Ancient Meteor Blast Peppered Mammoths With "Shrapnel"
Anne Minard in San Francisco, California
for National Geographic News
|December 14, 2007|
An ancient meteor impact in North America sent up waves of rock fragments that peppered prehistoric mammals with "space shrapnel" about 34,000 years ago, scientists say.
Many of the animals, particularly in the region near present-day Alaska, didn't survive.
That's the story being pieced together by a research team led by Richard Firestone of the Lawrence Berkeley National Laboratory in Berkeley, California.
The team had done previous work on a suspected impact that occurred 13,000 years ago. But while looking for evidence of that more recent blast in mammoth tusks, the scientists found traces of the much older event.
"The surprise was the tusks were dating back to 30,000 to 34,000 years ago," Firestone said.
"Nobody had thought of it before. It was serendipitous."
The work was presented this week at the fall meeting of the American Geophysical Union in San Francisco, California.
Tiny "Bullet Holes," Micrometeorites Found
While searching for evidence of the more recent cosmic impact, team member Allen West, an Arizona geophysicist, noticed an odd pattern as he was combing through thousands of ancient tusks.
He found that the top-facing sides of a few tusks were pockmarked by holes 0.08 to 0.11 inch (2 to 3 millimeters) across.
The pockmarks were found on seven mammoth tusks—most likely from near Alaska's Yukon River—and the skull and horns of a Siberian bison (see a map of Alaska).
That the holes were found on only one side of the bones indicated that the impacts came from a single direction, Firestone said.
The researchers believe the damage was caused by micrometeorites, each less than a millimeter in size, thrown off by an object that collided with Earth.
The researchers found fossilized meteorites in the holes and determined that they were rich in iron and nickel but poor in titanium—chemical traits that suggest celestial and not earthly origins, they say.
West also discovered that the tiny rocks were highly magnetic. When he suspended a magnet on a string over the holes, the magnet held fast.
The team additionally found that the bison skull showed bone growth around the holes, suggesting that the animal had survived the blast.
Firestone said the evidence corroborates previous work done by Ian Barnes, a paleobiologist at Royal Holloway University of London.
In a paper published in the journal Current Biology in June of this year, Barnes reported molecular evidence for a die-off of mammoths around the time of the newly suspected impact.
"We are pretty certain there was one event that occurred in that region," Firestone said.
Barnes agreed the data so far could suggest a connection, but he said it's tenuous at best and more information is needed.
"Brown bears underwent a local extinction in Alaska 35,000 years ago, and there may be some turnover or reduction of the Alaskan mammoth population around 35,000 [years ago], but that data hasn't been published yet," he said.
"Whether it only happened in North America or farther afield, I don't have the data for that yet."
More Tusks Ahead
Firestone and his colleagues expect to submit their work for publication in a scientific journal in the coming year.
Meanwhile, the early evidence has already raised some perplexing questions.
"The big mystery is the nature of the impact," Firestone said.
"We don't know whether a meteor exploded or what happened. It's confounding the impact [experts]."
Barnes pointed out there have been plenty of hazards for Earth's creatures aside from meteor blasts, including volcanic eruptions and rapid climate change.
(Read related story: "Volcano Theory of Dino Die-Off Gets New Support" [November 5, 2007].)
Celestial pockmarks notwithstanding, Firestone hesitated to pin mass extinction on any one event.
"There are all sorts of events that might cause a reduction in population size, so I'm cautious about actually ascribing causation," he said.
Nevertheless, Barnes called the findings "pretty extraordinary," particularly because they were uncovered so unexpectedly.
"You got to give it to them—it's pretty exciting stuff," he said. "Pretty amazing."
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