SCIENCE
NEWS
This Week
Ice Age Ends
Smashingly
Did a comet blow up
over eastern Canada?
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 mil-lennium-long cold spell, scientists say.
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.
WEST; (MIDDLE INSET): CANNON MICROPROBE
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,
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).
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. —S. PERKINS
Early Start
Fetuses generate immune
response to vaccination
A fetus can manufacture immune cells and
antibodies in direct response to vaccine
given to the mother during pregnancy,
according to researchers studying flu shots.
Scientists had already established that
a pregnant woman can pass along certain
antibodies to her fetus and that those
immune proteins can protect a baby for
up to 6 months after birth. Other studies
had found that a fetus can muster an
immune response to an infection contracted by the mother. But there had been
little evidence indicating that a fetus can
generate immunity to a vaccine, says study
