When Life Nearly Ended

Sept. 12, 2007 — -- There's a place in south China where the Earth tells its own story of the cataclysmic end to most of the animals and plants that had once thrived in its oceans and on its land. For decades now, scientists have probed the steep slopes of the Great Bank of Guizhou, pulling out fossils to see what they can tell us about the largest mass extinction in the planet's history.

Are those long-dead critters talking to us today? Can they tell us anything about the possible consequences of a changing global climate?

For hundreds of millions of years, small animals and plants accumulated in the floor of a giant seabed, forming a massive field of limestone that today provides a rare window into the past. The abundance was great, and the biodiversity was large, until a relatively brief period 250 million years ago. It was at that time, the end of the Permian period and the beginning of the Triassic, that 90 percent of life on Earth was wiped out.

The evidence suggests that the atmosphere was extra rich with carbon dioxide, probably because of volcanic activity, that would have changed the concentrations of ambient gases in the atmosphere and in the oceans. A once hospitable planet became very unfriendly to most of its inhabitants.

To be sure, the situation then was quite different than the global climate change we see today, but scientists who are studying the record in south China are not real comfortable with what they are seeing.

"The parallels to what's going on now are astonishing and a little disquieting," said Jonathan Payne, assistant professor of geological and environmental sciences at Stanford University. "There are many similarities."

But, he quickly added, "the organisms alive today are quite different" from the animals that thrived then. "It's difficult to predict."

For more than five years now, Payne has climbed over sections of the Great Bank. Deposits on the ancient sea floor were laid down horizontally, but tectonic forces have tipped them over on their side, so it is theoretically possible to walk back in time for hundreds of millions of years.

But, it would be a difficult walk. The area of interest to scientists is more than 40 miles long, extending across the face of a forbidding mountain, and rising nearly 2,000 feet above a twisting mountain road.

"It's quite dramatic when you see it," Payne said. "It is an enormous scale. When you stand there, you are an ant looking at a mountain."

But, what's even more impressive is the story that mountain has to tell.

The line that separates the Permian period from the Triassic is so fine, "you can put your fingernail on it," Payne said. The rocks below your fingernail, from the late Permian, would be rich with marine fossils.

"In many areas, the proportion of the rock that's made up of shells of animals and algae is as much as 20 or 30 percent," Payne said. "When you think about the limitations of packing shells together, you realize you can't make it 100 percent of the volume. So, 20 or 30 percent basically means you have shells cheek-to-jowl, and stacked for 50 to 100 meters."

But, above your fingernail — the beginning of the Triassic — would be a very different story.

"Above (the line), you have very low diversity of fossils, very few fossils, and that remains the case for 4 to 5 million years," explained Payne. "We find that extremely intriguing, because decreasing the diversity in the world doesn't necessarily mean you decrease the abundance of animals.

"The few species that survive could proliferate so that there is just as much abundance as before, but less diverse. We actually see evidence that the abundance is much lower after the extinction."

So, the planet had lost many of its species, and those that survived were barely making it.

How long did all that take? It may seem likely that the greatest extinction ever must have taken millions of years. But, the evidence suggests that's not necessarily so.

That fine line in the rock that marks the extinction "probably doesn't represent more than 100,000 years," Payne said. "That's still a very long time scale for humans, but when you think about the geological time scales of hundreds of millions of years, then 100,000 years is nearly instantaneous."

A process that could have taken much longer may have fed upon itself in a runaway series of global changes. Some scientists believe that is what is happening today.

There is yet another story told in the rocks. The sequence at which various species succumbed is very revealing. The first to go were the trilobites, those primitive arthropods that had survived since the Paleozoic.

"There's never been a trilobite found since that extinction," Payne said. "The other groups that were hit hardest were two groups of corals that are distant relatives of modern corals."

Next, came the brachiopods, similar to clams, that stayed on the bottom as filter feeders. Some survived the extinction, and are still around, but they are very rare. Relatives of modern starfish and sea urchins and sea cucumbers, called crinoids, also lost most of their diversity.

"The group that made it through in the best shape were the mollusks, largely clams and snails," Payne said. And they came through big time. "Nowadays, if you go to the beach, they are a very large component in what you are going to find among the shells."

During the few million years after the extinction, some brachiopods and crinoids began to recover, but they never caught up with the mollusks.

Payne theorizes that the mollusks survived because they were more mobile, and, thus, better equipped to make it in a changing world. Life on Earth had survived its greatest challenge, though, perhaps, barely. But, what could have caused such a disaster?

The mass extinction coincided with the largest known volcanic event in the past 600 million years. Volcanoes erupted across Siberia, throwing gases and ash into the atmosphere, and covering an area the size of the United States with basalt nearly four miles high.

Some scientists believe magma intruded into vast fields of coal, sending enormous quantities of carbon dioxide and methane — known as greenhouse gases today — into the atmosphere.

The Earth probably cooled briefly because ash shaded the sun, then warmed. The oceans turned acidic from fallout, making life very difficult, both on land and on sea. It took millions of year for the planet to fully recover.

It's a bit sobering.

Lee Dye is a former science writer for the Los Angeles Times. He now lives in Juneau, Alaska.