Stable carbon and nitrogen isotope compositions of hydrocarbon-seep bivalves on the Gulf of Mexico lower continental slope

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 2800 m 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 delta C-13, reflecting differences in the local methane pool. Mussels from sites on the lower slope sitting atop the contiguous salt sheet generally had tissue delta C-13 values that reflected a stronger biogenic methane signal (-70.8 to -58.8 parts per thousand) than mussels on the upper slope or seaward of the Sigsbee Escarpment (-67.3 to -40.4 parts per thousand). Clams (Calyptogena ponderosa and Calyptogena sp. nov.) had a narrow range of delta C-13 values between -37.0 and -34.4 parts per thousand, indicating that their thiotrophic symbionts are fixing primarily seawater-dissolved organic carbon. The most depleted tissue delta N-15 values yet published for both mussels and clams are reported in this study at -23.7 and -9.2 parts per thousand, 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. (C) 2010 Elsevier Ltd. All rights reserved.