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

Pascal Philippot (1) , Mark van Zuilen (1) , Claire Rollion-Bard (2)

1 IPGP - Institut de Physique du Globe de Paris
2 CRPG - Centre de Recherches Pétrographiques et Géochimiques

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 2.45billion 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 2.45Gyr ago. Sulphates deposited episodically between about 3.5 and 3.2Gyr 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.

Domaines: Planète et Univers [physics]

Pascal Philippot, Mark van Zuilen, Claire Rollion-Bard. Variations in atmospheric sulphur chemistry on early Earth linked to volcanic activity. Nature Geoscience, 2012, 5, pp.668-674. ⟨10.1038/ngeo1534⟩. ⟨insu-03619306⟩