Scientists at the University of Chicago have found evidence that continent-sized firestorms raged across much of the Earth 65 million years ago, blackening the skies with soot and helping trigger a sudden global freeze that wiped out the dinosaurs.

The catastrophic event occurred at the time that other scientists have been saying a giant asteroid collided with the Earth. Such a collision would have generated enough heat to start the fires.

It was the asteroid theory that led researchers to ask whether a nuclear war could trigger a similar freeze, a phenomenon now known as nuclear winter. The new findings suggest that nuclear winter theorists have greatly underestimated the amount of soot that would enter the atmosphere from wildfires.

The researchers have found some of the soot, which eventually settled to the ground, in the same geologic layers that gave evidence five years ago of the asteroid impact. The soot is in the form of fluffy, black particles known to be produced by forest fires and the burning of fossil fuels.

The layer, sampled in regions as far apart as Europe and New Zealand, contains an amount of soot, which is pure carbon, equal to about 10 percent of the carbon now incorporated into all forms of life on the Earth. Nearly all molecules that make up living things contain carbon atoms.

The original impact theory held that an asteroid, probably at least six miles wide, slammed into the Earth, blasting enough rock dust into the atmosphere to darken the sky for weeks. Deprived of sunlight, the ground would have cooled, bringing on a global freeze that exterminated not only the dinosaurs but many other species all over a world that had been largely tropical.

Paleontologists long have known that the dinosaurs died out in the most wide-ranging mass extinction the Earth has ever seen. The worst hit region was between the Ural Mountains of central Eurasia and the Rocky Mountains of North America, where 69 percent of known fossil species died out.

Scientists working on the asteroid impact theory already had calculated that airborne rock dust alone would have been thick enough to bring on a devastating freeze.

The discovery that vast quantities of soot may also have been injected into the upper atmosphere indicates that the period of darkness and freezing would have lasted longer, perhaps months, although it would have been no darker or colder than originally thought. This is because soot washes out of the atmosphere more slowly than does rock dust.

The finding adds two more factors that could have contributed to the extinction: concentrations of fire-produced toxic substances, such as carbon monoxide, and the destruction of plants and animals by fire.

The scientists say the fires also would have added enough carbon dioxide to the atmosphere to trigger a so-called greenhouse effect. Once the sun came out again its heat would have been trapped near the ground, raising average world temperatures by an estimated 10 degrees Fahrenheit above pre-impact temperatures.

The findings by Edward Anders, Wendy S. Wolbach and Roy S. Lewis of the University of Chicago are being published in the Oct. 11 issue of the journal Science.

Anders said the discovery was an accident. He and his colleagues were examining ancient sediments laid down at the time of the impact, looking for traces of gases that had been part of the asteroid, he said. Instead, they found that the sediments contained about 10,000 times as much carbon as would have been normal.

The layer, sampled in Denmark, Spain and New Zealand, is the one, found all over the world, in which five years ago other scientists found unusually high concentrations of iridium, an element that is rare on Earth but abundant in meteorites and asteroids. It is thought that when the asteroid hit the Earth, the impact would have generated enough heat to vaporize it, sending its content of iridium into the atmosphere, where it spread around the world.

Gradually the iridium and the carbon settled to the ground, becoming incorporated in sediments that were forming then.

Anders said the impact would have scattered white-hot particles of rock dust as far as 800 and possibly 1,200 miles. The particles would have been hot enough to ignite forest fires over the entire area.

"Once started," the researchers wrote, "such a fire could spread over an entire continent, and the resulting winds may disperse the soot worldwide."

The impact, other scientists have calculated, would have left a crater 85 miles wide and 20 miles deep. Since no such crater has been found, many scientists have assumed the asteroid hit the ocean, vaporizing the water before it blasted into the sea floor.

Some researchers suspect that the impact was in the Bering Sea between Alaska and Siberia, a point from which the scattered, white-hot rock particles easily could have ignited the forests of North America and Asia, the areas where the extinction was most severe.