Influence of manganese cycling on alkalinity in the redox stratified water column of Chesapeake Bay

The alkalinity dynamics in coastal environments play a crucial role in controlling the global burial of carbonate minerals and the ocean's capacity to sequester anthropogenic CO2. This study presents results from high vertical resolution profiles obtained during two summers in the temperate Chesapeake Bay estuary, enabling detailed investigation of carbonate dynamics over salinity and redox gradients, along with measurement of the speciation of most redox-sensitive elements. Under oxygen-rich conditions, carbonate dissolution, primary production and aerobic respiration explain the evolution of total alkalinity (TA) versus dissolved inorganic carbon (DIC), once adjusted for fresh and oceanic water mixing. A significant flooding event in 2018 promoted carbonate dissolution. In oxygen-depleted waters, we observed a previously unreported 2.4 mol increase in DIC per 1 mol of TA production, which was consistent over the 2 years. Stoichiometric changes suggest that MnO2 reduction followed by Mn carbonate precipitation is responsible for this characteristic carbonate signature, likely produced in sediment pore water and then transferred to the water column along with other by-products of anoxic respiration at the onset of summer. Our findings highlight the critical role of Mn in alkalinity dynamics in the Chesapeake Bay and potentially other river-dominated environments where it can limit H2S oxidation to SO42- and promote sulfur burial.