Stable sulfur isotopic evidence for historical changes of sulfur cycling in estuarine sediments from northern Florida

Data on abundance and isotopic composition of porewater and sedimentary sul fur species are reported for relatively uncontaminated and highly contamina ted fine-grained anoxic sediments of St. Andrew Bay, Florida. A strong cont rast in amount and composition of sedimentary organic matter at the two sit es allows a comparative study of the historical effects of increased organi c loading on sulfur cycling and sulfur isotopic fractionation. In the conta minated sediments, an increase in organic loading caused increased sediment ary carbon/sulfur ratios and resulted in higher rates of bacterial sulfate reduction, but a lower efficiency of sulfide oxidation. These differences a re well reflected in the isotopic composition of dissolved sulfate, sulfide , and sedimentary pyrite. Concentration and isotopic profiles of dissolved sulfate, organic carbon, and total sulfur suggest that the anaerobic decomp osition of organic matter is most active in the upper 8 cm but proceeds at very slow rates below this depth. The rapid formation of more than 90% of p yrite in the uppermost 2 cm which corresponds to about 3 years of sediment deposition allows the use of pyrite isotopic composition for tracing changi ng diagenetic conditions. Sediment profiles of the sulfur isotopic composit ion of pyrite reflect present-day higher rates of bacterial sulfate reducti on and lower rates of sulfide oxidation, and record a profound change in th e diagenetic cycling of sulfur in the contaminated sediments coincident wit h urban and industrial development of the St. Andrew Bay area.