Once, dinosaurs had the run of the planet. Then, 65 million years ago, they disappeared. Scientists have been puzzling over dinosaurs' sudden demise ever since.
Now, a team of investigators from the University of Chicago offers an explanation: In a paper published today in the journal Science, the researchers say they have found evidence of a worldwide fire that may have caused the mass extinction at the end of a geologic era known as the Cretaceous Period. This 50-million-year period began 110 million years ago.
They also claim their findings suggest that nuclear-winter studies may be too optimistic. These studies hypothesize that fires from nuclear explosions would throw tons of soot in the air, blocking sunlight, and resulting in a catastrophic drop in global temperatures.
``Nature has left us a record of an experiment we could never duplicate,'' says Chicago chemist Edward Anders, who led the research effort. ``It can serve as a guide to predicting future disasters.''
The Chicago findings are only the latest of several scientific guesses that attempt to explain the disappearance of the dinosaur. But the current study reinforces a widely held theory that the mass extinctions were caused by the impact of a huge meteorite. Previous theories held that the impact of a meteorite -- perhaps some five miles across -- cast huge amounts of dust into the atmosphere, killing plants and animals by blocking the sun's rays for months.
The theory has leagues of detractors. They question, among other things, whether even vast amounts of dust could absorb enough of the sun's light to disrupt Earth's climate on a global scale.
But the Chicago team, which included two graduate students, says it found evidence of a worldwide layer of soot -- actually carbon -- in Cretaceous Period soil. They say those soot particles can only be produced in a flame or hot gas: in other words, continent-sized wildfires ignited by the impact of a large meteorite.
Such a scenario suggests the meteorite brought even greater destruction then previously imagined, Dr. Anders explained in a telephone interview. The wildfires would destroy anything in their way. Because soot absorbs sunlight much more efficiently then dust, it would exacerbate the darkening and cooling of the planet. In addition, the fires would pump vast amounts of poisonous gases into the air.
A key to the Chicago study, and what may be its most controversial aspect, is an estimate of how much soot the wildfires may have produced. Many nuclear-winter scenarios project that about 0.27 to 0.60 percent of burning materials are transformed to soot. But the Chicago data suggest that the soot produced in the Cretaceous fires amounts to 10 percent of the burning mass.
``The soot production by these large wildfires may have been 20 times more efficient then assumed by nuclear-winter studies,'' Anders says.
Few scientists expect this to be the last word on the subject. ``The most interesting papers help steer the scientific debate, not answer all the questions,'' says Carl Turekian, a Yale University geophysicist who has read the paper and believes it may belong in the former category. Dr. Turekian says he doubts the ``fantastically efficient conversion of combustibles into soot'' that the Anders team predicts. ``But if it's true, it's going to be enormously significant.''