Microbial sulphate reduction during Neoproterozoic glaciation, Port Askaig Formation, UK

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 parts per thousand) at the base of the formation to highly positive compositions in the overlying Bonahaven Dolomite (mean +44.8 parts per thousand). 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.