Benthic biogeochemical processes and fluxes in the hypoxic and acidified northern Gulf of Mexico, part II: Influence of Mississippi-Atchafalaya River system inputs on the differential cycling of Mn, Fe, and S in Louisiana continental shelf sediments

River-dominated ocean margins are carbon processing hot spots, but the spatiotemporal dynamics of the benthic biogeochemical cycling of Mn, Fe, and S under different discharge regimes and the impact of these variations on carbon cycling are not well characterized. Here, a comparative analysis of pore water profiles of redox-sensitive chemical species reveals the biogeochemical restructuring that occurred over a summer-spring-summer sequence on the Louisiana continental shelf in the northern Gulf of Mexico. Increases in depth-averaged pore water inventories, including exceptionally high Mn(II) concentrations, were associated with high river discharge and sediment inputs, suggesting that Mn reduction plays an important role in Louisiana continental shelf sediments, except during below-average discharge periods in summer when iron and sulfate reduction are promoted. The river effect was greatest and most consistent proximal to the Mississippi River and extended at least ~60 km westward when high winds and discharge enhanced plume dispersal, whereas the area where seasonal bottom water hypoxia often develops remained largely isolated from any riverine influence. A river imprint on biogeochemical patterns was also observed south of Atchafalaya Bay, suggesting the area periodically receives inputs from the bay throughout the year. Results demonstrate that Mn(II) often traces these hydrodynamic processes and that river loads of Mn (oxyhydr)oxides buffer the redox state of shelf sediments and moderate the biogeochemical response to low discharge and hypoxia. Stores of river-derived Mn may therefore function as an environmental redox capacitor. Based on the patterns observed between the two summers, a period of Mn reduction may persist for ~20 days after Mississippi River discharge subsides to below the threshold for net mobilization of sediment to the shelf.