Comet finished off North American big game animals, cooled the planet?

Woolly mammoth.

Fires from a comet shower may have killed off the big game in North America about 13,000 years ago, scientists say. Those extinctions have long been attributed either to an abrupt return to glaciation known as the Younger Dryas event or to over-hunting. But now, a team of scientists has found tiny, nanoparticle-sized diamonds in sediments from the Younger Dryas — signs that a cosmic impact may have ultimately been behind both the cold period and the extinctions.

About 12,900 years ago, a world that had been warming steadily as it climbed out of the last ice age suddenly cooled into a new ice age that lasted 1,500 years. Mammoths, horses, camels and ground sloths that had inhabited the forests and grasslands as glaciers from the last ice age retreated started dying off en masse. Populations of the early North Americans waned.

Causes of the Younger Dryas and the subsequent extinctions are debated, but explanations abound. Increased glaciation would have placed pressure on people and animals. Additionally, some researchers suggest that human hunting of big game animals might have finished them off. The prevailing explanation for the actual cooling has focused on Lake Agassiz — a glacial melt lake that may have dumped enough fresh water in the Atlantic to change its thermohaline circulation, possibly altering climate patterns. But these explanations have not entirely satisfied the data, according to Douglas Kennett, an archaeologist at the University of Oregon in Eugene and his colleagues.

The finding of nanodiamonds in Younger Dryas-aged sediments in sites across North America is just one line of evidence of the impact hypothesis, Kennett and his colleagues say. The team also found higher-than-normal traces of iridium and small magnetic particles near a charred layer, such as would have occurred with an impact-induced fire — glaring evidence of an impact event, they reported today in Science. But the nanodiamonds themselves are significant, Kennett says. “[Nanodiamonds] have been interpreted by impact [scientists] as a definitive marker of impacts,” he says. Diamonds form in the high-pressure, high-temperature environment of Earth’s core, not at the surface. “So we have to come up with some way of distributing them through the atmosphere to get them deposited in concentration in sediments of this age,” he says. “We think the cosmic impact is really the only way you could do that.”

The same friction that may have formed the nanodiamonds would have blasted the atmosphere with multiple-thousand-degree bursts of air, Kennett says, torching forests and grasslands from coast to coast and dooming many animals and people. The heating from the impact could have also melted part of the ice sheet near the Great Lakes, Kennett said in a separate presentation at the American Geophysical Union’s 2008 fall meeting, creating the freshwater run-off often blamed for the Younger Dryas cooling.

But the evidence for the impact just isn’t there, says Christian Koeberl, an impact geologist at the University of Vienna in Austria. For one thing, Koeberl says, the Younger Dryas sediment layer lacks other signs of a cosmic impact — like high-pressure-modified quartz, shattered crustal rocks, or a crater — that appear in sediments dating to more famous impact-driven extinctions such as that at the end of the Cretaceous 65 million years ago, which marked the end of the age of dinosaurs.

Impact-shocked minerals and craters are not linked to just some impacts, but all of them, Koeberl notes. Moreover, he says, “If there was a five-kilometer-diameter comet nucleus … then a body of this size does not just blow up without a trace in the atmosphere — physics won't have it.” Instead, such a comet would slice through the main part of the atmosphere in less than a second, punching out a 50- to 100-kilometer-wide crater that would still be smoldering today.

Koeberl also questions the interpretation of the nanoparticles as diamonds, noting that the electron diffraction patterns used to identify the particles are not unique to diamonds. And even if the particles were diamonds, he adds, there is a general consensus that diamonds are not a “definitive marker” of impacts, but are in fact a very rare phenomenon. To convince him that an impact caused the Younger Dryas, finding a large-enough crater dated to 12,000 years ago “would sure help,” he says, in the same way that finding the Chicxulub crater in the Yucatan Peninsula helped enlighten the end-Cretaceous debate.

One place to look for such a crater might be near the Great Lakes, Kennett says. But as for the mineralogical signs of a cosmic impact, Kennett argues it’s unfair to compare the Younger Dryas with the end-Cretaceous. “This is not the K-T [end-Cretaceous] boundary,” he says. “There are a range of different impacts that could produce very different kinds of signatures in the geological record.”

Brian Fisher Johnson
Friday, January 2, 2009 - 12:30