Early oxygenation of the terrestrial environment during the Mesoproterozoic

Geochemical data from ancient sedimentary successions provide evidence for the progressive evolution of Earth’s atmosphere and oceans1, 2, 3, 4, 5, 6, 7. Key stages in increasing oxygenation are postulated for the Palaeoproterozoic era (~2.3 billion years ago, Gyr ago) and the late Proterozoic eon (about 0.8 Gyr ago), with the latter implicated in the subsequent metazoan evolutionary expansion8. In support of this rise in oxygen concentrations, a large database1, 2, 3, 9 shows a marked change in the bacterially mediated fractionation of seawater sulphate to sulphide of Δ34S < 25‰ before 1 Gyr to ≥50‰ after 0.64 Gyr. This change in Δ34S has been interpreted to represent the evolution from single-step bacterial sulphate reduction to a combination of bacterial sulphate reduction and sulphide oxidation, largely bacterially mediated3, 7, 9. This evolution is seen as marking the rise in atmospheric oxygen concentrations and the evolution of non-photosynthetic sulphide-oxidizing bacteria3, 7, 10. Here we report Δ34S values exceeding 50‰ from a terrestrial Mesoproterozoic (1.18 Gyr old) succession in Scotland, a time period that is at present poorly characterized. This level of fractionation implies disproportionation in the sulphur cycle, probably involving sulphide-oxidizing bacteria, that is not evident from Δ34S data in the marine record1, 2, 3, 9. Disproportionation in both red beds and lacustrine black shales at our study site suggests that the Mesoproterozoic terrestrial environment was sufficiently oxygenated to support a biota that was adapted to an oxygen-rich atmosphere, but had also penetrated into subsurface sediment