An extraterrestrial object with a three-mile-wide girth appears to have exploded over southern Canada nearly 13,000 years ago, wiping out Ice Age megafauna such as mastodons and mammoths and killing uncounted Stone Age humans.
The blast could be to blame for a major cold spell called the Younger Dryas that occurred at the end of the Pleistocene epoch, the period of time spanning from about 1.8 million years ago to 11,500 years ago.
Research presented Monday at a meeting of the American Geophysical Union in Acapulco, Mexico could shed light on major questions about the megafauna extinction and an abrupt climate change.
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"Based on the distribution of material, it looks like this impact probably occurred in southern Canada near the Great Lakes, over what at that time would have been a major glacier, the Laurentide Ice Sheet," said one of the presenters, Richard Firestone of Lawrence Berkeley National Laboratory.
Researchers couldn't find a distinct crater, which suggests the comet burst in the air rather than slammed into Earth.
Even an airburst should leave its mark, so the scientists think the Laurentide Ice Sheet absorbed much of the impact.
A much smaller object burst in the air over a remote part of Siberia in 1908, flattening 800 square miles of forest in what is known as the Tunguska explosion.
Firestone and his colleagues investigated buried carbon-rich layers dating back 12,900 years and blanketing more than 50 areas that span from California through Canada and into Belgium.
They found a slew of extraterrestrial markers, including nanodiamonds, which are formed by energetic explosions in space, as well as elevated amounts of the rare element iridium and tiny capsules of glass-like carbon.
"Glass-like carbon is essentially carbon that's been melted at very high temperatures," such as those generated by a comet impact, Firestone explained.
During the last catastrophic animal extinction, more than three-fourths of the large Ice Age animals, including woolly mammoths, mastodons, saber-toothed tigers and giant bears, died out.
Scientists have debated for years over the cause of the extinction, with both major hypotheses — human overhunting and climate change — insufficient to account for the mega die-off.
An atmospheric explosion could have triggered a wave of massive wildfires that would have reduced the mastodons of the day to ashes, say the scientists.
At one site called Murray Springs in Arizona, a well-known site for artifacts of the Clovis tool-making culture, the scientists found the remains of megafauna covered by comet debris.
"This black mat [of wildfire ash] drapes over the bones of partially butchered mammoths as if somebody was in the process of working on these animals while they were actually killed," Firestone told LiveScience in a telephone interview. "And between this black mat and the bones of this mammoth, we find this ejecta layer. So it's as if the [impact] event occurred right on top of these mammoth bones, and then this black mat occurs on top of that."
Once put out, the fires would have left behind a barren landscape devoid of food for remaining animals.
"I would argue that most of the megafauna either died or starved after this thing," Firestone said. "But certainly, there must have been pockets of survival of large animals, even mammoths, that may have survived for thousands of years beyond that, ultimately to be hunted to death or whatever happened to them."
The comet theory could also explain the abrupt plunge in temperatures during the Younger Dryas period.
Presenters at this AGU symposium argue that the comet impact or explosion would have heated up the area, causing the Laurentide Ice Sheet to partly melt and sending massive amounts of fresh water into the Atlantic Ocean.
The input of light, fresh water onto heavier seawater would have affected ocean currents, which are responsible for keeping the higher latitudes at moderate temperatures.
The massive wildfires would also have loaded the atmosphere with light-blocking dust, soot, water vapor and nitric oxides. The result would be abrupt climate cooling.
The explosion would have been life-shattering for humans living in North America, particularly near the Great Lakes.
"It would have had major effects on humans," said one of the researchers, Douglas Kennett of the University of Oregon, in a telephone interview. "Immediate effects would have been in the North and East, producing shockwaves, heat, flooding, wildfires and a reduction and fragmentation of the human population."
Any survivors would have been driven into isolated groups in search of food and warmth.
Kennett said archaeological evidence at the Clovis sites is "suggestive of significant population reduction and fragmentation, but additional work is necessary to test the data further."
"It was a bad day in North America for those folks who were living there," Kennett added.
The evidence for a comet impact is substantial.
"I think the fact that there's an impact is pretty definite. There are too many markers there for it all to be coincidence or happenstance explanations," Firestone said. "What will be debated is whether the extent of the impact was sufficient, for instance, to kill all of the megafauna or whether other factors were also equally important."
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