Potential and limitations of using sulphur isotope abundance ratios as an indicator for natural and anthropogenic environmental change

Sulphur isotope abundance ratios can often be used to determine the relative magnitude of the contributions of natural and anthropogenic sources to atmospheric sulphur deposition. There is evidence that the δ34S values of pre-industrial sulphur deposition were in some areas up to 10 per mille higher than those of recent anthropogenic sulphur additions. Stable sulphur isotopes could therefore serve as a historical tracer of anthropogenic pollution, provided that the original isotopic composition of the deposited sulphur is preserved in historical archives. Ice cores and tree rings might provide the best records of changes in 34S/32S ratios caused by recent anthropogenic sulphur additions, because isotope fractionation during sulphur deposition and fixation in these historical archives is small. Only a few relevant data have been reported in the literature, since the ability isotopically to analyse microgram quantities of sulphur is needed for such investigations. Lake sediments, peat deposits, and (palaeo-)soils also have the potential to record chronologies of anthropogenic sulphur inputs. However, syn- and post-depositional sulphur transformations might significantly alter the sulphur isotopic composition in these archives. The isotopic integrity of the source signal might thereby be lost, but information on environmental changes in the catchment area might nevertheless be obtained. Although it is sometimes difficult to separate source versus process components of isotopic change, sulphur isotope abundance ratios can often be used as a valuable indicator of historical change in sulphur loading of natural ecosystems. (author)