Oxygen and sulfur isotopes insights into ongoing biogeochemical processes in sediments from the Argentine Basin

The Argentine Basin is characterized by dynamic depositional conditions, such as high sedimentation rates and gravity mass flows, where reactive iron and manganese mineral phases are rapidly buried. Sulfate pore water concentration profiles in this area reflect these variable depositional conditions. Interpretation of these patterns led to the conclusion that sulfate concentration profiles are controlled by methane fluxes, mass flows, diffusion or any combination of these processes (Hensen et al., 2003). Sulfur and oxygen isotope data of pore water sulfate provide additional constraints on the interpretation of sulfate concentration patterns and allow us to gain insight into ongoing biogeochemical processes in the sediment column. Samples were collected aboard the RV Meteor during a recent cruise to the Argentine Basin area (May-July 2009). We present concentration profiles of pore water sulfate and sulfide from several study sites in the Argentine Basin. We can identify three classifications of sulfate concentration profiles: Linear, Kink, and Concave Up (Hensen et al., 2003). We compare these profiles to the isotopic composition of oxygen and sulfur of sulfate and sulfur isotopic composition of sulfide. This allows us to identify zones with ongoing biogeochemical sulfur cycling. Based of sulfate isotope studies three primary observations can be made for our study sites: 1)oxygen isotopic composition of sulfate consistently plateaus near a value of 25‰, this is 4‰ lighter than expected if the sulfate is in equilibrium with normal seawater, 2)while oxygen isotopic composition approaches a constant value, the isotopic compositiion of sulfur continues to increase, indicative of ongoing sulfate reduction, 3)sulfate isotope cross plots from linear-type sulfate concentration profiles yield additional information about underlying processes contributing to the sulfate distribution within the sediment column. Hensen, C. et al. (2003) Geochimica et Cosmochimica Acta, 67, 2631-2647.