The impact would have destabilized an ice sheet over North America, which allowed large bodies of fresh water to drain into the ocean and alter circulation patterns, he explained.
The change in currents included a temporary halt to one that brings warm, tropical waters to the North Atlantic.
Scientists have long debated whether the cooling event or human hunters are responsible for the extinction of the so-called North American megafauna.
At least 17 species are thought to have gone extinct around the time of the proposed impact, according to the researchers.
(Read related story: "Humans to Blame for Ice Age Extinctions, Study Says" [August 10, 2005].)
"We find it rather unbelievable that human hunters would drive a fairly large variety of animals into extinction fairly suddenly at 13,000 years ago," Douglas Kennett, an archaeologist at the University of Oregon in Eugene, said in a telephone interview.
Douglas is James Kennett's son and a project team member.
"We think a better explanation is this dramatic [impact] event, which would be coupled with the fallout and major changes in climate and environments that would have associated with it," he continued.
According to Douglas, archaeological evidence suggests Clovis populations, which were spread out across the Americas prior to 12,900 years ago, became fragmented.
The archaeological record shows a time gap between the presence of Clovis people and later cultures, particularly in the Great Lakes region where the comet is thought to have struck, he added.
"The basic story seems to be, there's more of a gap in the East and less of a gap in the West," he said.
David Meltzer is an archaeologist at Southern Methodist University in Dallas, Texas, who is not part of the research team. He said the theory is far from proven.
First, he said, the team must prove a comet did in fact hit Earth 12,900 years ago, an issue that geologists will eventually resolve.
Then, if an impact is demonstrated, the team has to show what the effects were.
"At the moment, the issues are far more complicated than all animals died at once and people suffered tremendously," he said.
For example, some of the big animals went extinct well before the proposed impact, and others disappear later.
Nor does Meltzer see evidence for the disappearance of Clovis populations.
"At least out on the [Great] Plains, populations are booming [at the time of impact], they're not declining at all despite this horrific global conflagration," he said.
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