Scientists Wednesday published new evidence to bolster a controversial theory that dinosaurs went extinct because of a global firestorm sparked by the impact of a giant meteorite.

An international team of researchers, writing in the British journal Nature, said they found certain clays dating back about 65 million years - the era when dinosaurs died off - contain 100 to 10,000 times as much soot as should be expected.In addition, soot with the same composition has been unearthed at the same geologic levels at sites in Europe, and researchers say the findings lend support to a global fire.

But the strongest piece of evidence added to the researchers' theory, first presented in 1985, is the discovery of the rare metal iridium in the same seam of rock where the soot was found, near Woodside Creek, New Zealand. Iridium is considered a sign of meteorites because the metal is rare on Earth but relatively abundant in meteorites.

Skeptics of the firestorm theory have argued a meteorite impact would be unlikely to start a massive fire, because living trees do not burn well. Instead, they have speculated the forests were killed by the darkness and cold caused by dust following the impact, with the dead wood ignited by lightning well after the dust settled and the skies cleared.

But the finding of iridium and soot together shows that "the fire started well before all the ejecta (dust) settled," wrote the research team, composed of scientists from the University of Chicago and Los Alamos National Laboratory in New Mexico, as well as Switzerland and New Zealand.

The scientists said, "Perhaps the most likely possibility is that the trees were killed and dried by the (meteorite) impact, both by the prompt heating of the atmosphere and strong wind capable of flattening forests . . . and subsequent heating by the ejecta plume and hot fallout."

Soot from the ensuing fire, the researchers say, would absorb sunlight more effectively and would settle more slowly than rock dust from a meteorite, generating the darkness and cold thought to have contributed to the demise of dinosaurs.

Some critics claim the firestorm theory fails to account for variations in the extinction patterns of various dinosaurs and other creatures at the end of the Cretaceous period.

But in the Nature article, the researchers said a global firestorm would generate poisonous gases, such as carbon monoxide, and slower-acting organic toxins. That "may help explain the selectivity of extinction patterns . . . as different species often have very different tolerances for chemical toxins," they wrote.