submitted by coldrum -
University of Arizona researchers revisited evidence of a cataclysmic event thought by many scientists to have wiped out the North American megafauna and Clovis hunter-gatherer culture some 13,000 years ago. The team obtained findings following a multidisciplinary approach.
"The idea of an extraterrestrial impact driving the Pleistocene extinction event caused a stir in the scientific community," said C. Vance Haynes, School of Anthropology and the department of geosciences, the study's lead author. "We revisited the evidence for an impact scenario and discovered it just does not hold up."
When the last ice age came to an end a sudden cooling period, the Younger Dryas, reversed the warming process. Even though this cooling period lasted only for 1300 years, the large mammals disappearance. The big question, said Haynes, is ‘Why did those animals go extinct in a short geological timeframe?'"
One of the best Clovis sites is the Murray Springs where archaeologists have unearthed hundreds of artifacts. "When you look at the sediments, you see this black layer (Black Mat). It contains the fossilized remains of a massive algae bloom, indicating a 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 [animals], but above it - nothing."
Some ascribe it to the rapid shift toward cooler and dryer weather, causing widespread droughts. Haynes disagrees. "We find evidence of big changes in climate not associated with widespread extinctions."
The two attempts to account for the mass extinction prevailing at this point include humans and celestial bodies. Many deem it possible that humans hunted the big mammals to extinction. Alternatively, it is thought that a comet or asteroid slammed into the Great Lakes area. Dust kicked high into the atmosphere could have shrouded the Earth in a blanket, causing temperatures to plummet.
Haynes and his coworkers set out to test the evidence for impact scenario:
*High concentrations of spherical magnetic particles in soil samples taken at the Murray Springs Clovis site.
*A spike in the Black Mat's iridium content - an element rare on Earth but abundant in meteorites. Nanodiamonds had been suggested as evidence of an extraterrestrial origin.
*Supposedly abundant charcoal content, cited as evidence of widespread wildfires in the impact aftermath.
Haynes collected at the same locations in the Black Mat layer as the team proposing the impact scenario.
*The team did find abundant magnetic spherules. Was a meteorite the only possible source?
Magnetic spherules of terrestrial origin (exhaust, power plants,) are found in rooftop dirt. "We confirmed the other authors' findings that the magnetic spherules are concentrated in samples at the Clovis site, but when you study the topography, you see why: Rain washed them down into a river bed; they accumulated over time. Samples from the slopes do not have higher than normal concentrations."
*What about the charcoal indicating vegetation burning? "The only places we found charcoal were the hearths of the Clovis people, where they build their fires."
*Origin of the nanodiamonds? An ingredient of cosmic dust, nanodiamonds are constantly raining down onto the earth, making them unsuitable as unequivocal evidence of extraterrestrial impact.
Said Haynes. "We can say, that all of the evidence in support of the impact scenario can be sufficiently explained by earthly causes such as climate change, overhunting or a combination of both."
Does this mean Haynes and his coworkers' findings rule out a cosmic event?
"No, it doesn't," Haynes said. "It just doesn't make it very likely."
Paper: Proceedings - National Academy of Science
For more, http://www.physorg.com.
Something is not right. This message is just to keep things from messing up down the road