The Neoproterozoic Port Askaig Formation contains widespread pyrite within many diamictite beds, across Scotland and Ireland. The quantity of pyrite is anomalous for coarse-grained rocks, especially in rocks deposited at a time when seawater contained low sulphate levels owing to a continental ice cover, which inhibited weathering. Sulphur isotope compositions evolve from lightest values (down to −3.1‰) at the base of the formation to highly positive compositions in the overlying Bonahaven Dolomite (mean +44.8‰). This trend is consistent with progressive utilization of available sulphate by closed-system microbial sulphate reduction. Together with records from other contemporary diamictite successions, there emerges a picture of global microbial activity during Neoproterozoic ‘Snowball Earth’ glaciation.

Supplementary material: Details of the sulphur isotopic composition are given at

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