Sulfate reduction and the formation of 35S‐labeled FeS, FeS2, and S0 in coastal marine sediments

A comparative survey was done of the formation of radiolabeled end‐products during 3 5 SO 4 2− reduction measurements in coastal sediments of Denmark. The distribution of reduced 35 S in acid‐volatile sulfides (AVS), pyrite, and elemental sulfur in the upper 0–15 cm of sediment depended on the sulfur chemistry and on the overall sulfate reduction rates. In sediments with relatively low metabolic rates, undetectable H 2 S, and low FeS : FeS 2 ratio : 20), only 32–55% of the reduced 35 S was recovered in the AVS pool. In sediments with high metabolic rates, high H 2 S, and high FeS : FeS 2 ratio (>1 : 10), the 35 S recovery in AVS was 63–92%. The relative contribution of Fe 35 S 2 formation showed little depth dependence, whereas the formation of 3 5 S° was highest in the more oxidizing sediment layers near the surface. Inclusion of 35 S‐labeled FeS 2 and S° in the sulfate reduction measurement yields more accurate rate data than are obtained from AVS alone. Due to possible isotopic exchange reactions and unknown pathways of H 2 S transformation into AVS, S°, and FeS 2 , however, the radioactivities of the three pools cannot be used directly to calculate their differential rates of formation.