Sulfur (δ34S) and oxygen (Δ17O) isotopic measurements of sulfate from two Antarctic ice cores over a complete glacial/interglacial cycle

It has been shown that variations in Δ¹⁷O (Δ¹⁷O = δ¹⁷O - 0.512 x δ¹⁸O) of sulfate preserved in ice cores reflects changes in aqueous and gas-phase oxidation of reduced sulfur species in the atmosphere. The branching ratio of heterogeneous and homogenous oxidation will affect aerosol number densities thus having implications for the indirect climate effect. Such measurements also have the potential to give insight into changes in the oxidant concentrations themselves. The primary oxidant for S(IV) species in the gas phase is the hydroxyl radical (OH), while the primary oxidants in the aqueous phase are ozone (O₃) and hydrogen peroxide (H₂O₂). These three oxidants largely control the oxidizing capacity of the atmosphere and thus the lifetimes of pollutants and greenhouse gases such as CO and CH₄. The anomalous isotopic composition (Δ¹⁷O > 0) of sulfate originates only from aqueous-phase oxidation. The magnitude of the anomaly thus provides information on relative amounts of OH oxidation versus O₃ and H₂O₂. Analysis of ice cores from both Antarctica and Greenland reflect climatic and anthropogenic impacts on the oxidation of reduced sulfur species in the atmosphere. However, factors other than the oxidant concentrations themselves, such as aerosol loading, cloud processing efficiency and stratospheric intrusions, can affect the magnitude of Δ¹⁷O. This has prevented the quantitative analysis of paleo-oxidant variations. Sulfur isotopic measurements (δ³⁴S) have been used as source apportionment indicators for sulfate species both in aerosols and ice cores. Here, we present measurements of δ³⁴S to compliment previous measurements of Δ¹⁷O in Antarctic ice cores. Such measurements provide further support for Δ¹⁷O measurements as being representative of variations in paleo-oxidant levels.