Mass-independent fractionation of sulfur isotopes

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The single sharpest proxy for the composition of the Archaean atmosphere, and the reason the Great Oxidation Event has a date rather than a vague interval. Ordinary chemical and biological processes sort sulfur isotopes in proportion to their mass difference. Photochemical reactions driven by short-wavelength ultraviolet light do not — they produce a mass-INDEPENDENT signature, and that signature can only be generated and then preserved if ultraviolet light reaches the lower atmosphere and if the atmosphere carries no ozone shield and essentially no free oxygen. The signal is present in sedimentary sulfides and sulfates through the Archaean and then disappears, and the disappearance is effectively irreversible: once oxygen is present, the sulfur cycle passes through a homogenising oxidised reservoir that erases it. The interpretation was published by Farquhar, Bao and Thiemens in 2000 and it converted a qualitative argument about rusting into a datable stratigraphic horizon.