Did cosmic impact kill the mammoths?
March 6th, 2012, 10:33 am by Pat Brennan, science, environment editorA series of strikes by an asteroid or comet might solve a prehistoric mystery: Why did a long list of creatures, from mammoths and saber-toothed cats to horses and camels, vanish suddenly from North America nearly 13,000 years ago?
Relying on a meticulous analysis of fragments, including tiny diamonds of submicroscopic size, from a dark layer of sediment in a Mexican lake, a new study suggests that large objects from space might have caused the continental-sized catastrophe.
And while the impact is "dinky" compared to the massive strike that wiped out the dinosaurs 65 million years ago, it would have been enough to profoundly alter the course of evolution in the Americas.
"This is the standout event in terms of environmental and biotic change," said James Kennett, an earth science professor at UC Santa Barbara and an author of the new study. "All the horses in North America, all the camels, the mastodon, short-faced bear, giant beaver, dire wolf, saber-toothed cats — the disappearances converge very abruptly at or close to 12.9 (thousand years ago)."
Humans, too, suffered steep population declines, with the Clovis culture seeming to vanish, along with human presence in California, during a sharp cooling episode known as the Younger Dryas.
"It's a bit of a puzzle why the cooling even occurred," Kennett said. "We argued the cooling occurred as a result of the impact."
Kennett and an international, 16-member team examined "nanodiamonds" that form only during cosmic impacts, along with "impact spherules" that had crashed together at high speed and remained fused — again an effect seen only in impacts from space bodies.
And, while there are hints of a crater in the Gulf of St. Lawrence that might be related to the event, Kennett says his team's hypothesis does not require an impact crater.
Instead, the strikes — perhaps a series of strikes from a comet or asteroid that had broken up — could have taken place above the ground, much like the Tunguska event over Siberia in 1908 that leveled trees across 800 square miles.
Evidence of the strike, or strikes, is scattered across western Europe as well as North and South America, though the European material is made up of nanodiamonds so light they could have been carried there by wind.
The material analyzed by the team came from Lake Cuitzeo in central Mexico.
"It's really a cometary cloud that intersected Earth at that time," Kennett said. "That's our hypothesis. We're finding, as we do more work on this, essentially the larger it's getting."
Relying on detailed geochemistry as well as careful analysis of the lattice-like structure inside the nanodiamonds, the team was able to rule out other possible origins for the material — volcanoes or stray bits falling into the atmosphere from space.
While Kennett is careful to point out that the new study includes only evidence of the extraterrestrial impact, and makes no direct link to animal extinctions, the match between the timing of the many disappearances of large creatures and the timing of the impact is extraordinary.
It even includes the pygmy mammoths that once inhabited California's Channel Islands.
"The youngest known pygmy mammoth is dated to 12.9 (thousand years ago)," he said. "They go right up to 12.9, then they're gone."
More familiar creatures such as grizzly bears and elk might have reinvaded the continent after the cosmic impacts. But compared to the spectacular period before, the later North American fauna begins to look a little impoverished.
"What we have left is wimpish compared with Africa," Kennett said. "It's sad. But we lost all these wonderful animals, including the giant sloths."
The study, one of a series on the impacts 12,900 years ago, was published Monday in the Proceedings of the National Academy of Sciences.
Source: http://sciencedude.ocregister.com/2012/03/06/did-cosmic-impact-kill-the-mammoths/168824/--
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