OXIDATION AND REDUCTION OF RADIOLABELED INORGANIC SULFUR-COMPOUNDS IN AN ESTUARINE SEDIMENT, KYSING FJORD, DENMARK

35S-labeled SO42-, S2O32-, S0, ΣHS- (=H2S + HS- + S2-), and FeS2 were used to trace the oxidative and reductive pathways of the sulfur cycle. We studied the transformation of 35SO42-, 35S2O3-, 35S0, AV35S ( =ΣH35S- + Fe35S), and Fe35S2 in 8-cm long undisturbed estuarine sediment cores in time course experiments of up to 24 h. All the tracers, except for pyrite, were oxidized and/or reduced at all depths. More than 60% of the 35S from 35SO22- reduction was recovered as 35S0g in the top two cm of the sediment. At >2 cm, nearly all of the reduced 35SO42- was recovered as AV35S plus Fe35S2. Thiosulfate was not detected in the sediment. From the combined data of outer- and inner-labeled 35S2O32- injections, concurrent oxidation, reduction, and disproportionation of S2O32- were demonstrated. In oxidized surface sediment the three processes comprised 10, 24, and 66%, respectively, of the metabolized 35S2O32-. In reduced sediment these percentages were 16, 45, and 39%. Injections of SH35S- into S2O32- spiked sediment cores produced 71% 35SO42- and 29%35S2O32- (% of the total 35SO42- + 35S2O32- in the oxidized zone and 8% 35SO42- and 92% 35S2O32- in the reduced zone. A similar experiment with 35S0 produced 62% 35SO42- and 38% 35S2O32- in the oxidized zone and 22 and 78% in the reduced zone. We calculated that more than half of the 35S0 and ΣH35S- oxidation to SO42- proceeded through S2O32- in the reduced sediment. In the oxidized sediment these percentages were 15 and 21% for SH35S- and 35S0, respectively. Thiosulfate was thus a key intermediate in the S cycle. © 1990.