Quantification of oxidative sulphur cycling in marine sediments
The goal of this proposal is to quantify the rates and processes of sulphide oxidation in coastal sediments. The marine sulphur cycle is driven by the reduction of sulphate to sulphide coupled to microbial decomposition of organic matter, a process which is responsible for up to 50 % of carbon mineralisation in coastal sediment and is intimately connected to earth’s biogeochemical evolution. The sulphide produced by sulphate reduction may either react with Fe to be buried as pyrite or may be oxidised through different pathways and intermediates. The amount of sulphide that is oxidised in marine sediments is not known, yet oxidative sulphur cycling has critical implications for estimates of sulphate reduction, sub-seafloor carbon mineralisation, methane oxidation, microbial metabolism and the interpretation of ancient and modern stable isotope signatures. Quantification of oxidative sulphur cycling, including sulphide oxidation rates, the formation of key intermediate species and associated stable isotope effects is therefore fundamental to our understanding of sub-seafloor biogeochemistry. 
     The research proposed herein will address these yet unanswered questions through a novel combination of radiotracer experiments, stable isotope measurements, and analyses of inorganic sulphur speciation. This approach will be applied to (1) quantify rates of concurrent sulphate reduction and sulphide oxidation in coastal sediment, (2) quantify the formation of intermediate species formed during sulphide oxidation and (3) measure stable isotope effects associated with oxidative sulphur cycling. This work will result in a more accurate description of sediment biogeochemistry by resolving current debates regarding the role of sulphur for carbon mineralisation ad further constraining processes which affect the sulphur stable isotope distribution.