[meteorite-list] Second Mass Extinction Linked To Impact

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu Apr 22 10:22:37 2004
Message-ID: <200306130031.RAA09863_at_zagami.jpl.nasa.gov>

http://www.nature.com/nsu/030609/030609-12.html

Second mass extinction linked to impact

Rock from space might have hit life hard 380 million years ago.

JOHN WHITFIELD
Nature Science Update
13 June 2003

About 380 million years ago, a rock from space smashed into
the Earth, say geologists. They believe that the impact
wiped out a large fraction of life.

The idea could strengthen the controversial connection
between mass extinctions and impacts. Up to now, the only
candidate for a link is the meteor 65 million years ago
that some believe helped exterminate the dinosaurs.

Signs of an earlier catastrophe coincide with a disappearance
of many animals, says Brooks Ellwood of Louisiana State
University, Baton Rouge: "It doesn't mean that the impact
killed off the critters, but it's suggestive that it had
something to do with it." It's not known where
a rock struck, although it's possible that traces of a crater
might be found, he adds.

Other researchers agree that there was an impact around that time,
but feel the evidence for a mass extinction is much weaker.

Rocks in Morocco laid down about 380 million years ago bear a layer
of sediment that looks like the debris from a cataclysmic explosion,
Ellwood's team found. The sediment has unusual magnetic
properties, and contains grains of quartz that seem to have
experienced extreme stresses.

Around this time, about 40% of marine animal groups vanish from the
fossil record, say Ellwood's team. Ellwood posits an asteroid slightly
smaller than the 10-kilometre rock suspected of killing the dinosaurs.

The evidence for an impact is compelling, says geologist Paul Wignall
of Leeds University, UK. And linking it to a mass extinction would be
a major finding. "The potential lethality of impacts would be greatly
increased," he says.

But it's not clear how much disappeared around the time of the
impact - the death toll may be far lower than Ellwood's team
suggest, says Wignall. He thinks palaeontologists should search the
rocks for a better picture of what happened at that time.

Even a figure of 40% is a typical extinction rate for that period of the
Earth's history, agrees palaeontologist Norman MacLeod, who
studies mass extinctions at the Natural History Museum, London.
"It's not a mass extinction, it's part of a much longer-term pattern,"
he says.

MacLeod doubts that mass extinctions are the result of
extraterrestrial intervention. "Impacts are quite a common
phenomenon," he says. "But they don't correlate significantly with
peaks in extinction."

References

    1.Name, A.B.Impact ejecta layer from the mid-Devonian:
      possible connection to global mass extinctions. Science, 300,
      1734 - 1737, (2003)
Received on Thu 12 Jun 2003 08:31:42 PM PDT