杏吧原创

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鈥檚 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鈥檚 where the Clovis people鈥檚 doom came from. Citing several lines of evidence, the team suggests that a wayward comet hurtled into Earth鈥檚 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鈥檚 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. 鈥淭he entire continent was on fire,鈥� 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.

鈥淵ou might find some other explanation for these individually,鈥� says Firestone, 鈥渂ut taken together, it鈥檚 pretty clear that there was an impact.鈥� 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 controversy

The team鈥檚 findings will almost certainly stir intense controversy and debate, for many geologists remain sceptical of impact hypotheses in general.

鈥淭here is a tendency in this field to label any circular feature a crater,鈥� 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鈥檚 methods are sound, and finds the case for an extraterrestrial explosion convincing. 鈥淚 think it is a very straightforward case of an impact.鈥�

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鈥檚 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 鈥� 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, 鈥渨hatever hit us was a low-density object鈥� 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 鈥渨ould have been ice-walled and basically melted away at the end of the last ice age鈥�, leaving few traces.

If the team鈥檚 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鈥檚 mammals went extinct 鈥� 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鈥檚 plumbing. Near the end of the last ice age, meltwater from the continent鈥檚 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鈥檚 salinity-driven 鈥渃onveyor belt鈥� 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. 鈥淲hat we suggest is that the meltwater outflow from the proglacial lakes and from the temporarily melting ice sheet was the result of extraterrestrial impact,鈥� 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 鈥淢arks of a comet?鈥�, below), chemist Wendy Wolbach of DePaul University in Chicago has detected significant quantities of soot 鈥� 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 鈥� polycyclic aromatic hydrocarbons 鈥� in samples taken from three of the team鈥檚 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. 鈥淚 don鈥檛 want to sound catastrophic here,鈥� he says, 鈥渂ut this is wild stuff. There is significant evidence of massive biomass burning.鈥�

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. 鈥淚f the number of points are diagnostic of the number of people there, which is a pretty reasonable assumption,鈥� notes West, 鈥渢here was at least a 70 per cent decline鈥� 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鈥檚 sweeping claims. 鈥淚鈥檓 not convinced yet there were [widespread] wildfires,鈥� says Koch. 鈥淏ut if an impact just triggered the Younger Dryas, that in itself is a pretty big issue.鈥�

Gerta Keller, a Princeton University geologist, has similar reservations. 鈥淪ome of the conclusions may be a bit over the top,鈥� she says, particularly the claims of continent-wide fires. Kennett and his colleagues are braced for the critics. 鈥淵ou watch it,鈥� he jokes, 鈥渢here will be blood on the streets.鈥�

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