Deep Sediment-Sourced Methane Contribution to Shallow Sediment Organic Carbon: Atwater Valley, Texas-Louisiana Shelf, Gulf of Mexico

Coastal methane hydrate deposits are globally abundant. There is a need to understand the deep sediment sourced methane energy contribution to shallow sediment carbon relative to terrestrial sources and phytoplankton. 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 (average methane/ethane ratio similar to 11,000) and isotope ratios of methane on the mound (average delta C-13(CH4(g)) = -71.2 parts per thousand; Delta C-14(CH4(g)) = -961 parts per thousand) indicate a deep sediment, microbial source. Depleted sediment organic carbon values on mound (delta C-13(SOC) = -25.8 parts per thousand; Delta C-14(SOC) = -930 parts per thousand) relative to off-mound (delta C-13(SOC) = -22.5 parts per thousand; Delta C-14(SOC) = -629 parts per thousand) suggest deep sourced ancient carbon is incorporated into shallow sediment organic matter. Porewater and sediment data indicate inorganic carbon fixed during anaerobic oxidation of methane is a dominant contributor to on-mound shallow sediment organic carbon cycling. 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.