[meteorite-list] Deep In Crater's Core, Clues To Dinosaur Extinction

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu Apr 22 09:54:03 2004
Message-ID: <200202110410.UAA11897_at_zagami.jpl.nasa.gov>

http://www.dallasnews.com/texasliving/stories/dinocrater_11liv.ART0.66a49.html

Deep in crater's core, clues to dinosaur extinction

Scientists drilling at Mexican site where enormous meteorite struck

By ALEXANDRA WITZE
The Dallas Morning News
February 11, 2002

Near the city of Merida, on Mexico's Yucatan Peninsula, some drill-rig
workers are forgoing this week's Carnaval celebration. They're busy
investigating an even bigger blast - an explosion with the energy of 100
million megatons of TNT.

That big boom happened 65 million years ago, when a meteorite the size of
Richardson hit Mexico. The impact indirectly killed off the dinosaurs, along
with three-quarters of all other species living at the time. More obviously,
it left behind a giant pockmark in the ground, a crater spanning more than
100 miles both on land and offshore.

But geologists have never gotten a close-up look at this scar. Unlike Meteor
Crater, which draws tourists to Arizona, the Yucatan crater is buried under
sediment half a mile deep.

Now an international research team is drilling the first deep core into the
crater. The findings, scientists say, will provide new insight into what
happened on one of the worst days in the history of life.

Already, the team has drilled more than 3,200 feet down and retrieved a
jumble of rock fragments from the impact. But on Jan. 20 the rig's pipe got
stuck. Drilling stopped for two weeks, but the workers have just threaded a
new pipe into the hole and begun bringing up more of the core.

"There's still an enormous potential for exciting discoveries even though
we've had this minor setback," says Richard Buffler, a University of Texas
geophysicist and a co-leader of the project.

The drilling results could provide further clues to the impact and the mass
extinctions that followed. Standard theory, for instance, holds that animals
died out because the impact kicked up enough tiny dust particles to encircle
the globe and block out sunlight. But one new paper challenges that idea.

The impact has triggered controversy for more than two decades.

Nobody knew about the Chicxulub crater (pronounced "CHICK-shoo-loob," Mayan
for "tail of the devil") back in 1980, when physicist Luis Alvarez, his
geologist son, Walter, and colleagues proposed that the dinosaurs might have
died because of a meteorite impact. As evidence, the scientists pointed to
high levels of iridium - an element rare on Earth but more common in
meteorites - that appeared around the globe in 65 million-year-old
sediments.

Because those sediments marked the boundary between the Cretaceous and
Tertiary periods of geologic time, the event became known as the "K-T
extinctions." (The "C" was already taken by the earlier Cambrian period.)

But the controversial theory took more than a decade to be accepted. Slowly,
geologic evidence began to accumulate that something catastrophic had
happened in the Gulf of Mexico region. Layers of churned-up sediments along
the Brazos River, for instance, were interpreted as remains of a giant
tsunami possibly caused by a meteorite impact.

Only in 1991 did geologists gather enough proof to publish a formal
suggestion that the buried Chicxulub structure was the scar of the
dinosaur-killing blast. (Oil-exploration geologists had found the odd
circular feature during surveys, but had no conclusive evidence that it was
an impact crater.)

Scientific interest in drilling the crater grew. In 1994 and 1995,
scientists from the Universidad Autonóma de México, or UNAM, drilled eight
shallow holes in and around the crater.

But then the idea became mired in financial and management delays. In
November 2000, the $1.5 million project was reorganized under six lead
investigators.

On Dec. 3, 2001, the governor of Yucatan and the president of UNAM helped
kick off the drilling at the site, 25 miles southwest of Merida. Workers hit
the first rock on Dec. 12 and worked straight through Christmas and New
Year's Day, says team leader Jaime Urrutia Fucugauchi of UNAM.

For the first 2,600 feet, nothing but limestone emerged - not surprising,
since limestone is commonly formed in shallow water as the shells of tiny
marine creatures fall to the sea floor and turn into rock. Then, just where
scientists had predicted, the core hit the impact-jumbled rocks.

"It was a very beautiful moment," says Dr. Urrutia, who hurried from Mexico
City to the drill site two days later.

These rocks, known as breccias, are made of other rock shattered into
fragments by the impact. Their jumble of different colors, textures and
shapes is very different from the silty gray limestone drilled until then,
says Dr. Urrutia.

