According to a new study, a comet impact didn't set off a 1,300-year cold snap that wiped out most life in North America about 12,900 years ago. Though no one disputes the frigid period, more and more researchers have been unable to confirm a 2007 finding that says a collision triggered the change, known as the Younger Dryas. The drop in temperature, plus fires from the impact, may have caused the decline of an early civilization known as the Clovis culture.
     The 2007 research was based on a combination of archaeological artifacts and extraterrestrial magnetic grains in soil samples found in a thin layer of sediment throughout North America. The original team also found what he said are traces of charcoal and microscopic bits of carbon from intense fires ignited by the collision. However new research, presented at a meeting of the Geological Society of America in Portland, Oregon, has taken aim at all of these findings.
Wetlands and Mini-Meteorites.
     Nicholas Pinter, a geologist at Southern Illinois University, argued that black mats described as charcoal in the 2007 research weren't actually charcoal. Instead they were from ancient, dark soil formed in a long-ago wetland, Pinter said. Likewise, the small amounts of carbon "are not uniquely associated with high-intensity fire," he said. As for the magnetic grains, they may well have come from outer space, he said. But the grains are likely from the 30,000 tons of tiny meteorites that fall to Earth each year. He found such grains in equal or greater concentrations at many other layers dating to other time periods.
     Vance Holliday, an archaeologist at the University of Arizona, added that there is no sign that the demise of the Clovis culture was caused by a comet crash. Around the time of the cold snap, the style of spearpoints changed - which Firestone and colleagues argued was evidence that the Clovis peoples had declined due to the comet impact. But Holliday said it reflects a normal evolution in preference. "An extraterrestrial impact is an unnecessary solution for an archaeological problem that doesn't exist."
     Firestone, the original proponent of the comet theory, is sticking to his guns. Though no one can prove the Clovis disappeared, the total number of spearpoints plummeted after the impact, he said. Likewise, other research has shown a gap of more than a hundred years between the Clovis and the ensuing Folsom culture, Firestone pointed out. As for the other evidence, the comet-impact layer is very thin, and is easy to miss if samples are diluted with soils from adjacent layers. University of Southern Illinois' Pinter is also wrong about that the charcoal and carbon fragments, he added. These substances are not widespread as soil would be, and are found only in a narrow band of sediment. In addition, the magnetic grains are not the same composition as what's found in typical tiny meteorites, he added.

Source: National Geographic News (23 october 2009)