Earth's ancient atmosphere trapped in rocks
July 13, 2000
By Environmental News Network staff
Scientists on a quest to characterize the long-term chemical
evolution of Earth's atmosphere need to understand what the air
was like millions of years ago. To do this, they've come to
realize they can leave no stone unturned.
Uncovering the signature of so-called "fossil air" in terrestrial
rocks and sediment is reported for the first time in today's
issue of Nature.
The signature is in the form of an irregular isotope of oxygen
that gets transferred from ozone and other atmospheric oxidants
to sulfate during the oxidation of reduced sulfur gases,
according to Huiming Bao, a geochemist at the University of
California, San Diego.
These oxidized gases become incorporated into sulfate minerals in
solid deposits on Earth's surface. Scientists have searched for
this signature for decades. They finally found it in gypsum
deposits from the Namibian desert in Africa and in volcanic ash
deposits in Nebraska and South Dakota.
Detection of the isotope anomaly gives scientists an important
new tool to answer questions about the composition of Earth's
early atmosphere, the atmospheric processes of ancient volcanic
eruptions, past ocean circulation patterns and early biological
productivity.
"No one has found a way you can measure the ancient atmosphere in
solid examples," said Mark Thiemens, a professor of chemistry at
the University of California at San Diego. "Ice cores don't go
back far (about 250,000 years). Now one can go back hundreds of
millions of years or billions of years."
Thiemens said the ability to characterize the long-term chemical
evolution of Earth's atmosphere will help scientific, economic
and political leaders in their discussions of global warming.
"One always hears the argument, 'Isn't this (global warming) all
part of a natural cycle?'" he said. "To answer that question, you
really want to have a large-scale record. This will give it to
us. We really need to understand the past to understand the
present and the future."
The scientists believe the signatures in the volcanic ash from
Nebraska and South Dakota could provide geologists with more
information about the chemistry of volcanic plumes and the nature
of the eruptions that produced them.
Because the coast off central Namibia is a major zone of
upwelling with intense biological activity, the researchers were
able to tie the anomalous sulfate deposits to the activity of
nearby sulfur-producing marine micro-organisms and the unique
desert environment that is able to preserve the signature.
However, the upwelling current may not have been constant during
the past several millions of years and may be intimately tied to
the change of ancient climate conditions. The researchers said
that if such a connection can be made, it might provide insight
to previous ocean circulation and biological productivity.
The long-term chemical evolution of the atmosphere of other
planets such as Mars may also be preserved in terrestrial
sediment, said Thiemens. As such, presumed Martian meteorites and
future samples returned from Mars may provide information about
the chemical evolution of the Martian atmosphere.
SOURCE: Environmental News Network (ENN)
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