[meteorite-list] New Evidence that Earth's Greatest Extinction Caused by Ancient Meteorite or Comet

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu Apr 22 10:27:52 2004
Message-ID: <200311210231.SAA03280_at_zagami.jpl.nasa.gov>

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November 20, 2003

New Evidence that Earth's Greatest Extinction Caused by Ancient Meteorite or Comet

Long before the dinosaurs ever lived, the planet experienced a mass extinction
so severe it killed 90 percent of life on Earth, and researchers at the
University of Rochester think they've identified the unlikely culprit.

"An ancient meteorite body, one from the days when the solar system was still
forming, struck the Earth 251 million years ago," says Asish Basu, professor of
earth sciences in today's issue of Science. The research is the latest volley in
a decades-long debate over what caused "The Great Dying," the greatest
elimination of life in the planet's history.

While scientists have been wrangling over whether a meteor caused this great
extinction ever since a meteor was fingered with the blame for the later
dinosaur extinction, these new findings add weight to the argument that a major
meteorite did strike the Earth 251 million years ago, likely triggering climate
change and unprecedented volcanic activity. That one-two punch so affected the
composition of the atmosphere that it took thousands of years to recover --
leaving only a relative handful of plants and animals alive.

Two decades ago, Nobel laureate Luis Alvarez and his son, Walter, detected huge
concentrations of iridium throughout the world in rock dated to the end of the
dinosaur era. Iridium is only found in such concentrations in asteroids, so they
concluded that a giant asteroid had struck the Earth at that time, likely
leading to the downfall of the dinosaurs. The Alvarez claims were at first
largely dismissed, but the evidence grew and today it is accepted that their
interpretation was largely correct.

Basu added weight to the Alvarez claims in 1988 when he announced the discovery
of "shocked quartz" -- special crystals that have split along certain planes
indicative of a large impact -- immediately beneath the Deccan Traps of India.
The Deccan Traps are areas of huge volcanic deposits that have been dated to 65
million years ago, the time of the dinosaur extinction, so finding shocked
quartz immediately beneath them suggests that a giant impact preceded these
giant lava flows.

While a meteorite has been largely accepted as the source of the dinosaurs'
demise, the root of The Great Dying has been a mystery. In 1991, however, Basu
published a study in Science that showed a massive and ancient lava flow in
Siberia dated precisely to that greatest of extinctions 251 million years ago.
The lava did not shoot out of the Earth like a giant volcano, but oozed molten
rock for thousands of years -- so much lava, in fact, that if spread evenly, it
would bury the surface of the Earth under 10 feet of magma.

Further testing by Basu and Robert Poreda, professor of earth and environmental
sciences at the University, and also co-author of the current Science research,
showed that both the Siberian and Indian lava had come from as deep as 1,800
miles beneath the surface.

"These were not just examples of local magma bubbling through the crust,"
explains Poreda. "Something brought this lava all the way up from near the
Earth's core."

To find what might have caused the Siberian flows meant finding rock samples 251
million years old-not an easy prospect since oceanic tectonic plates that make
up 70 percent of the Earth's surface are younger than that. Oceanic plates slide
underneath continental plates as they move, thus carrying any evidence far
beyond the reach of humans. From an area in Antarctica called Graphite Peak,
Basu and Poreda took rock from a stratum that sat between a layer that contained
many fossils and a layer nearly devoid of fossils called the Permian/Triassic,
or P/T boundary. One of the fossils that had gone from prominence to sudden
disappearance was Glossopteris flora, a plant that was widely known to have been
wiped out in The Great Dying. This reassured the team that they had the right
rock from the right period. Previous tests by Poreda on this same layer found
shocked quartz and fullerenes, cage-like molecules, containing atoms of
extraterrestrial gases, which again hinted at a meteorite or comet strike. These
results, however, were disputed by some researchers.

Coming at the problem from another angle, Basu and Poreda separated out the
magnetic particles from the samples from Graphite Peak and from a source of P/T
strata in Meishan, China, and Japan. To their surprise they found that the
grains that sorted out contained an iron alloy that does not occur on Earth.
Some 40 pieces were tiny fragments of meteorite 4.56 billion years old, while
other grains displayed metallic characteristics that were more indicative of
being formed by extreme heat, such as that in a severe meteorite impact. The
very fact that these grains had not deteriorated from weathering means they must
have been buried quickly under sedimentary deposits, again, indicative of a
major impact.

"At the end of the Permian era, Antarctica was close to its present position as
the southernmost part of the ancient supercontinent, Pangea, while south China
was at the equator and Japan was to the north of the equator," explains Basu.
"Such a wide, global distribution of these metal grains in the P/T boundary
strongly suggests that these grains mark a major impact of a celestial body at
that time."

Critics of the P/T impact theory may point to the lack of iridium, the element
that is so rare on Earth but common in asteroids and which alerted Alvarez to
the possibility of a meteorite as the death knell for the dinosaurs. The
Rochester team's work shows strong evidence that not all collisions with
extraterrestrial bodies will leave an iridium footprint. Basu suggests that a
collision with a comet, which may have a meteoric core, would be low in iridium.
Thus the culprit that wiped out nine of every 10 creatures on the Earth and
nearly ended life when it was just taking hold may have been created before the
Earth itself was fully formed.

Basu and Poreda plan to continue searching for evidence of a catastrophic impact
in the P/T layer in different sites around the world. They hope that if enough
samples from enough locations show evidence of a major impact, then scientists
will be able to construct the exact scenarios of how the two largest mass
extinctions in history were caused by meteorite collisions.

Along with Basu and Poreda, the co-authors of the paper are Michail I. Petaev
and Stein B. Jacobsen of Harvard University, and Luann Becker of the University
of California, Santa Barbara.
Received on Thu 20 Nov 2003 09:31:48 PM PST