[meteorite-list] Scientists Pinpoint Pre-Dinosaur Extinction

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Tue Oct 26 19:44:58 2004
Message-ID: <200410262344.QAA07639_at_zagami.jpl.nasa.gov>


Scientists pinpoint pre-dinosaur extinction
By Brendan O'Keefe
The Advertiser (Australia)
October 27, 2004

SCIENTISTS in California and at the University of New England in
northern NSW have combined to pin down a date for a mass extinction that
came tens of millions of years before the catastrophe that wiped out the

Researcher Ian Metcalfe of UNE and others from the Berkeley
Geochronology Centre developed a technique that more precisely pinpoints
the age of minerals and rocks, using uranium-lead dating.

They say the first mass extinction came 252.6million years ago, give or
take 200,000 years, after about a million years of volcanic eruptions in
what is now Siberia.

They measured the amount of lead remaining in zircon crystals found in
volcanic ash in China.

Metcalfe says the new technique, described in the journal Science,
allows more accurate dating and obviates subjectivity. He says the
finding was significant.

"The clear, unambiguous, robust evidence we now have for the temporal
coincidence of those things, the extinction and the massive vulcanism,
provides a very strong argument," he says.

The new technique avoids lead loss, the leading problem in using
uranium-lead dating on zircon crystals.

A previous technique of wearing away the crystal surface using air
abrasion was thought to expose pristine particles inside and to give the
correct age.

"We've been dating single crystals and finding different ages from one
population in the ash layer. You could see the trail of ages going
younger and younger," Metcalfe says.

But the new process, called anealing, in which the crystal is subjected
to heat (up to 800C) and extremely high pressure, "locks the good parts
of the crystal very tightly".

"Then the crystal is subject to leaching with hydrochloric acid at high
temperature and parts are eaten away ... the parts which have been
damaged by weather, leaving behind the good, pristine crystal, from
which there has been no lead loss."

The leached material was shown to have 40 per cent less lead than that
which remained, which would translate to a given age 40 per cent younger
than reality.

Metcalfe says anealing gave "very tight clusters of ages ... you don't
have to reject any of the data".

"Previously, you had to subjectively reject some of these crystal ages
and use what was left to statistically come up with an age.

"The subjectivity has been taken away and that is the real contribution
of this paper."

The vulcanism brought most life on the ancient continent of Pangaea to
an end.

It is believed about 5000 million cubic kilometres of material was thrown

Dust and toxic gases filled the sky, blacking out the sun.

Acid rain fell.

A period of global cooling was overtaken by an era of warming.

A drop in sea levels possibly related to tectonic movements combined
with the vulcanism to spell doom for billions of trilobites (relatives
of prawns and crabs), brachiopods (sea shells) and pig-like reptiles
called tetrapods.

Only 2 per cent of life forms survived to fill niches in the new world.

>From this group came the reptiles that became dinosaurs, which met their
end in the better known extinction 65million years ago, when an asteroid
is thought to have struck the Earth.
Received on Tue 26 Oct 2004 07:44:40 PM PDT

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