MANGANESE, IRON, AND SULFUR CYCLING IN A COASTAL MARINE SEDIMENT, AARHUS BAY, DENMARK

The seasonal variation in oxidized and reduced pools of Mn, Fe, and S, as well as the rates of SO4(2-) reduction, were studied in a fine-gra ined sediment. Below the 1-5 mm thick oxic zone, a zone of net Mn redu ction extended to 1-2 cm depth, while iron reduction was found to 4-6 cm depth. Although the reactive Mn oxide pool was ten times smaller th an the reactive Fe(III) pool, the average ratio between depth gradient s of Fe and Mn oxides was only 1.7, which implied that rates of Mn and Fe reduction were similar. Sulfate reduction was maximal near the bot tom of the suboxic zone, but fine-scale measurements showed that it ex tended to the upper 0-2.5 mm during summer, when the zones of Mn and F e reduction were compressed towards the surface. Most of the H2S produ ced precipitated as iron sulfides and S0 by reaction with Fe. Both Fe( III) and a nonsulfur-bound authigenic Fe(II) pool reacted efficiently with H2S. The authigenic Fe(II) pool was present at one hundredfold hi gher concentration than dissolved Fe2+. Only 15% of the precipitated s ulfide was buried permanently. Most of the reoxidation of reduced S oc curred within 1 cm of the sediment-water interface and was supported b y upward bioturbation. All of the estimated Mn reduction could be coup led to the reoxidation of reduced S and Fe. Partial oxidation of H2S, forming S0 and pyrite, accounted for 63% of the estimated Fe reduction . The remaining Fe reduction was coupled to complete oxidation of redu ced S or to C mineralization. The settling of a diatom spring bloom ca used distinct maxima in SRR and Mn2+ at 0.5-1 cm depth within two week s. In autumn, the reactive Mn oxides were depleted due to a net releas e of Mn2+ to the water column. Thus, the Mn cycle extended significant ly into the water column, while a constant Fe pool over the year sugge sts that the Fe cycle was restricted to the sediment.