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Crater Debates: Is the Chicxulub crater the K/T
boundary killer or Not?
Chicxulub Crater Debates: Is the Chicxulub crater the K/T boundary killer or Not?
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(6) MORE THAN ONE ASTEROID CAUSED K-T EXTINCTION
The Daily Princetonian, 23 March 2004
Princetonian Staff Writer
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 theory.
"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 undiscovered.
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 asteroids.
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 disturbances.
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 entrenched.
"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 recognition.
"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.
Copyright 2004 Daily Princetonian Publishing Company, Inc. All Rights Reserved.
MODERATOR'S NOTE: Let me see whether I get that right: Scientists who hold that "Chicxulub is
the K-T impact killer" are dogmatic "true believers", obstinate pig-heads who will never
change their views regardless of the geological evidence, horrid people who normally
use "character assassination" instead of arguments. For goodness' sake, get out of your
glass houses or stop throwing stones. BJP
Look here for the Yaxcopoil-1 core segment 793.85 to 794.60 m: the transition of the impact to post-impact crater infill,