ICE AGE IDEA SURE TO MAKE AN IMPACT ON SCIENTISTS.

Byline: Greg Bolt The Register-Guard

For more than 200 years, scientists have debated the cause of a sudden and catastrophic change in climate 13,000 years ago that sent the thawing Earth abruptly back into an Ice Age that lasted a thousand years and ended as quickly as it began.

Now, two scientists at the University of Oregon are part of a team that is proposing a radical new theory for the sudden freeze, a deadly event that killed off most of the large mammals in North America and set early human expansion back by a millennium. Their theory: An extraterrestrial intruder, possibly a comet, may have hit near the Great Lakes and triggered continent-spanning wildfires followed by a kind of nuclear winter.

It's a startling idea, and one that's almost certain to spawn the same kind of controversy that greeted the first scientist to propose a similar theory, that a meteor impact about 65 million years ago finished off the dinosaurs.

"It blows me away," said UO archaeologist Jon Erlandson, who with university colleague Doug Kennett are on the 26-member team offering the theory. Kennett presented the first public paper on the idea Wednesday at the American Geophysical Union's joint assembly in Acapulco, Mexico.

Although cautious to say it's only a theory and that much work lies ahead - the team has not yet published any findings in a peer-reviewed journal - Erlandson called it one of the most intriguing ideas he's come across. And he's bracing for the reaction.

"Because we all started out skeptical, I think we understand the skepticism and controversy that will come with this," he said. "I'm actually trying not to get too invested in the theory because I don't know what the outcome will be."

The key will be putting together a convincing argument that something from outer space hurtled through the atmosphere on a collision course with an Earth just emerging from the last glacial period of the Pleistocene Ice Age.

Unlike the dinosaur killer, though, this bullet left no crater. But it might have left marks. And like the meteor, it seems to have left telltale traces hidden in a layer of soil dating back precisely to the start of the hemispheric cooling event.

The cause of that sudden cooling, a period known as the Younger Dryas Event, has never been explained to scientists' satisfaction. But its effects were enormous: It marked the end of mammoths, mastodons and other big animals in North America, forever changing it from a continent with as many large mammals as modern Africa to one dominated by lesser animals.

Humans, too, would have been seriously affected. People had only recently begun to spread to the continent, and the return of the glaciers and aftereffects of an impact likely would have set the early migration reeling.

At the time, most of North America down to what is now the northern tier of the United States lay beneath a massive but melting blanket of ice, known as the Laurentide ice sheet. Assuming the theory is correct, the comet would have slammed into the ice at unimaginable speed.

"The theory at this point - and it's only a theory - is that a comet may have hit the ice sheet and probably caused an enormous shock wave and heat wave that would have triggered wildfires, possibly continental in scale, and then thrown up a great deal of debris," Erlandson said.

All life over hundreds of square miles would have been destroyed instantly, and the shock wave and fires might have spread from coast to coast. Debris would have been thrown into the atmosphere and, along with the soot from massive fires, would have blanketed at least the Northern Hemisphere, if not the planet, in a cloud that could have produced a "year without summer."

Also, Erlandson said, the impact would have sent incalculable amounts of melted ice water into the North Atlantic, shutting down the system of currents that warm northern latitudes by bringing water from the Gulf of Mexico. That would have brought a return of the just-departed Ice Age.

The combination would have disrupted the food chain that large animals depend on for survival. Hunting by the remaining humans and smaller predators, desperate for food, might have finished off the bigger beasts, the theory suggests.

Then, after about 1,000 years, the climate suddenly returned to the warm, interglacial period that continues today.

So why blame it all on a comet? First, there's the soil. The team has documented some 50 sites from the Channel Islands off Southern California to the Eastern Seaboard where there's a layer of soil full of ashes and soot that dates to the start of the big cool-down.

But it also contains more curious bits: tiny spheres of metal and carbon, the rare element iridium and the odd-shaped carbon molecules known as fullerenes, all things best explained as the products of a massive impact.

"What they think they're finding are signatures that absolutely do not form on Earth under normal conditions," Erlandson said.

When Kennett first broached the theory, he asked Erlandson if he had any soil samples from that time period from excavations he'd done a decade earlier in the Channel Islands. Erlandson did, and after finding the impact debris in those, he went back for more and saw the evidence with his own eyes.

And then there are the geologic features known as the Carolina bays, a half-million oval-shape ponds and lakes that dot the Eastern Seaboard from New Jersey to the Carolinas, all aligned on a northwest-to-southeast axis that points directly at the Great Lakes region. The team believes they were dug by debris falling back to Earth after the comet's impact.

But given his own reaction to the theory the first time Kennett described it, Erlandson said he doesn't blame anyone who finds the idea tough to swallow.

"My initial reaction was one of complete disbelief," he said. "It's like, `Yeah, right. A comet hit the Earth in North America 13,000 years ago and nobody ever noticed it before.' It was very hard to imagine."

That's what Don Grayson said. Grayson, an archaeology professor at the University of Washington who's not involved in the research, is a leading expert on the disappearance of large mammals, or megafauna, at end of the Ice Age. He has a lot of doubts, but he's not ready to dismiss the idea.

"It's hard for me to understand how it might explain all the things they're saying that it might explain," he said.

For Erlandson and Kennett, the research is just too compelling, too mind-boggling, to ignore. "All I know is something really interesting happened about 13,000 years ago," Erlandson said. "I'd like to know what it is."