Transformations of Sulfur in Sediment Microcosms

Laboratory microcosms were used to investigate rates of S movement into and out of sediments and factors affecting its net retention and speciation. Rates of sulfate reduction, calculated from movement of 35S into and out of the sediments, were much higher than net rates of sulfate movement into the sediments. Results suggested that high rates of sulfide oxidation accompanied high rates of sulfate reduction; net flux across the interface depended on the balance between these two processes. Both pyrite and acid-volatile sulfides were dynamic pools undergoing rapid formation and oxidation. Measurement of S in fulvic acid, humic acid, and humin fractions revealed differences between organic S formed by algae and that resulting from sulfate reduction and subsequent reactions in sediments; sulfate esters formed by algae were hydrolyzed in hot acid to a greater extent than sulfate esters formed within the sediments. The organic S species also reflected the extent of oxygenation of the overlying water; low oxygen in overlying water resulted in a high sulfate ester content in the humic acid fraction. Addition of Fe to sediments did not cause increased S retention, but did lead to greater formation of pyrite relative to iron monosulfides.