A comet or some other extraterrestrial object appears to have slammed into northern Canada 12,900 years ago and triggered an abrupt and catastrophic climate change that wiped out the mammoths and many other prehistoric creatures, according to a team of U.S. scientists.

Evidence of the ecological disaster exists in a thin layer of sediment that has been found from Alberta to New Mexico, say the researchers, whose work adds a dramatic and provocative twist to the decades-old debate about the demise of the mammoths, mastodons and sloths that once roamed North America.

The sediment layer contains high concentrations of iridium, fullerenes and other compounds associated with space rocks and impacts, says Luann Becker, a geologist at the University of California, Santa Barbara, who has been analyzing the sediments.

"We have evidence for distribution of impact debris over several thousands of miles over the North American continent," says Becker, whose group will detail the findings at the American Geophysical Union meeting May 24.

The sediment layer formed 12,900 years ago coincides with both the extinction of the animals and the onset of a mini-ice age that lasted more than 1,000 years, say Becker and her colleagues from several U.S. universities and research labs. They say they are increasingly convinced the impact, extinction and cold snap are all related.

According to their scenario, a comet or large meteoroid generated a shock wave and threw massive amounts of debris, heat and gas into the atmosphere. This set off wildfires that raced across grasslands in southern North America, depriving the mammoths and other grazing animals of food.

The impact and heat also destabilized the ice sheet that blanketed Canada at the time, creating a flood of melt water that poured into the North Atlantic, according to their theory. The pulse of fresh water then shut down the ocean currents carrying heat from the tropics to the Northern Hemisphere, leading to an abrupt cooling. The resulting "mini-ice age" in the Northern Hemisphere, known as the Younger Dryas, lasted more than a thousand years.

Until now, one of the leading explanations for the disappearance of the mammoths and other animals is that they were hunted to extinction by the people who arrived in North America from Asia at least 13,000 years ago.

But Becker and her colleagues doubt there could have been enough people to drive the creatures to extinction with spears. "It would have been a real challenge to slaughter all the animals," she says.