The extinction of the dinosaurs has long been considered a crime committed by a lone gunman: an incoming asteroid that struck the earth 65 million years ago, filling the air with sun-blocking dust. Now, however, controversy is being stirred anew as evidence suggests that the asteroid might have had a partner in crime: volcanoes, massive ones, blasting clouds of toxic gas from the bowels of the earth and poisoning much of the planet's life.
It was nearly 30 years ago that physicist Luis Alvarez and his son Walter, a geologist, proposed the giant-impact theory of dinosaur extinction. Their evidence was compelling: a thin layer of iridium in the earth's sediment dating to about the time of the die-off. Iridium is rare on Earth but common in asteroids. The iridium layer, mapped by the Alvarezes in scattered sites around the world, suggested an asteroid that vaporized on impact, spreading a cloud throughout the stratosphere. The argument seemed sealed in the 1990s, when geologists realized that a huge crater centered near Chicxulub, Mexico, was almost certainly caused by a giant impact at just the time the extinctions occurred.
So what role would volcanoes have had to play in all this? A big one, argues Gerta Keller, a Princeton University paleontologist, who recently made her case at a meeting of the Geological Society of America. Geologists have known for centuries that a swath of central India was buried by a series of eruptions at around the time of the dinosaurs' demise. The remains of the flows, known as the Deccan Traps, still cover some 193,000 sq. mi. (500,000 sq km). The eruptions would have poured carbon dioxide and sulfur dioxide into the air, triggering runaway global warming and acid rain. That's bad news for living things, but it has never been clear if the eruptions were sufficiently well timed to cause the extinctions.
Keller believes she has evidence that they were, thanks to microscopic fossils of foraminifera, a type of plankton that largely died with the dinosaurs. Precisely dating the dinosaurs' demise is tough because most bones disintegrate before they can be fossilized. Plankton, by contrast, are preserved in ocean sediments. Keller studied sediments near the impact site in Mexico, where a massive bloom of new plankton species should have emerged within a handful of millenniums after the impact, taking advantage of the evolutionary room the extinctions created in the ocean. She found her bloom, all right, but 300,000 years later than expected. She then went to Texas and India and found the same thing.
In Keller's scenario, the Deccan eruptions began perhaps half a million years before the mass extinction. "This leads to greenhouse warming that puts a major stress on the environment," she says. Then came the asteroid impact, which pushed things further toward catastrophe. Finally, 300,000 years later, the eruptions reached their climax, sealing the dinosaurs' fate. "We've shown convincingly," she says, "the mass extinction came about 300,000 years after the asteroid impact."
Keller may be convinced, but others aren't. Sediment samples off the coasts of Senegal, Florida and Antarctica contradict her timeline, suggesting the mass extinction came right after the asteroid impact. "We've got beautiful sediments," says Brian Huber, a curator of paleontology at the Smithsonian Institution. "We have a continuous record of the event." Even 65 million years after the crime, the identity of the real perp is once again in dispute. And with eyewitnesses out of the question, the debate could go on for a while. India 65 million years ago and today
Mass extinctions 65 million years ago enabled larger mammals to take over the land. At the time, India was not yet part of the Asian landmass.
Walk of life A chronology of the first appearance of various life-forms on earth
[This article contains a complex diagram. Please see hardcopy of magazine.]