Not so non-marine? Revisiting the Stoer Group and the Mesoproterozoic biosphere

Abstract

The Poll a’Mhuilt Member of the Stoer Group (Torridonian Supergroup) in Scotland has been heralded as a rare window into the ecology of Mesoproterozoic terrestrial environments. Its unusually high molybdenum concentrations and large sulphur isotope fractionations have been used as evidence to suggest that lakes 1.2 billion years ago were better oxygenated and enriched in key nutrients relative to contemporaneous oceans, making them ideal habitats for the evolution of eukaryotes. Here we show with new Sr and Mo isotope data, supported by sedimentological evidence, that the depositional setting of this unit was likely connected to the ocean and that the elevated Mo and S contents can be explained by evapo-concentration of seawater. Thus, it remains unresolved if Mesoproterozoic lakes were important habitats for early eukaryotic life.