SULFUR AND IRON CYCLING IN A COASTAL SEDIMENT - RADIOTRACER STUDIES AND SEASONAL DYNAMICS

The seasonal variation in sulfate reduction and the dynamics of sulfur and iron geochemistry were studied throughout a year in sediment of A arhus Bay, Denmark. A radiotracer method for measuring sulfate reducti on rates was applied with incubation times down to 15 min and a depth resolution down to 2 mm in the oxidized surface layer of the sediment. The radiotracer data were analyzed by a mathematical model which show ed that, due to partial, rapid reoxidation of radioactive sulfide duri ng incubation, the actual reduction rates in this layer were probably underestimated 5-fold. In the deeper, sulfidic zone, measured rates ap peared to be correct. Sulfate reduction followed the seasonal variatio n in temperature with maximum activity at 1-2 cm depth in late summer. In spite of its rapid production, free H2S was detectable in the pore water only below the depth of free Fe2+ at 6-7 cm throughout the year. Following the massive sedimentation from a spring phytoplankton bloom , anaerobic degradation of phytoplankton detritus was strongly stimula ted over several weeks. A transient reversed redox zonation developed with a thin, black zone on top of the brown, oxidized sediment layer d ue to intensive sulfate and iron reduction. Mineralization through sul fate reduction was equivalent to two thirds of the annual net sediment ation of organic matter.