[meteorite-list] Professors Revise Theory of Dinosaurs' Extinction

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu Apr 22 10:32:52 2004
Message-ID: <200403231628.IAA29117_at_zagami.jpl.nasa.gov>


Professors revise theory of dinosaurs' extinction

Study finds more than one asteroid caused extinction

Raj Hathiramani
The Daily Princetonian
March 23, 2004

After years of detective work, University geosciences professor Gerta Keller
and her colleagues have found that an intensive period of volcanic eruptions
and a series of asteroid impacts likely ended the dinosaurs' reign on Earth,
challenging the dominant theory that a single cataclysmic asteroid hit
caused their extinction.

Though an asteroid or comet could have struck Earth at the time of the
dinosaur extinction, it most likely was, Keller said, "the straw that broke
the camel's back" and not the sole cause.

For more than a decade, scientists have believed the Chicxulub crater
- submerged off Mexico's Yucatan peninsula and and more than 110 miles wide
- was the remnant of the dinosaur-killing event.

The time of the dinosaur extinction is known as the Cretaceous-Tertiary
boundary (K-T). Keller's team, working on the problem since the Chicxulub
was identified as the K-T impact crater, accumulated data and evidence from
more than 100 localities that simply did not fit the popular K-T impact

"We wanted to find out just what the kill-effect of this impact was, how
quickly the mass dying occurred and whether there were environmental changes
preceding it that may have contributed to the mass extinction," Keller said.

Keller's findings revealed that the asteroid which formed the Chicxulub
crater crashed 300,000 years before dinosaurs died out. According to Keller,
the fateful dinosaur-killer probably struck Earth somewhere else and remains

Keller's team, including Thierry Adatte of Switzerland's University of
Neuchatel and Wolfgang Stinnesbeck of Germany's University of Karlsruhe,
obtained samples from a new core drilled within the Chicxulub impact.

Using radiometricand fossil-dating techniques, the team examined microscopic
rock slices and a 20-inch-thick layer of limestone sandwiched between
impact-formed glass beads and iridium, an element commonly found in

The new core, Keller said, was the decisive factor in concluding that the
Chicxulub impact predated the K-T boundary, and had nothing to do with the
dinosaurs' extinction.

"We discovered that the impact glass spherule layer that marks the Chicxulub
impact in Mexico and all over Central America was embedded in the late
Maastrichtian sediments that predated the K-T mass extinction by 300,000
years," Keller said.

However, it was Keller's detailed analysis of microorganisms that gave the
team's work real credibility, University geophysicist Jason Morgan said.

"It's not like finding an isolated dinosaur bone," Morgan told the Princeton
Weekly Bulletin. "You have thousands of organisms in a single sample. You
can do real statistics on them."

The team also determined that the ecological disruption from the Chicxulub
impact may not have been as severe as originally believed.

Keller found normal marine sediments directly on top of the fallout layer,
indicating there were no tsunami waves or other major environmental

These results were confirmed by other studies conducted throughout Mexico,
Guatemala and Haiti, which exposed signs of as many as three meteorite
impacts: the Chicxulub impact; the K-T impact with its iridium layer and
mass extinction; and a third smaller impact, evidenced by another iridium
layer about 100,000 years after the extinction.

Keller is now studying the effects of volcanic eruptions that began 500,000
years before the K-T boundary and produced a period of global warming.
Findings suggest that volcanism caused biotic stress almost as great as the
mass extinction itself.

"Asteroid impacts and volcanism may be hard to distinguish based on their
effects on plant and animal life, and the K-T mass extinction could be the
result of both," Keller said.

Her results have also helped other scientists understand the probable
effects of greenhouse effect warming resulting from volcanism and other
causes. However, Keller mentions that her team became unpopular in the
process of their work for challenging a theory that had become so

"We plowed on despite opposition, searching for the truth, searching for
what really happened more than 65 million years ago," Keller said.

According to geophysicist Vincent Courtillot at Universite Paris, the true
believers in the "Chicxulub is the K-T impact killer" theory are strongly
opposed by a number of renowned scientists. Keller does not expect to ever
change the former's minds but foresees her ideas gaining greater

"The large majority of us are happy that finally the spell is broken and
that other theories can be considered with less fear of character
assassination that has been the norm," she said.
Received on Tue 23 Mar 2004 11:27:58 AM PST

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