"You can immediately notice that they are completely different," he says.

But then came a mystery. The scientists had expected to drill into about 650
feet of breccias. After only 330 feet, though, they were back into
limestone.

Two possible explanations exist, says David Kring, a planetary geologist at
the University of Arizona. The deep limestone could represent a large chunk
sandwiched between breccia layers. Or it could be a "mega-block" formed at
the edge of the crater as the initial hole collapsed outward.

Paleontologists are now searching for tiny fossils in the core that might
provide clues to the deep limestone's age, Dr. Urrutia says.

As the researchers drilled through that limestone, the pipe got stuck and
possibly broken at a depth of 3,260 feet. They had to send away for new,
narrower pipe that could be threaded through the old one.

The new system, which was installed last week, will be able to drill to
nearly 5,000 feet - less than the 5,900 feet expected from the original
pipes, says Dr. Urrutia. The narrower diameter also means that less rock
will be extracted from the hole.

"We will have less material for the laboratory studies, which is a pity," he
says.

Detailed analysis of the cores should reveal how the local geology was
forever altered by the massive impact. Plans also call for two more cores to
be drilled offshore in 2004.

But neither land nor sea drilling may resolve the debate about exactly how
the dinosaurs died.

The Alvarezes' original theory suggested that dust enshrouded the planet in
darkness and halted photosynthesis. That theory is wrong, says geologist
Kevin Pope, of Geo Eco Arc Research in Aquasco, Md.

Earth would plunge into darkness only if more than 10 million billion grams
of tiny dust particles - those smaller than 1 micrometer, or a thousandth of
a millimeter across - blasted into the air. In the current issue of Geology,
Dr. Pope calculates that less than one one-hundredth that amount would have
been kicked up by the impact.

"These findings," he writes, "indicate that the original K-T impact
extinction hypothesis - the shutdown of photosynthesis by submicrometer-size
dust - is not valid, because it requires more than two orders of magnitude
fine dust than is estimated here."

The global cooling may have been caused by other material put into the air,
he acknowledges - such as sulfate particles from the limestone rock, or soot
from wildfires that swept the globe.

The threat of future asteroids harming life on Earth is also exaggerated,
Dr. Pope adds: "The dust effects from smaller impacts are probably
negligible."

His paper triggered a cyberspace outcry on a leading Internet discussion
board devoted to the threat of near-Earth asteroids.

David Morrison, an impact expert whose work was cited in Dr. Pope's paper,
said that the new calculations had no basis in reality.

"Kevin Pope did not measure the dust particles in the K-T boundary layer,"
Dr. Morrison wrote. "In fact, no one has detected or measured this dust. All
estimates, including that of Pope, are based on theoretical modeling and
extrapolation from the larger particles measured in the K-T boundary layer."

Dr. Kring, of the University of Arizona, agrees. Scientists have tried
searching for that elusive dust.

Sediments sometimes preserve dust grains kicked up by the impact, Dr. Kring
says, but the results on their sizes are mixed. On one hand, two 65
million-year-old rock samples from Europe contain dust particles 250
micrometers in size - so large that they would have settled quickly to the
ground, clearing the air. But other samples from Canada, recorded by fern
fossils at the time of impact, suggest that much of the dust may have been
smaller than 1 micrometer, Dr. Kring says.

"Because we have contradictory observations," he says, "I do not think the
issue is resolved."

The complex geology of the Chicxulub crater is buried under half a mile of
sediment. The cross-sectional diagram above shows some of the features left
behind 65 million years ago after a 5-mile-wide rock crashed into the
ground, melting some of the local rocks and fracturing others into giant
tilted blocks. At left, a cross-section of the actual drilled core shows the
contact between the overlying limestone (indicated on the diagram by gray)
and the impact-jumbled rocks called breccias (indicated on the diagram by
purple). Based on the core that has been drilled so far, the scientists
think the drill rig may actually be located a bit left of where it is
pictured above.Magnetic studies of the buried Chicxulub crater reveal a
rainbow-colored collection of anomalies. Purples and blues roughly outline
an inner ring. This data, combined with gravity and seismic studies, has
helped scientists pin down the crater's size to roughly 110 miles in
diameter.
Received on Sun 10 Feb 2002 11:10:37 PM PST