What caused the extinction of mammoths and the decline of Stone Age people about 13,000 years ago remains hotly debated. Overhunting by Paleoindians, climate change and disease lead the list of probable causes.
     A team of international researchers reports evidence that a comet or low-density object barreling toward Earth exploded in the upper atmosphere and triggered a devastating swath of destruction that wiped out most of the large animals, their habitat and humans of that period. "The detonation either fried them or compressed them because of the shock wave," said Ted Bunch, NAU adjunct professor of geology and former NASA researcher who specializes in impact craters. "It was a mini nuclear winter." Bunch and Jim Wittke, a geologic materials analyst at Northern Arizona University, are co-authors of the paper, which fingers an extraterrestrial impact 12,900 years ago for the mass extinctions at the end of the Ice Age. The paper was just released online in the Proceedings of the National Academy of Sciences.
     No one has found a giant crater in the Earth that could attest to such a cataclysmic impact 13,000 years ago, but the research team offers evidence of a comet, two and a half to three miles in diameter, that detonated 30 to 60 miles above the earth, triggering a massive shockwave, firestorms and a subsequent drastic cooling effect across most of North America and northern Europe. "The comet may have broken up into smaller pieces as it neared the Earth and then these pieces detonated in various places above two continents," Bunch said.
The evidence for multiple detonations comes from a four-inch-thick 'black mat' of carbon-rich material that appears as far north as Canada, Greenland and Europe to as far south as the Channel Islands off the coast of California and eastward to the Carolinas.
     Evidence of mammoths and other megafauna and early human hunters, known as the Clovis culture, are found beneath the black mat but are missing entirely within or above it. This led the research team to conclude an extraterrestrial impact wiped out many of the inhabitants of the Late Pleistocene. Bunch notes that some animals may have survived in protected niches.
     The black mat was formed by ponding of water and algal blooms and contains carbon, soot and glassy carbon—remnants of burned materials. Some of these remnants are extraterrestrial in nature. The black mat also has turned up nanodiamonds, which are formed in the interstellar medium outside the solar system, by or by a high-explosive detonation. The magnitude of the detonations would have been huge. "A hydrogen bomb is the equivalent of about 100 to 1,000 megatons," Bunch said. "The detonations we're talking about would be about 10 million megatons."
     The research team believes the detonations destabilized a vast ice sheet, known as the Laurentide Ice Sheet, that covered most of what was then Canada and the northern United States. Heat from the detonation and firestorms would have melted much of the ice sheet, releasing water vapor into the atmosphere. "The result was rapid cooling of about eight degrees over the next 100 years," Bunch said. The melting of the ice sheet and subsequent climate change would explain the water-based nature of the black mat. Bunch says impact airbursts may be more common than previously thought with possibly two or three such events having occurred over the last 100,000 years. And more are sure to follow.

Source: EurekAlert! (24 September 2007)