Did a comet impact really kill off megafauna such as the mammoth, mastodan and sabre-tooth tiger?
Credit: Wikimedia
PORTLAND, OREGON: Debate on the existence of a Younger Dryas comet impact, 12,900 years ago, and whether it is linked to mass extinctions of large mammals and early humans in North America reopened this week.
The Younger Dryas was a 1,300-year-long cold snap that affected climate in much of the Northern Hemisphere. In 2007, a team led by Richard Firestone of Lawrence Berkeley National Laboratory, in the U.S., argued that it was caused by the impact of a comet.
This then wiped out such Pleistocene megafauna as the sabre-toothed cat, the mastodon and the mammoth, along with the Clovis people, one of the earliest American cultures.
Impact markers
In support, Firestone's team cited a number of impact markers, dated about 12,900 years ago, as evidence of the exploding comet and its impact on cooling patterns resulting in mass extinction. These included tiny spheres, called spherules, made of metals and carbon and minute, shattered diamonds caused by the detonation. They also found unusual concentrations of helium 3 and iridium, common in impact sites, but rare in the Earth's crust.
"The impact layer at some sites is directly covered by a black algal mat that is in direct contact with mammoth bones and Clovis artefacts," Firestone told Cosmos Online. "The mat was literally peeled off of the bones with dental picks and adhering to the bones were impact markers."
But the study has proved to be controversial. "When I saw the study, I found it compelling," said Todd Surovell, an anthropologist at the University of Wyoming in Laramie. "But I wanted to see this patterning for myself."
Miscounted spherules
What Surovell and other scientists found wasn't what they'd initially expected. "Using the same methods on similar samples, we should have found the same results - we did not," he said. The results were revealed this week at the Geological Society of America's annual meeting in Portland, USA.
Part of the problem may have been methodology. Spherules, which are smooth glassy metallic grains common at impact sites, may have been miscounted, depending on how they were categorised. "When is a sphere a sphere?" said Surovell. "Definitional issues may have gotten in the way when it wasn't clear what counted as evidence."
Surovell's own study counted only those grains that were well rounded, highly spherical, and exhibited a smooth glassy or metallic surface. "Magnetic grains that are irregularly shaped and often sub-rounded are more abundant than micro-spherules," said Surovell, the implication being that counting them might have skewed the earlier study's results. "This [later] study employed a conservative definition."
Neither was a subsequent study able to duplicate the high iridium levels initially found at the sites, commented Vance Holliday, an anthropologist at the University of Arizona in Tucson. Iridium is an element that is common in asteroids but not in the Earth, therefore making it a good marker for an impact.
Firestone acknowledged that there is controversy but argues that there is an explanation for the hard-to-duplicate results.
"Some people have had trouble reproducing our results because they are taking sediment samples that are too wide. Typically people take sample sediments in five-centimetre intervals. In the case of the Younger Dryas impact, the layer was sometimes as little as a millimetre thick."
"The recent studies that found nothing used samples as much as 15 centimetres thick, which highly diluted the results," he said.
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