Climate change and productivity variations recorded by sedimentary sulfur in Lake Edward, Uganda/D. R. Congo

We investigate changes in the sulfur geochemistry of sediment cores from Lake Edward, Uganda/Congo to constrain the effects of recent climate changes in the region on the partitioning and isotopic composition of reduced sedimentary sulfur. We measured the concentration and isotopic composition of kerogen sulfur, chromium-reducible, and acid-volatile sulfides, as well as the abundance and partitioning of sedimentary phosphorus and iron, and the abundance and isotopic composition of organic carbon and nitrogen. Results indicate an approximately 150% increase in sulfur concentrations in sediments from Lake Edward over the past 200 years, during which time rainfall in the basin increased and the salinity of the lake fell. Increased sulfur concentrations are largely accounted for by increasing concentrations of kerogen sulfur, which is likely caused by environmentally-driven changes in organic matter quality and concentration, including increased anoxia and nutrient supply to the lake during wet intervals. The isotopic compositions of sedimentary sulfur pools support our interpretation that organic sulfur is derived primarily from sulfurization reactions in the sediment rather than primary bio-sulfur, and gradual isotopic enrichment of the δ 34S of sedimentary pyrite and kerogen sulfur could result from decreased lake salinity and sulfate availability during the past 200 years. These results highlight the importance of understanding environmentally-mediated changes in organic sulfur accumulation in interpreting the sulfur geochemistry of sediments and sedimentary rocks.