Sediment organic carbon composition and provenance have been extensively studied in the Gulf of Mexico. Generally, inputs of terrestrially-derived organic carbon dominate near shore sediments and decrease with distance offshore where water column phytoplankton dominate contributions to shallow sediments. For this study shallow sediment and porewater samples were collected from Atwater Valley, Texas-Louisiana Shelf, Gulf of Mexico near a seafloor mound feature identified in geophysical surveys as an elevated bottom seismic reflection. Geochemical data revealed off-mound methane diffusion and active fluid advection on-mound. Gas composition and isotope ratios of methane on the mound indicate a deep sediment, microbial source. Depleted sediment organic carbon isotope values on mound relative to off-mound suggest deep sourced ancient carbon is incorporated into shallow sediment organic matter. A simple stable carbon isotope mass balance suggests carbon fixation of dissolved inorganic carbon (DIC) associated with anaerobic oxidation of hydrate-sourced CH4 contributes up to 85% of shallow sediment organic carbon. Coastal methane hydrate deposits are globally abundant. There is a need to understand the deep sediment sourced methane contribution to shallow sediment carbon relative to terrestrial sources and phytoplankton. → Go to Full ReportSediment organic carbon composition and provenance have been extensively studied in the Gulf of Mexico. Generally, inputs of terrestrially-derived organic carbon dominate near shore sediments and decrease with distance offshore where water column phytoplankton dominate contributions to shallow sediments. For this study shallow sediment and porewater samples were collected from Atwater Valley, Texas-Louisiana Shelf, Gulf of Mexico near a seafloor mound feature identified in geophysical surveys as an elevated bottom seismic reflection. Geochemical data revealed off-mound methane diffusion and active fluid advection on-mound. Gas composition and isotope ratios of methane on the mound indicate a deep sediment, microbial source. Depleted sediment organic carbon isotope values on mound relative to off-mound suggest deep sourced ancient carbon is incorporated into shallow sediment organic matter. A simple stable carbon isotope mass balance suggests carbon fixation of dissolved inorganic carbon (DIC) associated with anaerobic oxidation of hydrate-sourced CH4 contributes up to 85% of shallow sediment organic carbon. Coastal methane hydrate deposits are globally abundant. There is a need to understand the deep sediment sourced methane contribution to shallow sediment carbon relative to terrestrial sources and phytoplankton. → Go to Full Report