SULFUR ISOTOPE EXCHANGE BETWEEN S-35 LABELED INORGANIC SULFUR-COMPOUNDS IN ANOXIC MARINE-SEDIMENTS

Isotope exchange reactions between S-35-labeled sulfur compounds were studied in anoxic estuarine sediment slurries at 21-degrees-C and pH 7.4-7.7. Two experiments labeled with radioactive elemental sulfur (S-35-degrees) and one labeled with radioactive sulfate ((SO42-)-S-35) were performed as time-course experiments of up to 10 days. More than 85% of the added S-35-degrees was distributed into the reduced sulfur pools comprising SIGMA-HS-(=H2S - HS- + S2-), iron sulfide (FeS) and pyrite (FeS2) in less than 10 min after S-35-degrees labeling. When 1.2-mu-mol SIGMA-HS- cm-3 was present in the slurry, 64% of the total S-35 was recovered in the SIGMA-HS- pool in less than 1.5 h. With no detectable SIGMA-HS- (less than 1-mu-M) in the slurry, 58% of the total S-35 was observed in the pyrite pool within 1.5 h. The FeS pool received up to 31% of all S-35 added. The rapid S-35 incorporation from S-35-degrees into SIGMA-HS- and FeS pools was explained by isotope exchange reactions. In contrast, there was evidence that the radioactivity observed in the 'pyrite pool' was caused by adhesion of the added S-35-degrees to the FeS2 grains. In all S-35-degrees-labeled experiments we also observed oxidation, in the absence of oxygen, of reduced sulfur to (SO42-)-S-35 which reached more than 24% of the total S-35 after 72 h. In the (SO42-)-S-35 experiment, (SIGMA-HS)-S-35 was formed and we observed a significant isotope exchange between (HS-)-S-35 and S-degrees as well as precipitation of (FeS)-S-35. As a result of isotope exchange, specific radioactivities of the reduced sulfur pools were poorly defined and could not be used to calculate their rates of formation. Such isotope exchange reactions between the reduced inorganic sulfur compounds will affect the stable isotope distribution and are expected to decrease the isotopic fractionations. All S-35, however, that was incorporated into AVS (i.e. SIGMA-HS- + FeS), S-degrees and FeS2 during short-term (SO42-)-S-35 incubations originated from the formed (H2S)-S-35 and realistic sulfate reduction rates therefore were measured from the sum of the radioactivities of reduced sulfur compounds.