OXYGEN ISOTOPES AS TRACERS OF SULFUR CYCLING IN DYNAMIC SEDIMENTS

In sediments, sulfur is heavily used by microbes for energy yielding (dissimilatory) processes. Dissimilatory sulfate reduction (DSR) coupled to the oxidation of organic matter plays a key role in the re-mineralization of organic matter once other energetically more favorable electron acceptors such as oxygen or nitrate are consumed. When sulfate is depleted methane becomes the end product of organic matter degradation. At the interface between downward diffusing sulfate and upward diffusing methane (sulfate-methane transition) sulfate is consumed by sulfate reduction coupled to the anaerobic oxidation of methane (SR-AOM). In this view, DSR is confined to the main sulfate zone, a redox zone where the more favorable electron acceptors are absent. The products of DSR and SR-AOM, sulfide and zero-valent sulfur can be oxidized back to sulfate if oxidants become available. Sulfur in an intermediate oxidation state, such as zero-valent sulfur or polysulfides can be disproportinated into sulfate and sulfide. In conjunction with the sulfate reduction these sulfate-producing processes constitute a closed sulfur cycle, which due to the lack of observable net fluxes, has been reffered to as cryptic. Dynamic sedimentary systems with changing sedimentation rates and fluid flows induce fluctuations in geochemical gradients and thus can be the ultimate drivers of such closed-loop, cryptic sulfur cycling.

The stable oxygen isotope signature of sulfate retains information about coupled reductive and oxidative sulfur cyling, and provides insight into the circumstances that allow such a combination of reactions to occur. However, to be able to take advantage of this tool, we need know what isotope signature we should look for. To address this need we compare two different sulfur and oxygen isotope data sets, one from marine sediments and the other from sediment incubation studies with stable oxygen isotope labels.