Stable isotope compositions of cold-seep bivalves can illuminate processes that affect the chemical and isotopic compositions of seeping fluids along the continental slope of the Gulf of Mexico as well as provide insight into the physiological ecology of these species. Carbon and nitrogen isotope compositions were analyzed in mussels and clams from 14 seep sites spanning a depth range of 1000 to 2800m along the lower Louisiana slope of the Gulf of Mexico. Mussels of three species found on the lower slope, Bathymodiolus childressi, B. brooksi, and B. heckerae, showed site-specific differences in tissue δ13C, reflecting differences in the local methane pool. Mussels from sites on the lower slope sitting atop the contiguous salt sheet generally had tissue δ 13C values that reflected a stronger biogenic methane signal (-70.8 to -58.8%) than mussels on the upper slope or seaward of the Sigsbee Escarpment (-67.3 to -40.4%). Clams (Calyptogena ponderosa and Calyptogena sp. nov.) had a narrow range of δ13C values between -37.0 and -34.4%, indicating that their thiotrophic symbionts are fixing primarily seawater-dissolved organic carbon. The most depleted tissue δ15N values yet published for both mussels and clams are reported in this study at -23.7 and -9.2%, respectively. These depleted values have implications for the assimilation of inorganic nitrogen by these symbioses and the concentrations of particular inorganic nitrogen sources in the local environment.
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