Science News Online
Week of June 2, 2007; Vol. 171, No. 22
Ice Age Ends Smashingly: Did a comet blow up over eastern Canada?
Sid Perkins
http://www.sciencenews.org/articles/20070602/fob1.asp
Evidence unearthed at more than two dozen sites across North America
suggests that an extraterrestrial object exploded in Earth's
atmosphere above Canada about 12,900 years ago, just as the climate
was warming at the end of the last ice age. The explosion sparked
immense wildfires, devastated North America's ecosystems and
prehistoric cultures, and triggered a millennium-long cold spell,
scientists say.
IT'S IN THERE. A layer of carbon-rich sediment (arrow) found here at
Murray Springs, Ariz., and elsewhere across North America, provides
evidence that an extraterrestrial object blew up over Canada 12,900
years ago. The hallmarks include lumps of glasslike carbon (top),
carbon spherules (middle, in cross section), and magnetic grains rich
in iridium (bottom).
West; (middle inset): Cannon Microprobe
At sites stretching from California to the Carolinas and as far north
as Alberta and Saskatchewan—many of which were home to prehistoric
people of the Clovis culture—researchers have long noted an enigmatic
layer of carbon-rich sediment that was laid down nearly 13 millennia
ago. "Clovis artifacts are never found above this black mat," says
Allen West, a geophysicist with Geoscience Consulting in Dewey, Ariz.
The layer, typically a few millimeters thick, lies between older,
underlying strata that are chock-full of mammoth bones and younger,
fossilfree sediments immediately above, he notes.
New analyses of samples taken from 26 of those sites reveal several
hallmarks of an extraterrestrial object's impact, West and his
colleagues reported at the spring meeting of the American Geophysical
Union in Acapulco, Mexico.
Samples from the base of the black mat yield most of the clues to its
extraterrestrial origin, says Richard B. Firestone, West's coworker
and a nuclear physicist at the Lawrence Berkeley (Calif.) National
Laboratory. Some of the particles there are small, magnetic grains of
material with higher proportions of iridium than are found in Earth's
crust, he notes.
Also in the mat's base are tiny lumps of glasslike carbon that
probably formed from molten droplets of the element. These lumps, as
well as little spheres of carbon with a different microstructure,
contain nanoscale diamonds formed under intense pressure.
A host of unusual geological features, collectively known as Carolina
Bays, hints at the cataclysm's location, says team member George A.
Howard, a wetland manager at Restoration Systems, an
environmental-restoration firm in Raleigh, N.C. Around 1 million of
these elliptical, sand-rimmed depressions, measuring between 50 meters
and 11 kilometers across, scar the landscape from New Jersey to
Florida. In samples taken from 15 of the features, Howard and his
colleagues found iridium-rich magnetic grains and carbon spherules
with tiny diamond fragments similar to those found at Clovis
archaeological sites.
The long axes of the great majority of the Carolina Bays point toward
locations near the Great Lakes and in Canada—a hint that the
extraterrestrial object disintegrated over those locales, says Howard.
Because scientists "haven't discovered a large, smoking hole" left by
the event, the object that blew up in the atmosphere probably was a
comet, says West.
Heat from the event would have set off wildfires across the continent,
the scientists suggest. The heat and shock from the explosion probably
broke up portions of the ice sheet smothering eastern Canada at the
time, they add. The flood of fresh water into the North Atlantic that
resulted would have interrupted ocean currents that bring warmth to
the region, and thick clouds of smoke and soot in the air would have
intensified cooling across the Northern Hemisphere.
The inferred date of the event matches the beginning of a
1,200-year-long cold spell that geologists call the Younger Dryas,
which in its first few decades saw temperatures in the Northern
Hemisphere drop as much as 10°C.
--
There may come a time when the CO2 police will wander the earth telling
the poor and the dispossed how many dung chips they can put on their
cook fires. -- Captain Compassion.
Wherever I go it will be well with me, for it was well with me here, not
on account of the place, but of my judgments which I shall carry away
with me, for no one can deprive me of these; on the contrary, they alone
are my property, and cannot be taken away, and to possess them suffices
me wherever I am or whatever I do. -- EPICTETUS
Celibacy in healthy human beings is a form of
insanity. -- Captain Compassion
"Civilization is the interval between Ice Ages." -- Will Durant.
Joseph R. Darancette
daranc@NOSPAMcharter.net
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