Geochemistry

Oxidative Trigger

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Science  08 Nov 2002:
Vol. 298, Issue 5596, pp. 1137
DOI: 10.1126/science.298.5596.1137c

Most recent evidence implies that Earth's atmosphere was reducing for nearly the first half of the planet's history—lacking abundant free oxygen—and became oxidizing fairly abruptly about 2.3 billion years ago. What caused this change, which determined the subsequent geologic (by initiating oxidative weathering) and biologic (by allowing higher life in the oceans and on land) history of the planet?

One idea is that this change was biologically driven by the appearance of cyanobacteria, which produce oxygen during photosynthesis, but recent data imply that cyanobacteria evolved many hundreds of million years earlier. Another idea is that the oxygen fugacity of gases emitted from Earth's mantle via volcanoes and vents increased, but other data have shown that the fugacity of Earth's mantle has not changed much during its history. Holland reanalyzes this second proposal and shows that only a small change in the composition of gases is needed, related to the escape of hydrogen to space by about 2.3 billion years ago, to trigger a sudden increase in atmospheric oxygen content. — BH

Geochim. Cosmochim. Acta66, 3811 (2002).

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