When the last ice age came to an end approximately 13,000 years ago and the glaciers covering a large portion of the North American continent began melting and retreating toward the north, a sudden cooling period known as the Younger Dryas reversed the warming process and caused glaciers to expand again. Even though this cooling period lasted only for 1,300 years, a blink of an eye in geologic timeframes, it witnessed the disappearance of an entire fauna of large mammals.
"When you go out and look at the sediments deposited during that time, you see this black layer we call the Black Mat," said C. Vance Haynes, an anthropologist at Arizona State University. "It contains the fossilized remains of a massive algae bloom, indicating a short period
of water table rise and cool climate that kept the moisture in the soil. Below the Black Mat, you find all kinds of fossils from mammoths, bison, mastodons, Dire wolves and so forth, but when you look right above it – nothing."
But why did those animals go extinct in a very short geological timeframe?
University of Arizona researchers analyze the Black Mat Layer during a sampling excursion to the Murray Springs Clovis site in Southern Arizona.
(Photo Credit: Douglas J. Kennett; Courtesy of the Center for the Study of the First Americans)
Scientists have suggested several scenarios to account for the rapid Pleistocene extinction event. Some experts believe that a comet or asteroid slammed into the glaciers covering the Great Lakes area, unleashing firestorms that consumed large portions of vegetation. In addition, the dust and molten rock kicked up high into the atmosphere during the impact could have shrouded the Earth in a nuclear winter-like blanket of airborne dust, blocking sunlight and causing temperatures to plummet.
Unusually high concentrations of spherical magnetic particles in the soil samples taken at the Murray Springs Clovis site have been interpreted as indication of an extraterrestrial source. Another hint in this direction was a spike in the Black Mat's iridium content – an element rarely encountered on Earth but quite abundant in meteorites. In addition, the occurrence of nanodiamonds had been suggested as evidence of an extraterrestrial origin. Finally, a supposedly abundant charcoal content in the soil samples had been cited as evidence of widespread wildfires ravaging the land in the aftermath of the impact.
The authors of the new PNAS paper set out to put these lines of evidence to the test. To ensure their samples were comparable, Haynes collected at the same locations in the Black Mat layer as the team proposing the impact scenario: "I sampled where they sampled and at the same times they sampled."
Using highly sensitive and sophisticated analytical methods, researchers in the department of geosciences and UA's Lunar and Planetary Lab then analyzed their samples for the evidence that had been presented in support of the impact scenario.
The team did find abundant magnetic spherules. But where did they come from? Was a meteorite the only possible source?
"Researchers have only begun to study those magnetic spherules recently, so we still don't know much about them," Haynes said. "What we do know is that they occur in exhaust from vehicles and power plants."
To determine whether the magnetic spherules found at Murray Springs could be of terrestrial origin, Haynes took a sample of dirt from the rooftop of his house and examined it under the microscope.
Haynes remembers looking at the soil samples on a microscope slide, and "sure enough, there they were – among all the dust and grains and grit, they appeared like tiny, shiny ball bearings."
"We did confirm the other authors' findings that the magnetic spherules are concentrated in the samples at the Clovis site, but when you study the topography on which the sediments were laid down, you immediately see why: Rainwater washed them down into a river bed, where they accumulated over time.
Since this is where the samples with the increased spherule content came from, we were not surprised to find more of the spherules there. The samples we took from the slopes do not have higher than normal concentrations of spherules."
What about the charcoal indicating vegetation burning? "The only places we found charcoal were the campsites of the Clovis people, where they build their fires."
But where could the nanodiamonds come from? A common ingredient of cosmic dust, nanodiamonds are constantly raining down onto the earth's surface, rendering them unsuitable as unequivocal evidence of an extraterrestrial impact.
"Something happened 13,000 years ago that we do not understand," said Haynes. "What we can say, though, is that all of the evidence put forth in support of the impact scenario can be sufficiently explained by earthly causes such as climate change, overhunting or a combination of both."
Citation: Haynes Jr. et al., 'The Murray Springs Clovis site, Pleistocene extinction, and the question of extraterrestrial impact', March 2010, 107(9), 4010-4015; doi:10.1073/pnas.0908191107
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