Preferential dissolution of carbonate shells driven by petroleum seep activity in the Gulf of Mexico
Authigenic carbonates are common at deep-sea petroleum seeps as a result of excess bicarbonate production during microbial degradation of hydrocarbons coupled to sulfate reduction. Consequently, these seep environments are supersaturated with respect to carbonates. This finding conflicts with taphonomic data that dissolution is the most pervasive mode of shell alteration at seeps. We provide here the first study linking the preservational process with the chemical characterization of the taphonomically-active zone at petroleum seep sites. This characterization is made possible using fine-scale porewater carbonate chemistry data and skeletal material deployed for 8 years at petroleum seep sites in the northwestern Gulf of Mexico. 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.