Adapted from J. S. Pigati et al., PNAS Early Edition (2012)

It's in there. The purported markers of an extraterrestrial impact found in a dark layer of sediment at Murray Springs, Arizona (left), also appear in similar yet older layers elsewhere, including Chile's Atacama Desert (right), suggesting the markers are actually formed on Earth by natural processes.

No Love for Comet Wipeout

Sid is a freelance science journalist.

Did a comet wipe out woolly mammoths and an ancient Indian culture almost 13,000 years ago? Geologists have fiercely debated the topic since 2007. Now a new study says an extraterrestrial impact wasn't to blame, though the scientists who originally proposed the impact idea still aren't convinced.

Three unexplained phenomena happened on Earth around 12,900 years ago. An extended cold spell known as the Younger Dryas cooled the world for 1300 years. Large creatures such as mammoths, mastodons, and their predators went extinct. And the Clovis culture—a group defined by the distinctive stone and bone tools that they manufactured, and presumed by many archaeologists to be the first inhabitants of the New World—suddenly disappeared.

In 2007, a team of researchers tried to tie together these seemingly disparate events to a single cause: an extraterrestrial object, possibly a comet, exploded above eastern Canada, they speculated. Their claimed evidence, which has been much disputed since it was first reported, included several types of "impact markers" sometimes found after an extraterrestrial object strikes Earth. These purported markers include unusual grains of a titanium-rich form of the mineral magnetite; tiny magnetic spherules; and elevated levels of iridium, a relatively rare element that's more common in extraterrestrial objects than in Earth's crust. The researchers found all of these markers embedded within unusual layers of dark, organic-rich sediments that scientists often call "black mats." These strata are the remains of ancient marshes and swamps, and at many sites across North America, especially in the American Southwest, black mats began accumulating at the beginning of the Younger Dryas, the researchers noted. Many paleontologists have noted that black mats are often a sort of dividing line between older sediments containing fossils of ice-age megafauna, and younger sediments that don't. And many archaeologists have observed that black mats seem to mark the demise of the Clovis culture, because the distinctive spear points that they produced are common in sediments below the layers but nonexistent above.

According to the 2007 comet-strike hypothesis, large amounts of heat generated by the explosion of the comet shattered and melted much of the region's ice sheet, suddenly flooding the North Atlantic with fresh water that interrupted ocean circulation, which in turn triggered an extended cold snap that wiped out the Clovis culture and polished off the last remaining ice age megafauna.

"It's an appealing idea because it links all of these things together," says Jeffrey Pigati, a geologist with the U.S. Geological Survey in Denver, who isn't a proponent of the comet-strike idea. Unfortunately for that hypothesis, he and his colleagues report online today in the Proceedings of the National Academy of Sciences, not all of the purported "impact markers" are produced solely by extraterrestrial objects striking Earth.

Pigati and his team studied black mats at 13 sites in the American Southwest and in the Atacama Desert of northern Chile—including some sites where the mats began accumulating 40,000 years ago. At 10 of the 13 sites, regardless of a site's age or location, the researchers found all three of the "impact markers" described in the 2007 study, says Pigati. Although those presumed markers had also been found at a site in Belgium, he notes, they probably wouldn't have dispersed to the South American sites he and his colleagues sampled in their new study, because effects of the purported impact likely would have been limited to the Northern Hemisphere.

Also, Pigati says, he and his team found supposed markers even at sites much older than 12,900 years—indicating that the purported impact couldn't have been the source of those markers. Finally, he notes, the ratios of the concentrations of several rare-earth elements and other trace elements, including iridium, in the spherules embedded in the black mats match those found in Earth's crust, not in extraterrestrial objects. Rather than coming from an extraterrestrial impact, the spherules were formed on Earth and then trapped in the ancient wetlands by natural processes, the team concludes. The dense spherules then sank to the base of the mat because they're heavier than other windblown dust, sand, and silt. The chemical composition and location of the spherules, as well as their presence in black mats of many different ages, are more easily explained by natural processes than by extraterrestrial impacts, the team contends.

But the new study has several flaws, says Richard Firestone, an isotopic chemist at Lawrence Berkeley National Laboratory in Berkeley, California, and a member of the team that originally proposed the impact hypothesis in 2007. For one thing, he notes, Pigati and his colleagues didn't use a scanning electron microscope to scrutinize the surface of the spherules—the only way to distinguish impact generated spherules, he says, that were melted at high temperatures and had a distinctive pattern inscribed on their surface as they splashed through the air, from the spherules commonly found in wetland sediments. Also, Firestone notes, Pigati's team didn't scrutinize all of the spherules, only the ones that were truly spherical—thus discarding many of the tiny markers that might have been generated by the impact, including teardrop-shaped blobs that cooled in midair as well as misshapen blobs that formed when one near-molten droplet bumped into another.

Pigati says he accepts Firestone's criticisms but stands by his team's findings and interpretations. "We admit in our paper that we can't disprove the impact hypothesis," he notes. "Our point is that some of the spherules and other markers [cited in the 2007 report] aren't uniquely produced by impacts."

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