Geochemical data from ancient sedimentary successions provide evidence for the progressive evolution of Earth’s atmosphere and oceans<sup>1, 2, 3, 4, 5, 6, 7</sup>. 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 expansion<sup>8</sup>. In support of this rise in oxygen concentrations, a large database<sup>1, 2, 3, 9</sup> shows a marked change in the bacterially mediated fractionation of seawater sulphate to sulphide of Δ<sup>34</sup>S < 25‰ before 1 Gyr to ≥50‰ after 0.64 Gyr. This change in Δ<sup>34</sup>S 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 mediated<sup>3, 7, 9</sup>. This evolution is seen as marking the rise in atmospheric oxygen concentrations and the evolution of non-photosynthetic sulphide-oxidizing bacteria<sup>3, 7, 10</sup>. 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 Δ<sup>34</sup>S data in the marine record1, <sup>2, 3, 9</sup>. 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
