September 1, 2010
Study Challenges Mammoth Space Impact Theory
A new study claims to debunk the theory that mammoths, giant bears and other large mammals in North America were wiped out by a space impact 13,000 years ago.
Scientists have long theorized that these large beasts, which disappeared quickly from the fossil record, were killed off by a comet or asteroid strike.
But researchers now say that the tiny diamonds believed to have been created in the collision may have been misconstrued.
Without these diamonds, the theory falls apart, they say.
"This was really the last pillar for this theory and I think it's time now everyone moved on," said Professor Andrew Scott from the University of London, who co-authored the study, during an interview with BBC News.
Scientists have long debated on what exactly caused the extinction of the creatures and humans living in North America at the start of a millennium-long climate-cooling period dubbed the "ËYounger Dryas'.
Conventional wisdom held that an abrupt release of fresh water from a vast glacial lake into the North Atlantic had disturbed the ocean's circulation, resulting in plunging temperatures within just a few short years.
However, this theory came under fire by a group of scientists who pointed to what they called telltale signs in the 12,900-year-old sediments at archaeological sites of an ancient impact from space.
These sediments were said to contain materials such as tiny carbon spheres, nanodiamonds and the rare element iridium in too high of concentrations to have occurred naturally on Earth. The sediments were also said to include a layer of charcoal deposited by the immense fire that would have engulfed the continent after the event.
Although a crater has never been identified, advocates of the space impact theory say the impactor may have simply disintegrated in the atmosphere as it came in. As proof, they produced mammoth tusks that seem to have meteoritic pieces embedded in them.
But researchers conducting the current study say they have now unraveled much of this evidence, and have refuted the theory's most compelling evidence - the nanodiamonds.
These tiny, hexagonal bits of diamond, known as lonsdaleite, are created as a result of the intense pressure and heat of space collisions, and can be good tracers for impacts.
However, upon rigorous examination of the carbon spherules professed to contain the nanodiamonds, Professor Scott and his colleagues Tyrone Daulton and Nicholas Pinter say there has been a misinterpretation.
"We looked for these diamonds and we couldn't find them," Scott said.
"But not only that, [the proponents of the theory] have misinterpreted what are really just aggregations of carbon."
"There were frequent low-temperature fires all through this period - this is no big deal. And what happens is that the carbon in molecules gets re-ordered and this happens in very small domains, less than micron-sized areas."
"It's not a high-temperature phenomenon; it happens at low temperatures. Obviously, what they've done is take that material and identified these domains as diamonds when they're not."
Even before the current study, Dr. Jay Melosh of Purdue University, a leading expert on impacts, had demonstrated that an airburst was not capable of producing the pressures necessary to create nanodiamonds.
However, proponents of the space impact theory are not ready to abandon their ideas just yet.
Dr. Douglas Kennett of the University of Oregon told Science Magazine that the current research had been looking in the wrong places.
"The Daulton et al claim that we have misidentified diamonds is false and misleading," Science quoted him as saying.
Meanwhile, geoscientist Allen West said that since Daulton and his colleagues had not followed the same protocols, it was not surprising their research produced different results.
"They looked at charcoal but we never mentioned that we ever found diamonds in the charcoal," BBC News quoted West as saying.
"They did say that they looked in some carbon spherules but we looked at 10-15 per layer and specified that in our methodology, and they only looked at 'one to several' - that's their quote. They didn't understand what they were supposed to be looking for."
West said that further nanodiamond evidence supporting the space impact theory would be forthcoming in the weeks ahead.
The current study was published in the Proceedings of the National Academy of Sciences.
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