Two mud volcanoes in the Gulf of Mexico were examined to understand barium cycling in shallow seafloor sediment at regions of intense methane expulsion. Due to anaerobic oxidation of methane and sulfate reduction, barium interacts with methane and sulfate, producing barite fronts at the sulfate-methane transition and barium-rich pore fluids underneath. Formation waters likely feed volcanoes and deliver large amounts of Ba2+ to the system from below. Locally elevated Ba2+ concentrations amplified cycling, producing sigmodial pore water profiles in the shallowest sediment. The expulsion of Ba-rich fluids directly to the water column concentrates barite in surficial sediments, which can also enhance cycling. Further, two types of barium-rich, carbonate nodules were recovered. Rocky nodules resemble buried carbonate crusts, diagenetically altered by pore fluids. Smooth nodules are likely preserved barite fronts marking past sulfate-methane transitions. Both types of nodules can serve as a modern analogue for barite deposits in the geologic record.