The Carbon-Sulfur Link in the Remineralization of Organic Carbon in Surface Sediments

Here we present the carbon isotopic composition of dissolved inorganic carbon (DIC) and the sulfur isotopic composition of sulfate, along with changes in sulfate concentrations, of the pore fluid collected from a series of sediment cores located along a depth transect on the Iberian Margin. We use these data to explore the coupling of microbial sulfate reduction (MSR) to organic carbon oxidation in the uppermost (up to nine meters) sediment. We argue that the combined use of the carbon and sulfur isotopic composition, of DIC and sulfate respectively, in sedimentary pore fluids, viewed through a delta C-13(DIC) vs. delta S-34(SO4) cross plot, reveals significant insight into the nature of carbon-sulfur coupling in marine sedimentary pore fluids on continental margins. Our data show systemic changes in the carbon and sulfur isotopic composition of DIC and sulfate (respectively) where, at all sites, the carbon isotopic composition of the DIC decreases before the sulfur isotopic composition of sulfate increases. We compare our results to global data and show that this behavior persists over a range of sediment types, locations and water depths. We use a reactive-transport model to show how changes in the amount of DIC in seawater, the carbon isotopic composition of organic matter, the amount of organic carbon oxidation by early diagenetic reactions, and the presence and source of methane influence the carbon and sulfur isotopic composition of sedimentary pore fluids and the shape of the delta C-13(DIC) vs. delta S-34(SO4) cross plot. The delta C-13 of the DIC released during sulfate reduction and sulfate-driven anaerobic oxidation of methane is a major control on the minimum delta C-13(DIC) value in the delta C-13(DIC) vs. delta S-34(SO4) cross plot, with the delta C-13 of the organic carbon being important during both MSR and combined sulfate reduction, sulfate-driven AOM and methanogenesis.

Automated summary

The study investigates the coupling of microbial sulfate reduction to organic carbon oxidation in sedimentary pore fluids through carbon and sulfur isotopic composition analysis. Systemic changes in the isotopic composition of dissolved inorganic carbon and sulfate are observed across various marine sediment types, locations, and water depths. A reactive-transport model demonstrates the influence of various factors such as seawater DIC concentration, organic carbon isotopic composition, and methane presence on the isotopic composition of sedimentary pore fluids.