Manganese fluxes from the seafloor were measured in situ in Aarhus Bay, Denmark, with a free operating benthic flux-chamber lander (ELINOR). Constant effluxes were observed during 3 h incubations. The benthic Mn flux resulted in bottom water concentrations of dissolved Mn up to 0.6 μM, whereas concentrations above the pycnocline were about 0.1 μM. Similar fluxes were observed from sediment cores incubated in the laboratory under in situ conditions. A large variation between cores was attributed to the small area covered by each core. Manganese reduction in the upper 0-1 cm of the sediment supported steep porewater gradients of Mn towards the surface. However, calculated diffusive Mn fluxes towards the sediment surface were 3-16 times higher than the benthic effluxes. This demonstrated high rates of Mn oxidation in the 1-2 mm thin oxic surface layer with turnover times of 2 h or less. Model calculations including measured microdistributions of O 2 and pH yielded rate constants more than 1000 times higher than those reported for abiotic Mn oxidation implying that Mn oxidation in the sediment was microbially mediated. The rapid oxidation was, as an internal source of oxidized Mn, essential to the intense redox cycling of Mn in the surface sediment.