[meteorite-list] Meteor Theory Gets Rocky Ride From Dinosaur Expert

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Tue May 24 16:21:30 2005
Message-ID: <200505242020.j4OKKrT15858_at_zagami.jpl.nasa.gov>

http://www.nature.com/news/2005/050523/full/050523-2.html

Meteor theory gets rocky ride from dinosaur expert
Rex Dalton
Nature
24 May 2005

US palaeontologist amasses data against Mexican crater hypothesis.

The widely held theory that a particular meteorite strike on Mexico
wiped out the dinosaurs is under sharp attack, again.

The asteroid that created the Chicxulub crater in the Yucatan peninsula
in Mexico arrived too early to have caused the Cretaceous-Tertiary mass
extinction, according to evidence given on 23 May at an American
Geophysical Union conference in New Orleans, Louisiana.

A team led by palaeontologist Gerta Keller of Princeton University, New
Jersey, reported that a sediment core drilled in east Texas emphatically
confirms a study that the group released two years ago. Sediments of
glass sprayed out by the Chicxulub impact are separated from fossils
killed during the mass extinction by a 300,000-year gap, they argue.

"I believe this is the mortal wound for the Chicxulub theory," says
Keller. Scientists should mount a search for the crater left by the
meteorite that was really responsible for the mass extinction, she adds.

Many geophysicists remain unswayed. Sean Gulick of the University of
Texas at Austin doubts the report because it means another huge asteroid
must have hit the Earth in the same era, about 65 million years ago.
"The odds of that are highly unlikely," said Gulick, who chaired the
Chicxulub symposium at the conference.

However, some sedimentologists are being persuaded by the core
specimens. Paul Wignall of the University of Leeds in Britain calls
Keller's evidence "quite convincing", although he didn't attend the
meeting.

Original thought

Two years ago, Keller stunned a symposium at an American Geophysical
Union meeting in Nice, France, with an analysis of a section of a
1,500-metre core drilled in the Yucatan, only 60 kilometres from the
Chicxulub crater.

The Yucatan core, called Yaxcopoil-1, was the result of an international
project designed to provide the most advanced record of events at the
Cretaceous-Tertiary boundary. But it was beset by strife over access to
the core and subsequent interpretations (see 'Hot Tempers, Hard Core
<http://www.nature.com/cgi-taf/DynaPage.taf?file=/nature/journal/v425/n6953/full/425013a_fs.html>').

Keller claimed the crater preceded mass extinction by 300,000 years[1].
Her critics say the sediment layers she sees are actually rubble
from collapsing crater walls. But her team argues that palaeomagnetic
dating and minute fossil analysis rules this out.

Northern exposure

To settle the dispute, Keller drilled 2,000 kilometres north of the
crater to get a sedimentary view unaffected by backwash.

The Brazos River Valley, Texas, is widely accepted as the best location
to check Chicxulub impact debris from afar. In March, three
50-metre-deep holes were drilled near the small town of Rosebud to
extract sediment from the time of the mass extinction.

>From a 2-metre section of the best core, the Keller team charted what
they say shows the 300,000-year gap. First, there is a 2
centimetre-thick layer of altered glass called bentonite that is the
ejected material from the Chicxulub impact. About 50 centimetres above
that lie sediments beds from the tsunami set off by the asteroid.
Finally, a full 1.2 metres above these beds, there is the detritus of
the mass extinction, represented by fossils of tiny plants and animals
that died.

The National Science Foundation has given Keller US$40,000 to drill
another core in autumn 2006. This one will be on the opposite side of
the Chicxulub crater, some 7,800 kilometres south near the city of
Recife in Brazil. Keller hopes to find evidence that will finally quiet
her critics.

References

   1. Keller G., et al. Proc. Natl. Acad. Sci. USA, 101. 3753 - 3758
      (2004).
Received on Tue 24 May 2005 04:20:53 PM PDT