Deadly carbon dioxide.

What killed almost every living animal 250 million years ago? Scientists may have the answer.

Could the bubbles in your Sprite or Coke wipe out most of the animals on Earth? Some scientists think so, and they have a new--well, prehistoric--theory to support it.

Flash back 250 million years, to the Permian period, long before the first dinosaur ever laid tracks: All land on Earth is massed together into one huge, perhaps tropical supercontinent called Pangaea. More vast still, is a single unbroken ocean that covers almost the entire planet. Then, a global catastrophe destroys 90 percent of all animal species on land and sea. Why?

One team of paleontologists (scientists who study prehistoric life through fossils) think they've solved the mystery of the biggest mass extinction of all time: Carbon dioxide ([CO.sub.2]), the gas that makes your soda fizz, could be the culprit. The Earth's huge ocean, they think, belched up enough [CO.sub.2] to kill nearly every animal in sight.

Why is the catastrophe so important in Earth's history? It may have altered evolution's long course, giving way to the rise of dinosaurs and maybe even human ancestors.

ENTER THE CARBON CYCLE

How do scientists come up with a novel theory, a scientifically acceptable idea, on what happened in a lost world eons ago? Sometimes, totally by accident.

Paleontologists Andrew Knoll of Harvard University and John Grotzinger of the Massachusetts Institute of Technology were studying changes in the ocean's chemistry over millions of years. Grotzinger accidentally came upon a photographic slide of an ancient sedimentary rock encrusted with tiny crystals. He recognized that the crystals had formed when the compound calcium carbonate precipitated, or separated, from salty sea water and "grew" on the ocean floor.

Calcium carbonate forms crystals when excessive amounts of carbon dioxide are present in sea water. (The compound also helps form seashells.) So Grotzinger deduced, or reasoned, that the Permian ocean was rich in [CO.sub.2].

Today, carbon dioxide has little chance to collect in Earth's oceans, mainly because constant, circulating currents prevent the build-up of the gas. Ice masses at the north and south poles chill ocean water, making it dense and driving it to the ocean deep. The frigid water flows along the ocean floor toward the equator, where the sun heats it up. Warmer, less-dense water rises, and currents flow back to the poles along the surface.

But in the Permian period, Earth's single ocean may have been an immense stagnant sea, thanks to a long cycle of beach-towel weather 250 million years ago. There was no polar ice to generate ocean currents. Result: The ocean became a huge storage bin for [CO.sub.2].

To find out how so much carbon dioxide amassed back then, the team turned to the carbon cycle (see carbon cycle diagram). Prehistoric marine plants absorbed [CO.sub.2] from the atmosphere through photosynthesis, the process in which plants draw in [CO.sub.2] to make their food. When vegetation died and sank to the ocean bottom, so did the carbon dioxide. For the next 10 million years, [CO.sub.2] slowly loaded up on the ocean floor.

SLOW DEATH

With so much [CO.sub.2] trapped underwater, levels of the gas declined in the atmosphere. (Excessive atmospheric [CO.sub.2] warms the Earth's climate.) Global temperatures started to drop, and ice caps formed in the Northern Hemisphere. Sinking icy water forced up the mass of carbon dioxide close to the ocean's surface where most sea animals lived. Catastrophe struck!

But a few heartier species survived in the sea. That critical fact helped the scientific team test its theory. Using a database of prehistoric critters, Knoll and paleontologist Richard Bambach of the Virginia Polytechnic Institute studied the animals' biology to predict which ones could survive excessive carbon dioxide in their systems.

Bambach figured that passive animals that merely sat on the seafloor--like brachiopods, then the most abundant animals--would die. But more-active sea animals with specialized respiratory system (like gills), could probably expel excess [CO.sub.2] and fare better.

After classifying Permian animal species as active go-getters or passive couch potatoes, the team went back to the database to see which sea animals would likely have died. Of the go-getters, about two-thirds of the species had been wiped out. But the couch potatoes lost more than 96 percent of their species.

What killed the land animals? Prehistoric global warming. When the ocean belched up so much [CO.sub.2], concentration of the gas increased six times in the atmosphere. "What we're worried about today is an increase in temperature between 5 and 10 degrees [Fahrenheit] just by doubling the [CO.sub.2] concentration in the atmosphere," Bambach explains. Imagine what a 600 percent increase of carbon dioxide in the atmosphere did to land animals.

Could the catastrophe happen again? Bambach doubts it. Because of Earth's geology today, it's highly unlikely [CO.sub.2] will wipe out the planet's population. Now that's ages!

RELATED ARTICLE: Lethal Lake

One summer night in 1986, as villagers in Cameroon, Africa, got ready for bed, the normally peaceful Lake Nyos exploded! A fountain spouted 80 meters (260 feet) in the air, releasing a deadly cloud of carbon dioxide from the lake's bottom. The following morning, more than 1,700 people and 8,000 animals in the area were dead.

Some scientists think that massive amounts of carbon dioxide were stockpiled on the lake's bottom when gas seeped in from volcanic vents. (The lake sits on the crater of a dormant volcano.) Others think the carbon dioxide had amassed from decaying plant and animal matter over many years.

But what caused the lake to belch out the gas? Scientists still aren't sure. Did a small volcanic eruption churn up the [CO.sub.2]-rich water? Could it have been a landslide or low-magnitude earthquake?

Either way, the sudden release of carbon dioxide squelched the air's oxygen and suffocated victims within a 16-km (10-mi) radius. Scientists continue to monitor Lake Nyos. Some predict that Nyos will erupt again within 30 years. By then scientists should be able to warn villagers before disaster strikes.