High-resolution sulphur isotope analysis of speleothem carbonate by secondary ionisation mass spectrometry

Sulphur emitted into the atmosphere from industrial activity is regarded as a key mechanism in forcing recent climate and has impacts upon the environment and human health at both local and regional scales. Trace amounts of sulphate present within speleothem carbonate can be used to provide an emissions inventory for regional sulphur loading to the atmosphere where sulphur isotopes can be extracted and used as a tool for provenance. However, speleothem sulphur concentrations are low and manual techniques for sulphur isotope analysis allow only low sampling resolution. SIMS (Secondary Ionisation Mass Spectrometry) is used here to develop records of sulphur isotopic change in speleothems from the Italian and Austrian Alps, where annual laminae are less than 100 µm thick. Analysis at annual resolution over the past 100 years demonstrates an excursion in sulphur isotopic composition from values close to the carbonate bedrock end-member composition in the pre-industrial era, to values isotopically depleted in 34 S. This trend is mirrored by increasing concentrations of sulphate and is suggested to reflect the increase in SO2 emissions with industrial activity. Subsequent decline in speleothem sulphate concentrations in one of the stalagmites reflects the recent European decline in industrial atmospheric SO2 content and is matched by a rebound in sulphur isotopic composition to values more enriched in 34 S. Superimposed on this general secular trend are smaller (annual) variations in concentration reflecting cave environmental conditions. High magnitude events of short duration, potentially reflect volcanic inputs of SO2 or the incorporation of dust particulates into the speleothem carbonate. This is the first micro-analysis of carbonate-associated sulphate (CAS) sulphur isotopes by SIMS. SIMS analysis of δ34S–SO4 in speleothem carbonate reveals a high resolution archive depicting a regional sulphur emissions inventory for SO2 loading to the atmosphere.