Effect of Sediment Inputs from the Mississippi River on Diagenetic Processes Across the Northeastern Gulf of Mexico Continental Margin

The Mississippi River provides large inputs of freshwater and sediment to the Gulf of Mexico, especially during peak discharge in winter and spring. Depending on riverine discharge, the frontal zone of the Mississippi extends toward the mouth of its distributaries or into the lower reaches of the river. In addition, deltaic sediments experience resuspension events during peak discharge, creating mobile muds and transporting altered terrigenous material to the adjacent continental slope that may affect sediment biogeochemistry and fluxes across the sediment-water interface. In this study, sediment cores were collected from the shelf near the Mississippi River mouth (60 m water depth) to the continental slope southward (1,000 m) to determine the effect of sediment transport during peak discharge on early diagenetic processes. Sediment cores were electrochemically profiled for O_2(aq), pH, Mn²⁺, Fe²⁺, and ΣH₂S, then sectioned for pore water analyses of DIC, total alkalinity, NH₄⁺, NO₃^-, SO₄^2-, and ΣPO₄^3-, and solid phase speciation of Mn, Fe, and S. Results indicate sediment transport influences the intensity of anaerobic carbon remineralization processes. Although sulfate reduction dominated anaerobic respiration processes close to the delta frontal zone, sulfate reduction was not evident in slope sediments. Instead, ferric iron accumulated in the solid phase, likely due to rapid reoxidation of iron sulfide or ferrous iron with dissolved oxygen during resuspension events. In turn, reduced manganese diffused from shelf sediments as a result of its slower oxidation kinetics and was likely entrained by mobile muds and ultimately precipitated as manganese oxides on the slope, where they were rapidly consumed during microbial reduction. Overall, the differential kinetics of manganese and iron redox cycling associated with resuspension events is likely able to keep sediments suboxic and affect the preservation of natural organic matter in these river-fed ecosystems.