Seasonally Recurring Water Column Hypoxia Tightly Controls Manganese Distribution, Flux, and Carbonate Precipitation in Sediments of the Eutrophic Coastal Ocean

Ocean deoxygenation, increasingly driven by human activities and climate change, severely threatens ocean health. Although manganese (Mn) plays a pivotal role in sediment biogeochemistry, the dynamics of Mn in sediments associated with water column hypoxia (WCH) are understudied. To elucidate the impacts of seasonally recurring WCH on the Mn distributions and benthic flux and precipitation of Mn minerals in coastal sediments enriched with Mn, we combined sediment incubation experiments with X-ray absorption near-edge structure (XANES) analysis. Under severe WCH conditions, enhanced sulfate reduction (SR) stimulated abiotic Mn reduction (MnR) coupled with H₂S oxidation, which promoted Mn²⁺ release into the overlying water column. An inverse relationship between benthic Mn flux and bottom water dissolved oxygen concentrations further suggested that severe WCH induces high dissolved Mn persistence in the water column for at least a quarter of the year, ultimately affecting the health of coastal ecosystems. The XANES analysis revealed simultaneous Mn(IV)-oxides depletion and MnCO₃ formation under severe WCH conditions. In particular, the increase in MnCO₃ precipitation is likely a result of enhanced SR and MnR generating bicarbonate and Mn²⁺, highlighting the potential enhancement of microbially induced carbonate precipitation and hence carbon sequestration in Mn-rich coastal sediments underlying hypoxic water columns.