Evaluating pathways of sulphate between the atmosphere and hydrosphere using stable sulphur and oxygen isotope data

Stable isotopes in the sulphate ion can be used to gain a better understanding of sulphur sources and transformations between the atmosphere and hydrosphere. Because of its low isotopic selectivity in internal soil processes in well aerated forest soils, δ34S is a suitable tracer for monitoring what happens to deposited sulphate, if it is isotopically different from the soil sulphur. The oxygen isotope composition of sulphate can be used as a tracer for geochemical reactions in the unsaturated zone, since during sulphate formation oxygen can be acquired from various oxygen reservoirs. Immobilization as organic sulphur and subsequent mineralization is the only known process in the biochemical soil sulphur cycle which affects the 18O content of sulphate. An irrigation experiment showed that sulphate in seepage water is a mixture of percolated atmospheric derived or desorbed sulphate and sulphate mineralized from organic soil sulphur. Sulphate participates actively in biogeochemical processes of the soil zone. Redox reactions, which can change the isotopic integrity of the deposition sulphate, are important for the turnover of sulphur in the unsaturated zone. Knowledge of the extent of the isotopic modification is important if the sulphur and oxygen isotope data of sulphate are to be used for interpretation of the past geochemistry of groundwater systems. (author). 24 refs, 1 fig., 3 tabs