History Forum / General / Archaeology / September 2007

http://www.brown.edu/Administration/News_Bureau/2007-08/07-040.html

   

"PROVIDENCE, R.I. [Brown University] — At the end of the Pleistocene
era, wooly mammoths roamed North America along with a cast of
fantastic creatures – giant sloths, saber-toothed cats, camels, lions,
tapirs and the incredible teratorn, a condor with a 16-foot wingspan.

About 12,900 years ago, these megafauna disappeared from the fossil
record, as did evidence of human remains. The cause of the mass
extinction and the human migration is a mystery. Now a team of
scientists, including Brown University planetary geologist Peter
Schultz, provides evidence that an asteroid impact likely caused the
sudden climate changes that killed off the mammoths and other majestic
beasts of prehistory.

In the Proceedings of the National Academy of Sciences, the
international team lays out its theory that the mass extinctions in
North America were caused by one or more extraterrestrial objects –
comets or meteorites – that exploded over the Earth or slammed into
it, triggering catastrophic climate change.

The scientists believe that evidence for these extraterrestrial
impacts is hidden in a dark layer of dirt sometimes called a black
mat. Found in more than 50 sites around North America, this puzzling
slice of geological history is a mere three centimeters deep and
filled with carbon, which lends the layer its dark color. This black
mat has been found in archaeological digs in Canada and California,
Arizona and South Carolina – even in a research site in Belgium.

The formation of this layer dates back 12,900 years and coincides with
the abrupt cooling of the Younger Dryas period, sometimes called the
“Big Freeze.” This coincidence intrigued the researchers, led by
Richard Firestone of Lawrence Berkeley National Laboratory, who
thought that the black mat might be related to the mass extinctions.

So the researchers studied black mat sediment samples from 10
archaeological sites dating back to the Clovis people, the first human
inhabitants of the New World. Researchers conducted geochemical
analysis of the samples to determine their makeup and also ran carbon
dating tests to determine the age of the samples.

Directly beneath the black mat, researchers found high concentrations
of magnetic grains containing iridium, charcoal, soot, carbon
spherules, glass-like carbon containing nanodiamonds and fullerenes
packed with extraterrestrial helium – all of which are evidence for an
extraterrestrial impact and the raging wildfires that might have
followed.

Schultz, professor of geological sciences at Brown and an impact
specialist, said the most provocative evidence for an extraterrestrial
impact was the discovery of nanodiamonds, microscopic bits of diamond
formed only from the kind of intense pressure you’d get from a comet
or meteorite slamming into the Earth.

“We don’t have a smoking gun for our theory, but we sure have a lot of
shell casings,” Schultz said. “Taken together, the markers found in
the samples offer intriguing evidence that North America had a major
impact event about 12,900 years ago.”

Schultz admits that there is little decisive evidence about the actual
details about the impact and its effects. Scientists suspect that a
carbon-rich asteroid or comets were the culprits. The objects would
have exploded over North America or slammed into it, or both,
shattering and melting ice sheets, sparking extreme wildfires, and
fueling hurricane-force winds – all of which could have contributed to
changes in climate that led to the cooling of the Younger Dryas
period.

“Our theory isn’t a slam dunk,” Schultz said. “We need to study a lot
more sediments to get a lot more evidence. But what is sobering about
this theory of ours is that this impact would be so recent. Not so
long ago, something may have fallen from the sky and profoundly
changed our climate and our culture.”

The U.S. Department of Energy and the National Science Foundation
funded the work.

Editors: Brown University has a fiber link television studio available
for domestic and international live and taped interviews and maintains
an ISDN line for radio interviews. For more information, call the
Office of Media Relations at (401) 863-2476.

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Eric Stevens