Origins of sulphate in Antarctic dry-valley soils as deduced from anomalous 17O compositions.

The dry valleys of Antarctica are some of the oldest terrestrial surfaces on the Earth. Despite much study of soil weathering and development, ecosystem dynamics and the occurrence of life in these extreme environments, the reasons behind the exceptionally high salt content of the dry-valley soils have remained uncertain. In particular, the origins of sulphate are still controversial; proposed sources include wind-blown sea salt, chemical weatherings, marine incursion, hydrothermal processes and oxidation of biogenic sulphur in the atmosphere. Here we report measurements of delta18O and delta17O values of sulphates from a range of dry-valley soils. These sulphates all have a large positive anomaly of 17O, of up to 3.4/1000. This suggests that Antarctic sulphate comes not just from sea salt (which has no anomaly of 17O) but also from the atmospheric oxidation of reduced gaseous sulphur compounds, the only known process that can generate the observed 17O anomaly. This source is more prominent in high inland soils, suggesting that the distributions of sulphate are largely explained by differences in particle size and transport mode which exist between sea-salt aerosols and aerosols formed from biogenic sulphur emission.