A meteorite hit Earth 380 million years ago with such force that it may
have been responsible for the great extinction that occurred around that
time, researchers have found.
In Friday's issue of the journal Science, geologists from
Louisiana, Morocco and the University of Texas at Arlington describe the
newfound remains of an extraterrestrial impact in northern Africa. The
impact took place at the same time that many marine animals died off –
suggesting the events are related, the scientists say.
If confirmed, the work would be the second time that researchers have
linked an extraterrestrial impact with a mass extinction on Earth.
The Chicxulub crater in Mexico is thought to be the scar of a giant
space rock that hit 65 million years ago, indirectly killing off the
dinosaurs and most other animals. Some scientists have proposed an
extraterrestrial cause for a mass extinction 250 million years ago,
based on a different type of geological evidence, but that has not yet
been widely accepted.
"There are a lot more of these extinctions that are related to impacts
than we have ever realized," said Rex Crick, a geologist at UTA and
member of the research team.
Dr. Crick has worked in the western Sahara for more than a decade,
studying rocks from the middle of the Devonian Period, about 380 million
years ago. To help with the project, he called on geologist Brooks
Ellwood, formerly of UTA and now a professor at Louisiana State
University in Baton Rouge, who specializes in the magnetism of ancient
rocks.
In the desert near Rissani, Morocco, Dr. Ellwood found something unusual
in the rocks' magnetism – an abrupt change in the layers that preserved
the evidence of the mass extinction. He found a similar change in rocks
in Oman that dated from the dinosaur-killing impact 65 million years ago.
Such magnetic clues help scientists hunt down extraterrestrial impacts,
said William MacDonald, a geologist at Binghamton University in New York.
"When you go to a rock outcrop and you try to find a meteorite impact
signature, it's very difficult," he said. Studying the magnetic
properties helps. "It's another tool to help you focus in and find
impact horizons."
Armed with the magnetic hints, the scientists began searching for other
signs of an impact. Eventually, they turned up several lines of
evidence, including quartz grains "shocked" by the impact; tiny rock
beads that resemble those sprayed outward by an impact; and a shift in
carbon chemistry that indicated environmental disaster.
The evidence "really looks good," said David Kring, an impact expert at
the University of Arizona in Tucson.
The researchers can't say exactly where the meteorite hit or how large
it was. But they say it's the best explanation yet for the mid-Devonian
extinction, in which many families of mollusks, ammonites and other
marine animals died out.
"Had this impact happened when there were terrestrial organisms around,
it would have been a major event," Dr. Crick said.
The team is looking at other, bigger extinctions to see whether they can
be linked with impacts as well. One good candidate is a major die-off
that took place 364 million years ago; early work suggests that it, too,
coincides with evidence of an extraterrestrial blast, Dr. Ellwood said.
Only by studying how frequently such impacts happened in the past can
scientists understand the threat posed by space rocks today, he said.
"The underlying question is, 'How often do these things hit us and how
severe is the damage?' " he said.
E-mail awitze@dallasnews.com
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