Black spots occur in the wadden sea of the southern North Sea as a result of the local burial of macro algae in tidal flats (Höpner and Michaelis, 1994). The anaerobic degradation of organic matter (Corg.) in the sediments is accompanied by the formation of CO2, H2S, and finally CH4. When H2S reaches the sediments surface a "black spot" is observed due to formation of FeSX. To follow the short and long-term effects of organic matter burial on pore water and sediment biogeochemistry, artificial black spots are investigated as part of the ecosystem research project "Elasticity of the Wadden Sea (ELAWAT)". The controlled formation of black spots was initiated on the Gröninger Plate (site RP63) in the back tide area of Spiekeroog island by the burial of different amounts of macro algae or starch. The chemistry of pore water profiles was investigated as a function of time with diffusion samplers (Höpner, 1981) or pore water lances up to 35 cm depths at 2 cm intervals. The results are compared to those of aerated and undisturbed reference areas. Dissolved sulfate (SO4) and reduced S-species (SHS = H2S + HS- + S2-) were precipitated quantitatively as BaSO4 and CdS, respectively, and the 34S/32S ratios were measured by means of combustion isotope-ratio-monitoring mass spectrometry (C-irmMS).
On areas with high amounts of buried Corg., SO4 (initial concentration: ~25 mmol/l) was extensively reduced and, due to the limited availability of iron in the sediments, SHS increased to values of more than 20 mmol/l. The d34S(SHS) values approached the isotopic composition of seawater sulfate (d34S = +21 vs. CDT) suggesting sulfate reduction under essentially closed system conditions. Residual SO4 was generally enriched in 34S with respect to co-existing SHS (Fig. 1), due to the kinetical preference of 32S upon bacterial sulfate reduction. Fractionation factors below 1.025 were observed in an area with high Corg. burden. As a function of time, the disturbed areas regenerated and sulfate reduction decreased leading to increasing sulfur isotope fractionation factors. Sulfate reduction in the pore waters of aerated or undisturbed areas was small compared to those with higher Corg. burden.
Höpner, Th. Env. Technol. Letters 2, 187-196 (1981).
Höpner, Th. & Michaelis, H., In Warnsignale aus dem Wattenmeer (Lozán et al., eds.) (1994).
Fig. 1: *34S(*HS) and *34S(SO4) values as a function of the SHS-fraction (F) for area A, 2 and 3 months after burial of 6 kg starch/m2 in about 15 cm depth.