MANGANESE OXIDATION AND IN-SITU MANGANESE FLUXES FROM A COASTAL SEDIMENT

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 M n flux resulted in bottom water concentrations of dissolved Mn up to 0 .6 muM, whereas concentrations above the pycnocline were about 0.1 muM . Similar fluxes were observed from sediment cores incubated in the la boratory under in situ conditions. A large variation between cores was attributed to the small area covered by each core. Manganese reductio n in the upper 0-1 cm of the sediment supported steep porewater gradie nts 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 t hin oxic surface layer with turnover times of 2 h or less. Model calcu lations including measured microdistributions Of O2 and pH yielded rat e constants more than 1000 times higher than those reported for abioti c Mn oxidation implying that Mn oxidation in the sediment was microbia lly mediated. The rapid oxidation was, as an internal source of oxidiz ed Mn, essential to the intense redox cycling of Mn in the surface sed iment.