"The absence of oxidized soil profiles and red beds
indicates that oxidative weathering rates were
negligible during the Archaean," the researchers
report in the Aug. 30 issue of Nature.
The ancient Earth should have had an oxygen atmosphere
but something was converting, reducing the oxygen and
removing it from the atmosphere. The researchers
suggest that submarine volcanoes, producing a reducing
mixture of gases and lavas, effectively scrubbed
oxygen from the atmosphere, binding it into
oxygen-containing minerals.
"The Archaean more than 2.5 billion years ago seemed
to be dominated by submarine volcanoes," said Kump. "Subaerial
andesite volcanoes on thickened continental crust seem
to be almost absent in the Archaean."
About 2.5 billion years ago at the Archaean/Proterozoic
boundary, when stabilized continental land masses
arose and terrestrial volcanoes appeared, markers show
that oxygen began appearing in the atmosphere.
Kump and Mark E. Barley, professor of geology,
University of Western Australia, looked at the
geologic record from the Archaean and the
Palaeoproterozoic in search of the remains of
volcanoes. They found that the Archaean was nearly
devoid of terrestrial volcanoes but heavily populated
by submarine volcanoes. The Palaeoproterozoic,
however, had ample terrestrial volcanic activity along
with continuing submarine vulcanism. Subaerial
volcanoes arose after 2.5 billion years ago and did
not strip oxygen from the air. Having a mix of
volcanoes dominated by terrestrial volcanoes allowed
oxygen to exist in the atmosphere.
Terrestrial volcanoes could become much more common in
the Palaeoproterozoic because land masses stabilized
and the current tectonic regime came into play.
The researchers looked at the ratio of submarine to
subaerial volcanoes through time. Because submarine
volcanoes erupt at lower temperatures than terrestrial
volcanoes, they are more reducing. As long as the
reducing ability of the submarine volcanoes was larger
than the amounts of oxygen created, the atmosphere had
no oxygen. When terrestrial volcanoes began to
dominate, oxygen levels increased.
The National Science Foundation, NASA Astrobiology
Institute and the Australian Research Council
supported this work.
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Source:-
Penn State
Published on
30th November 2007
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