The concentration of oxygen in Earth’s atmosphere dropped for an extended time about 1.9 billion years ago, following the Great Oxidation Event, researchers now report.
Evidence for the drop in oxygen levels comes from the analyses of minerals taken from banded iron formations, large repositories of iron oxide that accumulated billions of years ago (SN: 6/20/09, p. 24). Those minerals contain large amounts of trace elements, which can provide details about environmental conditions at the time, says Don Canfield, a geobiologist at the University of Southern Denmark in Odense. In particular, he and his colleagues argue in the Sept. 10 Nature, the ratio of stable isotopes of chromium reveal the level of oxygenation in the ancient atmosphere.
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