It's “;very unlikely”; that a meteor or asteroid colliding with the Earth caused an abrupt climate change leading to the extinction of the woolly mammoth and other large mammals 13,000 years ago, says the University of Hawaii at Manoa leader of a team that investigated the theory.

“;It was quite disappointing,”; said Francois Paquay, a doctoral student in the UH Department of Geology and Geophysics. “;It is a conceptually appealing theory. The problem is, we can't explain it.”;

Paquay led an international research team that explored the impact theory for what's known as the Younger Dryas, an abrupt cooling event that coincided 13,000 years ago with extinction of the woolly mammoth, saber-toothed jaguar and many sloths.

The cooling period generally was believed linked to global climate change, possibly resulting from a reduction or slowdown of the ocean circulation in North America.

But two years ago, a research group reported finding high iridium concentrations in sediments from that period and concluded an impact event caused the Younger Dryas because meteors and asteroids contain more iridium than the Earth's crust, Paquay said.

But no impact crater of that age is known and the researchers weren't able to replicate the data used to support the impact theory, he said.

Paquay said his team looked for other tracers of extraterrestrial material in rocks, such as osmium isotopes, and they broadened the search from lake deposits and rocks to high resolution sediment cores across North America. The Integrated Ocean Drilling Program provided sediment samples.

Paquay said “;it would have been wonderful”; to find evidence that an impact wiped out large mammals and created cooling in the northern hemisphere. “;The only definite answer is it was unlikely that an impact killed all these animals.”;

Greg Ravizza, UH-Manoa associate professor of geology and geophysics, said Paquay initiated the study because he thought the impact theory “;was exciting and he really wanted to learn what happened.

“;One of the reasons this (impact) hypothesis got so much attention was it presented a completely new idea about what might have happened.”;

Other theories about the extinction of the mammals, including human hunting and climate change, had been around for a while. “;Most people felt the data were inconclusive,”; Ravizza said.

“;Now, there seems to be limited evidence that supports the idea of an impact. We haven't completely ruled it out but it seems very unlikely, so we have returned to the state we were in before,”; he said.

Paquay's team included American, Belgian and Canadian researchers. Sediments from the same time period were analyzed at UH-Manoa and in Belgium with similar results.

Both marine and terrestrial sediment records support the finding that an impact event wasn't the trigger for the rapid transition in the cold period, Paquay said.

Results of Paquay's team work were published in the journal of the Proceedings of the National Academy of Sciences. The work was supported by the Geological Society of America and the National Science Foundation.

Paquay and his team and will discuss their findings in a special session this week at the American Geophysical Union's fall meeting in San Francisco.

The Younger Dryas, called the “;Big Freeze,”; lasted about 1,300 years, according to scientific reports. One explanation is that it was triggered by a huge outpouring of fresh water from glacial Lake Agassiz into the Atlantic and Arctic waters that caused an abrupt climate change.

Proponents of the impact theory suggested a meteorite may have hit a large ice sheet covering the northern half of North America that led to a big melt water event that flooded into the ocean, Ravizza said.