Comet Fire

By Natalie Angier

The world may someday end with a whimper, but evidence is mounting that the dinosaurs went out with a bang. According to the much debated theory proposed by the father-son team Luis and Walter Alvarez in 1980, an asteroid or comet slammed into the earth at the end of the Cretaceous period, 65 million years ago, spewing so much dust into the atmosphere that sunlight was blocked for months. Temperatures plummeted, plants withered, and many species, including the mighty dinosaurs, perished en masse.

Now chemists at the University of Chicago have added an important new twist to this version of the apocalypse. Edward Anders and his colleagues reported in the journal Science last week that they had found evidence of a global firestorm that raged about the time the dinosaurs disappeared. The conflagration, they say, suggests that the consequences of a great Cretaceous impact were even worse than the Alvarezes had dreamed.

Like many other spectacular discoveries, Anders' finding was serendipitous. He and his co-workers had simply hoped to elaborate on the Alvarez hypothesis by detecting trace amounts of rare noble gases, like neon and xenon, in the layer of Cretaceous clay deposited during roughly the same period that the dinosaurs became extinct. They were seeking to identify the nature of the object responsible for the impact. Because noble gases collect in carbon particles, the scientists isolated the carbon in Cretaceous sediment taken from Denmark, Spain and New Zealand. To their surprise, all three samples contained carbon that had been deposited at a rate 10,000 times as great as carbon in the layers immediately above and below them. It was bunched together in the fluffy patterns characteristic of common soot. Says Anders: "It's like the stuff you see in the flame of a candle." He believes that the soot almost certainly is a remnant of vegetation consumed by fires.

Disaster began when the celestial intruder crashed into what is now the Bering Sea, possibly creating a crater some 100 miles wide. The stupefying force of the impact, estimated at 100 million megatons, would have generated an enormous 3,000 degrees F fireball that would have spread outward at the speed of sound, igniting forest fires from North America to Asia. Several hundred billion tons of plants and animals would have been incinerated, sending great scarves of black smoke to join the impact dust in the stratosphere and circulate around the globe. What is more, because soot does not rain out as easily as dust, the protonuclear winter would have lasted much longer than it would through obscuring dust alone. Most plants and large animals that survived the blast, the fire and the lethal clouds of carbon monoxide would have succumbed to the climatic changes. But smaller creatures could have slipped into caves and hibernated until sunlight returned and they emerged to repopulate the earth.

With reporting by Robert Buderi/San Francisco