Pore water profiles and authigenic mineralization in shallow marine sediments above the methane-charged system on Umitaka Spur, Japan Sea

Umitaka Spur, situated on an unusual collisional plate boundary along the eastern margin of the Japan Sea, hosts gas seeps, pock-marks, collapse structures, and gas hydrates. Piston cores were recovered from this ridge to understand carbon cycling, pore fluid gradients and authigenic mineralization above a methane-charged system. We present the chemistry of fluids and solids from three cores adjacent to seep locations. High fluxes of CH4 and alkalinity transport carbon from a deep zone of methanogenesis toward the seafloor. Methane, however, reacts with SO42- across a shallow sulfate–methane transition (SMT), which generates additional alkalinity and HS−. A fraction of these CH4 oxidation products form authigenic carbonate and pyrite. These minerals are not readily apparent from visual inspection of split cores, because they exist as micritic coatings on microfossils or as framboidal pyrite. They are, however, readily observed in chemical analyses as peaks of “labile” Ca, Sr, Ba or S in sediment at or near the SMT. Carbon inputs and outputs nicely balance across the SMT in all three cores if one considers four relevant fluxes: loss of alkalinity to the seafloor, addition of methane from below, addition of alkalinity from below, and carbonate precipitation. Importantly, in all cores, the magnitude of the fluxes decreases in this order. Although some carbon rising from depth forms authigenic carbonate, most (>80%) escapes to the ocean as alkalinity. Nonetheless, authigenic fronts in sediment on Umitaka Spur are a significant reservoir of inorganic carbon. Given calculated pore fluid fluxes for Ca and Sr, the fronts require tens of thousands of years to form, suggesting that the present state and loss of carbon represent long-lived processes.