Isotope fractionation by sulfate-reducing natural populations and the isotopic composition of sulfide in marine sediments

Isotope fractionations during sulfate reduction hy natural bacterial popula tions were measured in seven different marine sediments and compared with t he isotopic composition of solid-phase sulfides in the same sediments. The measured fractionations during sulfate reduction could explain only between 41% and 85% of the S-34 depletion in the sedimentary sulfides, This result directly demonstrates th:lt the depletion of S-34 in solid sulfides is an expression of the combined activity of sulfate-reducing organisms with addi tional fractionations accumulated during the oxidative part of the sulfur c ycle. The only known process to significantly augment the fractionations cr eated during sulfate reduction is the microbial disproportionation of the i ntermediate sulfur compounds: elemental sulfur, thiosulfate, and sulfite, I n a simple model, we show how important each of these disproportionation pr ocesses could be in generating the isotopic composition of sedimentary sulf ides. The sulfate reduction rates in the sediments studied varied by a fact or of 1000 and did not correlate with the fractionation during sulfate redu ction, By contrast, a correlation was observed between sulfate-reduction ra te and the extent of S-34 depletion into sedimentary sulfides, the most S-3 4-depleted sulfides found in sediments supporting the lowest rates of sulfa te reduction. Thus, the fractionations imposed during the disproportionatin g processes are better expressed in sediments with low sulfate-reduction ra tes. The direct phototrophic oxidation of sulfide to sulfate, with minimal fractionation, may have been important in the sediments that had a high sul fate-reduction rate.