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.
Lead team member Richard Firestone, a nuclear analytical chemist at the Lawrence Berkeley National Laboratory in California, says the evidence lies in a narrow 12,900-year-old carbon-rich layer of sediment found at eight well-dated Clovis-era sites and a peppering of sediment cores across North America, as well as one site in Belgium.
In this layer the team detected several different types of extraterrestrial debris, including nanodiamonds that are only ever found on Earth in meteorites; tiny carbon spherules that form when molten droplets cool rapidly in air; and cage-like carbon molecules containing the rare isotope helium-3, far more abundant in the cosmos than on Earth.
"You might find some other explanation for these individually," says Firestone, "but taken together, it's pretty clear that there was an impact." The team says the agent of destruction was probably a comet, since the key sediment layer lacks both the high nickel and iridium levels characteristic of asteroid impacts.
Intense controversyThe team's findings will almost certainly stir intense controversy and debate, for many geologists remain sceptical of impact hypotheses in general.
"There is a tendency in this field to label any circular feature a crater," says geomorphologist Michael Oskin of the University of North Carolina in Chapel Hill. But Asish Basu, a geochemist at the University of Rochester, New York, thinks the team's methods are sound, and finds the case for an extraterrestrial explosion convincing. "I think it is a very straightforward case of an impact."
Exactly where the explosion might have occurred is uncertain, but several clues point to the north of the continent. Levels of the apparent extraterrestrial debris, for example, are highest at the Gainey archaeological site in Michigan, just beyond the southern reach of North America's primary ice sheet 12,900 years ago. Moreover, levels decrease the further you go from Gainey, suggesting that the comet blew up largely over Canada – perhaps over Ontario or the Hudson Bay region.
However, this cosmic wallop does not seem to have left behind any obvious crater. In all probability, says Arizona-based geophysicist and team member Allen West, "whatever hit us was a low-density object" that fragmented as it entered the atmosphere. The disintegrating pieces could then have blown up in a series of massive aerial explosions. Alternatively, some might have crashed into the 3-kilometre-thick ice sheet. West notes that such craters "would have been ice-walled and basically melted away at the end of the last ice age", leaving few traces.
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 spellAt 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 wildfiresMoreover, 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."
Gerta Keller, a Princeton University geologist, has similar reservations. "Some of the conclusions may be a bit over the top," she says, particularly the claims of continent-wide fires. Kennett and his colleagues are braced for the critics. "You watch it," he jokes, "there will be blood on the streets."
Comets and Asteroids - Learn more about the threat to human civilisation in our special report.
From issue 2591 of New Scientist magazine, 22 May 2007, page 28-33
MARKS OF A COMET?
If a comet really did blow up over North America 12,900 years ago, did the blast leave any traces other than microscopic extraterrestrial debris? Proponents have yet to find any, but they speculate that three areas might bear some traces – the Great Lakes, Hudson Bay and the Carolinas in the south-eastern US.
Most geologists believe that the Great Lakes were scoured out by glaciation, but the comet-blast team's principle author, Richard Firestone, notes that there are "four large holes in the lakes which are deeper than Death Valley, so we kind of suspect that pieces of this impact did penetrate them".
The team is also keen to investigate a 400-kilometre-long anomaly on the bottom of Hudson Bay, which they suggest may be part of a crater rim.
Then there are the Carolina Bays, thousands of oval depressions scattered along the Atlantic coast, most of them pointing towards the Great Lakes region. While many researchers are convinced that local winds scooped out these bays, team member Allen West suggests that a shock wave from the comet launched tornado-like winds that carved out the bays. Geomorphologist Michael Oskin of the University of North Carolina, Chapel Hill, flatly dismisses the idea. "In my opinion, the shock wave idea is grasping at straws – searching for an extraordinary origin for what are in fact quite ordinary [wind] features."