM

by Alan Boyle

George Wang / UW

A depiction of present-day Earth overlaid with simulated atmospheric oxygen of the early Triassic period. Because oxygen was low even at sea level, animals would have been restricted to very low altitudes, green or light-shaded areas. Red or dark-shaded areas are higher elevations where many animals could not have found sufficient oxygen and so could not have lived or even traversed.

What was the murder weapon for our planet’s biggest die-off? The Permian-Triassic extinction is the ultimate cold case, transpiring 250 million years ago. But some scientific sleuths have sketched out a scenario worthy of the trickiest mystery novel - involving a chain of volcanic eruptions, greenhouse-gas emissions, oxygen-deficient oceans, sulfur-loving bacteria and poisonous hydrogen sulfide, the compound that smells like rotten eggs. There are even those who would throw in an asteroid for good measure. The rotten-extinction theory gets a prime-time airing tonight on PBS’ “Nova ScienceNow.”

The event, which came at the transition between the Permian and Triassic geologic ages, ranks as the most severe of Earth's five mass extinctions (many people say we're in the midst of the sixth extinction). Based on the fossil record, more than 90 percent of marine species died out, along with about 70 percent of all land-based species.

Among the biggest losers were the mammal-like reptiles, which were thought to have given rise to mammals like us. The biggest winners were the dinosaurs, which came into their own during the Triassic and hung around until the Cretaceous-Tertiary extinction wiped them out 65 million years ago.

Most scientists have come to the view that the dinosaurs were doomed by the catastrophic impact of an asteroid or comet that blasted into Mexico's Yucatan Peninsula, though that claim is not beyond debate, and other factors may have been at work as well. The prime cause behind Permian-Triassic event is even more difficult to identify, though most suspect it had something to do with a million-year-long stretch of activity in a huge volcano field known as the Siberian Traps.

The timing was right for the Siberian Traps to play a role, and the volcanoes surely belched out enough carbon dioxide and other greenhouse gases to have an effect on climate. But the consensus is that the greenhouse effect by itself wouldn't explain the depth of the die-off.

This is the mystery that "Nova ScienceNow" traces in flashy TV fashion. For years, researchers such as the University of Washington's Peter Ward have been documenting the Permian-Triassic effect on land species, and looking for a climate-related mechanism that could deliver the killing blow.

"He was looking for a more potent killer," said Lee Kump, a geoscientist at Penn State University, "and we provided that to him."

Kump's research focuses on the tug of war between oxygen and hydrogen sulfide in the world's oceans. "One's going to win, and one's going to lose," he explained. "It's a fight to the death."

Right now, the world's oceans are well-oxygenated, and sulfide-producing marine bacteria are consigned to dismal dead zones where the water has gone anoxic. But Kump and his colleagues believe the situation was reversed at the end of the Permian era: Climate change warmed up the oceans so much that they couldn't retain oxygen, and most marine organisms that needed oxygen died out.

The sulfur-based ecosphere took over - so much so that toxic hydrogen sulfide bubbled up from the oceans into the atmosphere, killing off land species as well.

"What really looks like a universal way that this has happened is this global warming, leading to this terrible gas chamber atmosphere, killing off life in the ocean and land," Ward says during the "Nova" program. "It's not so much stuff from space that gets you, it’s your own planet."

Computer simulations of Permian climate change appear to confirm this scenario, Kump said.

"Probably the smoking gun for the role of hydrogen sulfide in this mass extinction was the discovery of particular traces of ancient bacterial life in China and Australia from this Permo-Triassic layer," he told me. Such bacteria would need light as well as hydrogen sulfide to survive, he said.

It would take millennia for the sulfurous sickness to run its course, and for Earth's oceans to bounce back to their oxygenated state.

Could it happen again, in an age when CO2-driven climate change is a concern?

"One of the concerns about global warming is that it will affect that tenuous balance of ocean oxygen," Kump said. He subscribes to the view that there's a critical threshold of carbon dioxide levels, and that passing the threshold can set off a cascade of unpleasant environmental changes.

"All sorts of positive feedbacks come into play once this threshold has been exceeded," he said. "Those feedbacks are lurking there. That's the problem with positive feedbacks, you don't know if they're going to kick in until it's almost too late."

So have scientists finally nailed down the case? Kump, Ward and many others are pretty confident about the "sick Earth" scenario, but other investigators aren't so sure. Luann Becker, an earth scientist at the University of California at Santa Barbara, said even a climate catastrophe wouldn't have been enough to push the Permian ecosystem over the edge.

"It just simply cannot be done without having more than one catastrophe happening at the same time," she told me today.

Becker has proposed that a cosmic collision, perhaps occurring off the coast of Australia, provided an additional push. Other researchers point to a possible Antarctic impact as the culprit.

A massive asteroid impact could have set off a global chain reaction, exacerbating the climate crisis and sparking continued volcanic activity, Becker said.

She said she's working on yet another TV show about the Permian-Triassic extinction - a documentary that will be part of the National Geographic's "Earth Shocks" series. Her perspective on the puzzle will be that it takes more than one long stretch of global warming to deliver a Permian-sized death blow: It also takes a space rock. 

"We're going to revisit the idea that the catastrophes at the end of the Permian and the end of the Cretaceous are things that were somewhat unique - but how can you call it a coincidence when it's happened more than once?" she said. "We have volcanism all the time, and yes, volcanoes have their effects. But we never see extinctions like the extinctions we're talking about, unless the two are coming together."

So don't close the file on the Permian-Triassic murder mystery just yet. For more background on the case, check out our "Earth's Timeline" interactive dossier.

"Nova ScienceNow" is offering much more in tonight's episode, including a look at the controversial effort to reconstruct the 1918 pandemic flu virus, the rise of sociable robots (featuring MIT's Cynthia Breazeal) and a tale of papyrus scavenging. For Webheads, the coolest part is that all these segments will be watchable online after tonight's premiere.