Sulfur mass-independent fractionation is proposed as an atmospheric oxygen proxy

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Farquhar, Bao and Thiemens reported that sulfur isotopes in sedimentary rocks older than roughly 2.4 billion years carry a mass-independent fractionation signature, and that this signature disappears in younger rocks. Because the signature is produced by ultraviolet photochemistry in an atmosphere with no ozone shield, and because it is homogenised away once an oxidised sulfate reservoir exists, its disappearance dates the point at which free oxygen became a persistent atmospheric constituent. The result converted the Great Oxidation Event from a qualitative inference drawn from red beds, banded iron formations and detrital uraninite grains into a stratigraphic horizon that could be located and dated in section — which is why the event now has a bracketed date at all.

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