Volcanism on early Earth should have had an impact on atmospheric chemistry, but that impact can be challenging to reconstruct. The isotopic composition of sulphur contained in rocks deposited more than 2.45 billion years (Gyr) ago shows both mass-dependent (as denoted by δ34S) and mass-independent (Δ33S) isotopic fractionation. This sulphur is predominantly in the form of suphides. These sulphides show a positive correlation between δ34S and Δ33S between 4.0 and 2.45 Gyr ago. Sulphates deposited episodically between about 3.5 and 3.2 Gyr ago indicate a more homogeneous sulphur reservoir and show no correlation to the sulphide trend. Here we report sulphur isotope values of sulphide from volcanic ash layers in the 3.2-Gyr-old Mapepe Formation of South Africa. We find a δ34S–Δ33S relationship that deviates from previous sulphide isotope records and instead overlaps with the range of values reported for sulphates. Coexisting sulphates and sulphides of a similar age found in Australia and India show a similar array of δ34S–Δ33S values. We suggest that the occurrence of this δ34S–Δ33S array reflects widespread, ultraviolet-light-triggered photodissociation of sulphur dioxide that was released into the atmosphere by short-lived but intense bursts of subaerial volcanic activity.
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