BioGeoChemistry of Tidal Flats


M. E. Böttcher, K. Bosselmann, S. Batel, B. Hespenheide, A. Theune

The biogeochemical sulfur cycle in intertidal surface sediments of the German wadden sea

The biogeochemistry of the sulfur cycle was studied in sandy, silty and muddy intertidal sediments of the German Wadden Sea (Spiekeroog, Sylt, Westerhever; North Sea). Sampling sites differ in grain sizes, organic matter (OM) and metal (Fe, Mn) contents. The study focusses on the seasonal dynamics of biogeochemical reactions in the S-Fe-Mn-C cycles. Main interest are the activity of sulfate-reducing bacteria, the corresponding development of S and O isotope signatures, and its relation to biogeochemical metal cycling. Especially, temperature and OM load were considered as process-controlling variables. Most sediments are characterized by the highest activity of sulfate reducers in the top 15 cm. In OM-rich sediments, temperature controls seasonal changes in microbial activity. Sulfur cycling leads to a characteristic partitioning of sulfur isotopes between sulfate and the reduced sulfur fractions. Sulfate and AVS (essentially FeS) reflect bacterial metabolism and re-oxidation of sulfur compounds, but pyrite compositions are additionally superimposed by transport processes. Depth-integrated sulfate reduction rates in sandy sediments were lower than in the muddy and silty sediments and controlled by the availability of reactive OM components besides temperature. Intense sulfide oxidation takes place in the top parts of the sediments linking the sulfur cycle partly to the dynamics in dissolved and solid phase Mn and Fe, as for instance AVS distributions.