The Canadian Comet of DOOM!

From NewScientistSpace:

Did a comet wipe out prehistoric Americans?

The Clovis people of North America, flourishing some 13,000 years ago, had a mastery of stone weaponry that stood them in good stead against the constant threat of large carnivores, such as American lions and giant short-faced bears. It's unlikely, however, that they thought death would come from the sky.

According to results presented by a team of 25 researchers this week at the American Geophysical Union meeting in Acapulco, Mexico, that's where the Clovis people's doom came from. Citing several lines of evidence, the team suggests that a wayward comet hurtled into Earth's atmosphere around 12,900 years ago, fractured into pieces and exploded in giant fireballs. Debris seems to have settled as far afield as Europe.

Jim Kennett, an oceanographer at the University of California, Santa Barbara, and one of the team's three principal investigators, claims immense wildfires scorched North America in the aftermath, killing large populations of mammals and bringing an abrupt end to the Clovis culture. "The entire continent was on fire," he says...


If the team's impact theory holds up under scrutiny it could help explain three mysterious events that coincided around 12,900 years ago.

Cold spell
At this key time, the climate changed abruptly in the northern hemisphere, suddenly cooling in a period known as the Younger Dryas. In addition, the distinctive Clovis culture seems to have vanished in North America, while at least 35 genera of the continent's mammals went extinct – including mammoths, mastodons, camels, ground sloths and horses.

For years, many researchers have chalked up the onset of the Younger Dryas to a major change in North America's plumbing. Near the end of the last ice age, meltwater from the continent's principal ice sheet flooded into proglacial lakes in the centre of North America, and from there drained southward into the Mississippi river.

But by 12,900 years ago, the ice had retreated sufficiently from the northern Atlantic coast to let meltwater rush suddenly eastward. As an estimated 9500 cubic kilometres of fresh water poured into the Atlantic, it switched off the ocean's salinity-driven "conveyor belt" current, shutting down the Gulf Stream that carries heat from the tropics to eastern North America. It was this that triggered the Younger Dryas cooling, say many palaeoclimate experts.

However, some of the comet proponents now propose a different trigger for the cold spell. The massive airbursts over Canada could have destabilised the continental ice sheet, opening new drainage channels to the east. Additionally, dust and debris from the explosions may have darkened the ice, absorbing solar heat and accelerating melting. "What we suggest is that the meltwater outflow from the proglacial lakes and from the temporarily melting ice sheet was the result of extraterrestrial impact," says Kennett.

The comet-strike also offers a third and radical hypothesis for the massive extinction of mammals, which for years palaeontologists have blamed on the sudden Younger Dryas freeze, combined with the hunting prowess of newly arrived Clovis bands. In the 12,900-year-old carbon-rich layer at Murray Springs, Arizona, and in sediment cores taken from the Carolina Bays (see "Marks of a comet?", below), chemist Wendy Wolbach of DePaul University in Chicago has detected significant quantities of soot – a product of the intense heat of wildfires.

Raging wildfires
Moreover, geologist Luanne Becker at the University of California, Santa Barbara, has detected a chemical signature of wildfire – polycyclic aromatic hydrocarbons – in samples taken from three of the team's study sites. Kennett and other team members say this suggests the cometary explosions ignited wildfires that swept across much of southern North America, wiping out large populations of animals. "I don't want to sound catastrophic here," he says, "but this is wild stuff. There is significant evidence of massive biomass burning."

If they are right, the cataclysm could also have devastated bands of Clovis hunters. Archaeologist Al Goodyear of the University of South Carolina, Columbia, reported at the Acapulco meeting that there is indirect evidence of a human disaster in what is now the south-eastern US. Chert points fashioned in the distinctive Clovis style disappear, and a new type of tool appears in the archaeological record: redstone points, judged on stylistic grounds to date from 12,750 years ago. Numbers of Clovis points outnumber redstones by 4 to 1. "If the number of points are diagnostic of the number of people there, which is a pretty reasonable assumption," notes West, "there was at least a 70 per cent decline" in the human population in the region.

Nonetheless, many researchers are likely to greet such apocalyptic scenarios with deep scepticism. Palaeontologist Paul Koch of the University of California, Santa Cruz, says he is intrigued by the new evidence of an impact, but he is far from persuaded by some of the team's sweeping claims. "I'm not convinced yet there were [widespread] wildfires," says Koch. "But if an impact just triggered the Younger Dryas, that in itself is a pretty big issue."


That's kind of staggering. Seventy percent of the people! To say nothing of the ecological devastation.

Who knows what North America might have grown into, if this disaster hadn't struck?