The sulfur biogeochemistry of chemosynthetic cold seep communities, gulf of Mexico, USA

Abstract

In hydrocarbon seep sediments associated with chemosynthetic communities along the continental slope of the northern Gulf of Mexico, the distribution of sulfate reduction rates is very heterogeneous, with rates ranging from values typical of off-seep Gulf environments to very high rates (up to ∼14.8 μmol cm −3 day −1). These extreme rates deplete porewater sulfate within 20 cm and are associated with increases in dissolved sulfide and DIC concentrations. Large depletions of porewater calcium and high concentrations of solid phase CaCO 3 indicate that carbonate precipitation is driven by sulfate reduction as well. Although total organic carbon concentrations (reflecting an unresolved mix of ambient biomass, crude oil and other seep hydrocarbons) are much higher than elsewhere in the Gulf, sulfate reduction rates are not related in any simple fashion to POC. In addition, apparent diffusion coefficients ( D S*) computed from sulfate gradients and reduction rates typically exceed molecular values. In tubeworm- Beggiatoa mat associations these excursions approach two orders of magnitude. We suggest that these elevated D S* values may indicate specific organism-sediment interactions that replenish sulfate, either through sediment irrigation or in situ sulfur recycling. Interactions between Beggiatoa and tubeworms may lead to large enhancements of the apparent flux of sulfate to the zone of SR, and give rise to very high SR rates that are otherwise probably limited in duration and lateral extent.