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Evidence for meteor in early mass extinction found
Lousiana State University Press Release
Ronald Brown, LSU Media Relations
225-578-3867

June 11, 2003

It's the stuff of science fiction movies. Bruce Willis, by a mighty
effort, saving the world from extinction by a huge meteor.

But Bruce Willis won't do it, and in our current state of readiness,
neither will anyone else. That is why LSU geophysicist Brooks Ellwood
is plumbing the geologic record, trying to correlate known mass
extinctions to meteor strikes.

"When we think about the human race and life in general, what do we
worry about? We worry about nuclear holocaust and major glaciation.
Then we worry about the giant chunks of rock that fly past Earth all
the time," Ellwood said.

"We can't see them till they're here, we can't stop one, so the
question is, how often do they hit the Earth and cause major mass
extinctions? Are extinctions often caused by impacts? If so, we want
to be sure we are prepared."

Ellwood and four other researchers have just published an article in
the journal Science in which they tie an early mass extinction to a
meteor strike. This extinction happened 380 million years ago in what
is called the middle Devonian. It was a time when only small plants,
wingless insects and spiders inhabited the land and everything else
lived in the sea. About 40 percent of all species disappeared from
the fossil record at this time. 

The extinction has been known to geologists for a long time but this
is the first time it has been tied to a meteor strike. This is also
the oldest known impact that has been tied to a mass extinction.

Ellwood is quick to point out that because the extinction and the
meteor strike happened at the same time does not prove the impact
caused the extinction -- but it certainly suggests it.

One of the great difficulties in determining whether an extinction
happened on a global scale, or was a local event caused by a volcano
or some other terrestrial force, is identifying the same strata of
rock at different locations on the globe. Finding a layer of earth in
Colorado, for example, and finding that same layer in Australia is no
simple task. 

"The same layer of earth is exposed to different conditions in
different parts of the world," Ellwood said. "Weathering, upheavals,
volcanos, earthquakes and flooding all confuse the geologic record,
making it incomplete and open to interpretation."

The layers can also be extremely thin, he said, showing a picture of
the location of his latest research. The layer he was looking at --
near the top of a barren plateau in the Anti Atlas desert near
Rissani in Morocco -- was about the thickness of a felt-tipped marker
and only distinguishable from the soil around it by its reddish
color.

What is unique about Ellwood's work, however, is the means he uses to
identify the different layers in the geologic record: induced
magnetism. 

"Everything is magnetic," he said. "If I put your finger in a
magnetic coil and turn it on, your finger will be magnetized."
Ellwood uses this phenomenon to take "magnetic signatures" of
geologic samples. The magnetic signature of a layer of earth will be
the same anywhere in the world, making it relatively easy to identify
strata, if they can be found. These signatures also make it easy to
identify meteor strikes. "The magnetic pattern associated with an
impact layer is often distinctive, making it easier to find in a
thick sequence of strata," he said.

Working with LSU graduate students Steve Benoist and Chris Wheeler;
structural geologist Ahmed El Hassani of the University of Rabat,
Morocco; and Devonian biostratigrapher Rex Crick of the University of
Texas at Arlington, Ellwood was able to find high concentrations of
shocked quartz, microscopic spherules and microcrysts in this layer,
sure signs of a meteor impact. Benoist is a paleontologist and
Wheeler is an isotope geochemist; both have since moved on.

The past 550 million years are divided up by geologists into about 90
"stages." Each stage is distinguished from another by a change in the
fossil record. To date, only four of these stages show strong
evidence of a meteor strike, Ellwood's discovery being the latest, as
well as the oldest. The most recent, best known extinction is the K-T
boundary at which the dinosaurs died out, about 65 million years ago.
There have been five major mass extinctions and many smaller ones
since then. 

"We know that meteors have struck the Earth hundreds of times,"
Ellwood said. "If I had to guess, I would say that once every 5
million years a meteor big enough to cause a mass extinction hits the
Earth. 

"We could protect ourselves if we wanted. We went to the moon, we can
figure out how to destroy or deflect a meteor. All it takes is the
political will -- and an awareness of the threat."

The work of Ellwood and his team, published in the prestigious
journal Science, is a step in that direction.

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