[meteorite-list] Mass Extinction from Buried Carbon?

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu Apr 22 10:21:08 2004
Message-ID: <200307241601.JAA05445_at_zagami.jpl.nasa.gov>

http://dsc.discovery.com/news/afp/20030721/carbon.html

Mass Extinction from Buried Carbon?
AFP/ABC Science Online
July 23, 2003

A vast reservoir of carbon is stashed beneath the Earth's crust
and could be released by a major volcanic eruption, unleashing a
mass extinction of the kind that last occurred 200 million years
ago, German geologists report.

Researchers know that carbon is stored in the mantle, a layer of
plastic-like rock beneath Earth's fragile crust, said Hans
Keppler of the Institute of Sciences at Germany's University of
Tuebingen, whose report appears in today's issue of Nature.

Exactly how much is down there is unknown. Most estimates, drawn
from analyses of gases emerging from the mantle, suggest the store
is many times more than all the carbon in the Earth's atmosphere,
soil and sea combined.

The concern is that if just a part of this gigantic
reservoir is quickly released as carbon dioxide, or
CO2, that could create a runaway greenhouse effect.
The CO2-soaked atmosphere would store up heat
from the sun, shrivelling plant life and destroying
species along the food chain.

"The [mantle] reservoir is just gigantic compared with
anything that we have on the Earth's surface," said
Keppler.

So he and his colleagues conducted an ambitious experiment
aimed at finding whether mantle rock is a stable storage for
CO2.

Most of the rock in the Earth's upper mantle is a crystalline
silicate called olivine. In a lab chamber, Keppler's team
replicated the fiery heat and intense pressures, of 1,200°
Celsius and 3.5 gigapascals, which are likely to exist in the
deeper parts of the upper mantle.

They used these conditions to create olivine crystals from raw
ingredients of magnesium oxide and silicon dioxide, and exposed
them to carbon and water.

The carbon turned out to be almost completely insoluble in olivine:
just a tiny amount, between 0.1 and 1.0 parts per million by weight,
was absorbed into the rock. So if the carbon is not in the olivine,
that leaves only one major source, Keppler said: "If you cannot
store the carbon in the olivine, then the only plausible place for
storing it are carbonates."

Carbonate rocks have a much lower melting point than olivine, which
is able to absorb the punishing furnace-like heat radiating from the
Earth's core and still not melt.

Heated to a molten state, carbonates are capable of squeezing through
cracks in the olivine, rising up towards the surface and absorbing the
free carbon as they go. They can pick up so much that as much as 10 or
20 percent of their mass is carbon.

The risk, said Keppler, is that this carbonate reservoir could suddenly
be breached in the event of a major volcanic eruption.

"Once the carbonate comes up to the surface, as soon as it is below
[a pressure of] 20 or 30 kilobars, which corresponds to a depth of 40 or
60 kilometers in the mantle," he said. "As soon as it comes up beyond
this depth, it will decompose and release carbon dioxide."

The nightmare scenario? Gigantic geysers of carbon dioxide, imperilling
life on the surface.

"There has been some evidence that something like this has happened in
the past. There is a very good correlation with [CO2] flooding that
coincides with several mass extinction events - some massive, sudden
change of carbon dioxide in the atmosphere," Keppler said.

One of these events occurred around 245 million years ago, at the end of
the Permian era, which saw the largest extinction event in Earth's history:
fossil evidence shows as many as 96 percent of all marine species were lost
and more than three quarters of vertebrate, or backboned, species on land.

The other, possibly a cluster of smaller events, was at the end of the
Triassic period around 208 million years ago, when around half of the
world's species suddenly died out.

That event essentially handed rule of the planet to the dinosaurs, which
began a long decline thereafter. They were ultimately consigned to history
65 million years ago by the cataclysmic impact of a 10-kilometer asteroid,
which struck what is now the Gulf of Mexico.
Received on Thu 24 Jul 2003 12:01:36 PM PDT