Variations in atmospheric sulphur chemistry on early Earth linked to volcanic activity

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 Delta S-34) and mass-independent (Delta S-33) isotopic fractionation. This sulphur is predominantly in the form of suphides. These sulphides show a positive correlation between Delta S-34 and Delta S-33 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 Delta S-34-Delta S-33 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 Delta S-34-Delta S-33 values. We suggest that the occurrence of this Delta S-34-Delta S-33 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.