Influences of iron and manganese cycling on alkalinity in the redox stratified water column of Chesapeake Bay

Abstract. The coastal alkalinity cycle controls the global burial of carbonate which modulates the ability of the ocean to trap anthropogenic CO2. Twelve high vertical resolution profiles from the temperate Chesapeake Bay estuary during two summers allow precise description of carbonate dynamics over the salinity and redox gradient along with the measurement of the speciation of most redox sensitive elements. In the presence of oxygen, carbonate dissolution, primary production and aerobic respiration are able to explain the evolution of total alkalinity (TA) versus dissolved inorganic carbon (DIC), once corrected for fresh and oceanic water mixing. A significant flooding event in 2018 prevented the trapping of atmospheric CO2 in the estuary and favoured carbonate dissolution to balance DIC consumption from photosynthesis. In oxygen depleted waters, a particularly high ratio of alkalinity versus DIC occurred (ΔTAex/ΔDICex = 2.4), that has not been previously reported in the literature, and that seemed invariant over the two years. The stoichiometric analysis agrees with Mn measurements to explain this carbonate signature by the critical role of MnO2 reduction followed by Mn carbonate precipitation. Our results underline that Fe and Mn are critical elements of the alkalinity cycle, especially due to their ability to limit the H2S oxidation into SO42- and by favouring sulphur burial.