Microelectrode measurements of pH and pCO2 identify a very restricted zone of CaCO3 undersaturation immediately below the sediment–water interface in otherwise supersaturated environments (i.e., sandwiched between supersaturated bottom seawater and sediment porewater). This zonation characterizes the taphonomically-active zone, and is a result of a highly compressed redox front between acid-generating aerobic oxidation of reduced chemical species including hydrocarbons, H2S, and planktonic-and-terrestrial organic carbon and base-generating sulfate reduction coupled to CH4 oxidation. Porewater geochemistry is spatially variable at seep sites, and produces variable shell-dissolution signatures. Shells deployed at seep sites have moderate to severe dissolution that is consistent with a much higher flux of total dissolved inorganic carbon (DIC) from the porewater to the bottom water. Therefore, a mosaic of preservational conditions is directly related to the spatially and chemically varying taphonomically-active zones found at seep sites. These findings support the variability of carbonate preservation reported for globally-distributed Phanerozoic fossil seeps and the view that data from field taphonomy can significantly upgrade and validate carbonate destruction rates used in geochemical and climatic models. Carbon mass-balance analyses also lead to an important conclusion that carbonate dissolution forms a very important mechanism for benthic carbon recycling, possibly accounting for 50%of the benthic DIC flux to the bottom water in northern Gulf of Mexico petroleum seep sediments.