[meteorite-list] New Blow for Dinosaur-Killing Asteroid Theory

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Wed, 29 Apr 2009 16:00:49 -0700 (PDT)
Message-ID: <200904292300.QAA07970_at_zagami.jpl.nasa.gov>

http://www.nsf.gov/news/news_summ.jsp?cntn_id=114648&org=NSF&from=news

National Science Foundation
Press Release 09-076

New Blow for Dinosaur-Killing Asteroid Theory

Impact didn't lead to mass extinction 65 million years ago, geologists find
April 27, 2009

The enduringly popular theory that the Chicxulub crater holds the clue
to the demise of the dinosaurs, along with some 65 percent of all
species 65 million years ago, is challenged in a paper to be published
in the Journal of the Geological Society on April 27, 2009.

The crater, discovered in 1978 in northern Yucutan and measuring about
180 kilometers (112 miles) in diameter, records a massive
extra-terrestrial impact.

When spherules from the impact were found just below the
Cretaceous-Tertiary (K-T) boundary, it was quickly identified as the
"smoking gun" responsible for the mass extinction event that took place
65 million years ago.

It was this event which saw the demise of dinosaurs, along with
countless other plant and animal species.

However, a number of scientists have since disagreed with this
interpretation.

The newest research, led by Gerta Keller of Princeton University in New
Jersey, and Thierry Adatte of the University of Lausanne, Switzerland,
uses evidence from Mexico to suggest that the Chicxulub impact predates
the K-T boundary by as much as 300,000 years.

"Keller and colleagues continue to amass detailed stratigraphic
information supporting new thinking about the Chicxulub impact, and the
mass extinction at the end of the Cretaceous," says H. Richard Lane,
program director in the National Science Foundation (NSF)'s Division of
Earth Sciences, which funded the research. "The two may not be linked
after all."

>From El Penon and other localities in Mexico, says Keller, "we know that
between four and nine meters of sediments were deposited at about two to
three centimeters per thousand years after the impact. The mass
extinction level can be seen in the sediments above this interval."

Advocates of the Chicxulub impact theory suggest that the impact crater
and the mass extinction event only appear far apart in the sedimentary
record because of earthquake or tsunami disturbance that resulted from
the impact of the asteroid.

"The problem with the tsunami interpretation," says Keller, "is that
this sandstone complex was not deposited over hours or days by a
tsunami. Deposition occurred over a very long time period."

The study found that the sediments separating the two events were
characteristic of normal sedimentation, with burrows formed by creatures
colonizing the ocean floor, erosion and transportation of sediments, and
no evidence of structural disturbance.

The scientists also found evidence that the Chicxulub impact didn't have
the dramatic impact on species diversity that has been suggested.

At one site at El Penon, the researchers found 52 species present in
sediments below the impact spherule layer, and counted all 52 still
present in layers above the spherules.

"We found that not a single species went extinct as a result of the
Chicxulub impact," says Keller.

This conclusion should not come as too great a surprise, she says. None
of the other great mass extinctions are associated with an impact, and
no other large craters are known to have caused a significant extinction
event.

Keller suggests that the massive volcanic eruptions at the Deccan Traps
in India may be responsible for the extinction, releasing huge amounts
of dust and gases that could have blocked out sunlight and brought about
a significant greenhouse effect.

-NSF-

Media Contacts
Cheryl Dybas, NSF (703) 292-7734 cdybas at nsf.gov <mailto:cdybas at nsf.gov>

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Received on Wed 29 Apr 2009 07:00:49 PM PDT