[meteorite-list] Asteroid Theory of Dinosaur Extinction Questioned

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu Apr 22 10:32:44 2004
Message-ID: <200403020122.RAA05116_at_zagami.jpl.nasa.gov>

http://www.reuters.com/newsArticle.jhtml?storyID=4472848

Asteroid Theory of Dinosaur Extinction Questioned
By Maggie Fox, Health and Science
Reuters
March 1, 2004

WASHINGTON (Reuters) - Scientists probing a vast crater off Mexico's
Yucatan peninsula questioned a popular theory about dinosaurs on
Monday, saying the collision that formed the crater happened too far
back in time to have caused their extinction by
itself.

Much evidence points to the idea that an asteroid or comet gouged the
Earth around 65 million years ago, triggering volcanic and climate
changes that eventually wiped out the dinosaurs.

When the huge, mostly underwater crater was found off Yucatan, it
seemed the perfect candidate.

"Since the early 1990s the Chicxulub crater on Yucatan, Mexico, has been
hailed as the smoking gun that proves the hypothesis that an asteroid killed
the dinosaurs and caused the mass extinction of many other organisms at
the Cretaceous-Tertiary (K-T) boundary 65 million years ago," the
researchers write in this week's issue of the Proceedings of the National
Academy of Sciences.

But they said a core drilled out of the middle of the crater suggests it dates
back more than 300,000 years before the K-T boundary and "thus did not
cause the end-Cretaceous mass extinction as commonly believed."

The researchers, led by Gerta Keller of Princeton University and including
experts from Germany, Switzerland and Mexico, studied a sample that
extends 5,000 feet below the current surface, in the middle of the more than
125-mile-wide crater.

Other samples have included tiny pieces of glass-like rock that could have
been melted during an asteroid impact, and which seem to date to the
65-million-year point, give or take a few hundred thousand years.

But their core sample showed fossils that suggest the crater was blasted
out 300,000 years before the K-T boundary. Magnetic evidence also
suggests it is older than previously believed.

ALTERNATIVE THEORY

This finding would support an alternative theory that the dinosaurs and other
forms of life were wiped out in a series of disasters that changed the Earth's
climate, Keller's team said.

They noted there are other craters dating to around this time. None is big
enough to have caused world-altering changes by itself.

But the meteors or asteroids hit at the same time of a busy period of
volcanic activity known as Deccan volcanism, as well as when
greenhouse-type atmospheric warming and major extinctions
occurred.

"The Chicxulub impact occurred at a time of massive volcanism which
led to greenhouse warming," Keller said in an interview conducted by
e-mail.

The name Deccan comes from an area of what is now India where a massive
amount of molten material surged up from near the Earth's core 65
million years ago.

It would have brought vast amounts of carbon gases to Earth's surface,
causing a warming effect that would have wiped out many species of
plants and animals.

"This finding suggests that the K-T boundary impact (and volcanism)
may have been the straw that broke the camel's back, rather than the
catastrophic kill of a healthy thriving community," the researchers
concluded.

Now they need to find the actual crater left by whatever made this
final blow. Perhaps one known as the Shiva crater in India, dating
to around the same period, is the one, they suggested.

"There is evidence for a third impact, which occurred about 150,000
years after the K-T mass extinction," Keller said.

This impact may have made it harder for plants and animals to recover
from the worldwide effects of the blasts from space and from within the
planet.
Received on Mon 01 Mar 2004 08:22:08 PM PST