Isotopic archives of sulphate in speleothems

摘要

The hemispheric impact of industrial emissions upon atmospheric sulphur loading is reflected in the sulphur depositional history recorded in cores from ice sheets. However, these do not reveal regional variations. Recently deposited speleothems are used here as archives of regional sulphur depositional history at two locations within the United Kingdom and Ireland. delta S-34-SO4 and delta O-18-SO4 present within speleothem carbonate are measured for the first time as part of a dual isotope approach to decode the speleothem sulphur record. The largely refractory nature of delta S-34-SO4 under oxidising conditions enables source provenance of atmospheric SO2, whereas the complex cycles of isotopic exchange and fractionation during incorporation of oxygen into sulphate molecules enable delta O-18-SO4 signatures to yield insights into ambient environmental conditions and biogeochemical cycling in the ecosystem above the cave. delta S-34-SO4 values extracted from speleothem carbonate formed within Browns Folly Mine, UK, range from +3.5 to +5.5 parts per thousand and delta O-18-SO4 +10.3 to +13.7 parts per thousand. Both signatures lie within the range expected from sulphate deposition in industrial locations and reflect the transfer of sulphate into speleothem calcite with little fractionation. However, delta O-18-SO4 signatures at Crag Cave, western Ireland, are isotopically heavier than expected and approach isotopic equilibrium with delta O-18-H2O Under reducing conditions. Dual isotope analysis of delta S-34-SO4 and delta O-18-SO4 optimises the correct identification of sulphur sources and biogeochemical cycling prior to incorporation into the speleothem record. At carefully selected cave sites where drip water flowpaths into the cave remain oxic, speleothems hold the potential to retain records of atmospheric sulphur loading at the local and regional scale. (C) 2008 Elsevier Ltd. All rights reserved.