[meteorite-list] Mammoth-Killer Impact Gets Mixed Reception From Earth Scientists

From: Darren Garrison <cynapse_at_meteoritecentral.com>
Date: Fri, 01 Jun 2007 09:31:43 -0400
Message-ID: <mr70639dl6uhgouito2m0if8ed74onjkcm_at_4ax.com>

News of the Week

Mammoth-Killer Impact Gets Mixed Reception From Earth Scientists

Richard A. Kerr


ACAPULCO, MEXICO--A headline-grabbing proposal that an exploding comet
wreaked havoc on man and beast 13,000 years ago got its first full
scientific airing at a meeting here last week.
<http://www.sciencemag.org/cgi/content/full/316/5829/1264#FN1#FN1> * Many
geoscientists who attended nearly a day of talks and posters on the putative
impact called the idea "cool." But they're not dashing off to rewrite the
textbooks yet.

A loose consortium of more than 25 scientists is arguing that a massive
comet exploding in the atmosphere over North America wiped out the mammoths,
terminated the founding Paleo-Indian culture, and triggered a
millennium-long reversion to an ice age climate. "We're quite sure there was
an impact," says analytical chemist Richard Firestone of Lawrence Berkeley
National Laboratory in California, one of the consortium's two leaders.

Not so fast, say veterans of decades-long wrangling over how cosmic
collisions have affected Earth and the life on it. "There is some
interesting evidence that deserves study," says cratering researcher Peter
Schultz of Brown University, a member of the consortium who did not attend
the meeting. But the evidence for an impact is too new and unconventional to
be conclusive.

The "impact wars" have been raging since scientists first began working out
geologic markers for ancient impacts in the 1960s. By the 1980s, researchers
found 65-million-year-old sediments that contained too much of the element
iridium--rare on Earth but enriched in meteorites. That discovery pointed
the way to mineral grains scarred by the shock of the impact that killed off
the dinosaurs. Geologists eventually found the crater from that impact.

In the 1990s, geochemist Luann Becker of the University of California, Santa
Barbara, and colleagues said they had found impact markers at the mother of
all mass extinctions, the Permian-Triassic 251 million years ago (Science,
23 February 2001, p.
<http://www.sciencemag.org/cgi/content/full/291/5508/1469> 1469). These
markers included metallic grains and molecular cages composed of
carbon--called bucky-balls or fullerenes--filled with extraterrestrial
helium. Three Science papers later, however, Becker's group has failed to
make its case for a Permian-Triassic impact. In fact, despite considerable
effort, no one else has found fullerenes or extraterrestrial helium at the
Permian-Triassic boundary.

Now Firestone and some of his consortium colleagues, including Becker, say
they have found nearly a dozen kinds of recent impact markers at 26 sites
from California to Belgium. Most of the supposed markers are new types; many
have never before been reported in the geologic record.

The consortium got its start in 1999 when retired archaeologist William
Topping of Deming, New Mexico, approached Firestone with unusual mineral
grains from sediments at Gainey, Michigan. The grains came from the base of
a black layer rich in organic matter left during the Younger Dryas, a cold
snap that began 12,900 years ago and lasted 1000 years. The "black mat" lies
just above the last arrowheads and spear points crafted by the Paleo-Indian
Clovis people, as well as the last bones of the mammoths the Clovis hunted.

From the odd composition of the Gainey samples, Topping and Firestone
inferred that the sediments had been tagged 12,900 years ago by radiation
from a nearby supernova that devastated the Western Hemisphere. In late
2004, Allen West, a retired geophysical consultant in Prescott, Arizona,
offered to help with the by-then-stalled project. Other specialists soon
came on board. West collected most of the samples and funded much of the
work with $70,000 of his own "fun money."

Given new evidence, the researchers have discarded the supernova scenario in
favor of a major collision. They believe the impacting object contributed
many of their proposed markers: iridium; irregularly shaped metallic grains,
some extraordinarily high in titanium; the same metallic grains melted into
microspherules; nanodiamonds; fullerenes carrying extraterrestrial helium;
and excess potassium-40. These markers have "no way of being produced except
by impact," Firestone said at a press conference at the meeting. The
collision with Earth, they propose, produced other markers: soot and
charcoal from global wildfires; vesicular carbon microspherules; and melted,
glasslike carbon. The latter two carry the nanodiamonds.

Because they have found no crater or shocked minerals, West and Firestone
say the alien object probably did not slam into the ground. They believe an
icy comet several kilometers in diameter and dirtied with rock and carbon
approached Earth and broke up into bits, as comet Shoemaker- Levy did before
it hit Jupiter in 1994. Each fragment exploded in the atmosphere over North
America before reaching the ground, in their scenario. The resulting shock
waves and heat would have devastated the plants, animals, and humans below.
The heat could also have melted enough of the ice sheet then on North
America to put a freshwater lid on the North Atlantic, shutting down the
warm-water ocean "conveyor" and plunging much of the hemisphere into the
Younger Dryas cold spell.

Most listeners at the meeting gave the Younger Dryas impact a polite,
sometimes welcoming reception. But the one specialist in impact markers who
heard out the presentations isn't so sanguine. "It's similar to the
situation with the Permian-Triassic" impact proposal, says David Kring of
the Lunar and Planetary Institute in Houston, Texas. "The proposed
signatures for an impact event shouldn't be dismissed, but they need to be
tested. Until they are, one has to look at them a little skeptically."
Iridium, for example, might have been concentrated by slowed sedimentation
or even by algae. The charcoal could well be from Clovis fire pits. And
Kring says the extreme titanium levels and the nanodiamonds embedded in
melted carbon make no sense to him.

A paper in review at the Proceedings of the National Academy of Sciences may
answer a few key questions about the comet clash--and perhaps lure
combat-weary impact specialists back into the fray.


*Joint Assembly of the American Geophysical Union, 22-25 May.
Received on Fri 01 Jun 2007 09:31:43 AM PDT

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