Sulfate reduction related to the early diagenetic degradation of organic matter and “black spot” formation in tidal sandflats of the German Wadden Sea (southern North Sea): stable isotope (13C, 34S, 18O) and other geochemical results

A stable isotope geochemical investigation (34S/32S, 13C/12C, 18O/16O) has been carried out to study the development of anoxic surface sediments (“black spots”) in the backbarrier tidal area of Spiekeroog island (southern North Sea). Microbial sulfate reduction related to the degradation of mainly marine organic matter under anaerobic conditions led to the accumulation of hydrogen bisulfide (up to 8 mM) in the interstitial waters of the sandy sediments. While the carbon isotopic composition of organic matter reflected the downcore preferential degradation of labile (marine) organic matter, sulfur and oxygen isotope variations in the dissolved sulfur species provided information about the microbiologically mediated complex interactions between the sulfur, carbon and oxygen cycles.