Sulfate reduction rates in the sediments of the Mediterranean continental shelf inferred from combined dissolved inorganic carbon and total alkalinity profiles

Microbial sulfate reduction in marine sediments is coupled either to anaerobic oxidation of methane (S-AOM) or organic material (organoclastic sulfate reduction, OSR). These two pathways of sulfate reduction are important components of the geochemical cycles of both sulfur and carbon in marine systems because they change the redox state of both elements and increase the dissolved inorganic carbon (DIC) and total alkalinity (TA) concentrations in sediment pore water. Here, we determine reaction rates of OSR and S-AOM in the sulfate methane transition zone (SMTZ) of the sediment based upon the different effect of each process on DIC and TA. Although TA has been used as a diagnostic proxy for the identification and determination of the governing chemical processes in numerous previous studies, we show that it can also be applied to disentangle and quantify net sulfate reduction rates in the SMTZ, provided that OSR and S-AOM are the principal processes that affect DIC and TA, and that the effects of other processes, such as carbonate mineral precipitation, are quantified by additional data. By integrating the obtained rates, we determine the contribution of each pathway to the total sulfate reduction. Calculated results from pore water profiles from the Southeastern Mediterranean continental shelf indicate that within the SMTZ, located at a depth of approximately 1 m, each pathway accounted for about half of the total sulfate reduction. The calculated OSR and S-AOM rates were similar to estimations of sulfate reduction rates based upon pore water sulfate profiles from studies in other sedimentary systems, in spite of the differences between these environments and the Southeastern Mediterranean continental shelf, showing that TA and DIC are a robust method for calculating net sulfate reduction. Furthermore, we show here that TA considerations can be used to quantitatively constrain the fractions of reduced sulfate that eventually precipitates as pyrite and FeS, versus the fraction that is oxidized and precipitates as elemental sulfur.