Dynamics of zero-valent sulfur species including polysulfides at seep sites on intertidal sand flats (Wadden Sea, North Sea)

Zero-valent sulfur species dynamics in tidal flat pools of Wadden Sea (North Sea) were studied. Concentrations of solid sulfur, colloidal sulfur of different particle sizes and distribution of inorganic polysulfide species were detected near the bottom of different pools as a function of time after detachment of the pool from sea. Concentrations of these species were detected also in the incubated sediment cores taken from the same site with the pumping of anoxic seawater, anoxic seawater spiked with sulfide and anoxic seawater spiked with lactate. Sulfide concentration in the pool increased during the first 2. h of pool detachment from the sea to a maximum of 273 μM. Polysulfides were present at the beginning of the detachment of the pool and higher concentrations of elemental sulfur and polysulfides lead to more rapid coagulation and precipitation of dispersed solid sulfur. The process of the formation of zero-valent sulfur began with the formation of polysulfides and colloidal sulfur with a particle size less than 5 μm. Further coagulation led to an increase in the amount of colloidal sulfur with the particle size more than 5 μm followed by crystallization of solid sulfur particles. Maximum concentration of colloidal sulfur detected in the pools was 73 μM and that of solid sulfur was 25 μM. Maximum concentration of the sum of all polysulfide species measured was 5.9 μM, which corresponded to 23.1 μM of detected polysulfide sulfur or to 27.7 μM of polysulfide sulfur taking into account the calculated concentrations of species with concentration under detection limit. Polythionates were not detected. Levels of total zero-valent sulfur usually reached the value of 40 μM, but can be even higher than 100 μM. This depended on the overall concentration of sulfide. In the tidal pools, polysulfide chain lengths were short and the polysulfide system was found not to be in equilibrium with solid or colloidal sulfur at any time of pool detachment from sea. Flow-through incubation experiments, however, exhibited polysulfide in equilibrium with respect to rhombic sulfur. No difference in the polysulfide speciation was observed in experiments where sulfide was artificially added and sulfide was produced through enhanced bacterial sulfate reduction. The differences were attributed to the kinetics and to competing oxidation processes present in the exposed Wadden Sea tidal pools as compared to the longer running and more restricted incubation experiments.