Stable sulfur isotopes in the water column of the Cariaco Basin

Abstract

Previous geochemical and microbiological studies in the Cariaco Basin indicate intense elemental cycling and a dynamic microbial loop near the oxic–anoxic interface. We obtained detailed distributions of sulfur isotopes of total dissolved sulfide and sulfate as part of the on-going CARIACO time series project to explore the critical pathways at the level of individual sulfur species. Isotopic patterns of sulfate ( δ 34 S SO 4 ) and sulfide ( δ 34 S H 2 S ) were similar to trends observed in the Black Sea water column: δ 34 S H 2 S and δ 34 S SO 4 were constant in the deep anoxic water (varying within 0.6‰ for sulfide and 0.3‰ for sulfate), with sulfide roughly 54‰ depleted in 34S relative to sulfate. Near the oxic–anoxic interface, however, the δ 34 S H 2 S value was ∼3‰ heavier than that in the deep water, which may reflect sulfide oxidation and/or a change in fractionation during in situ sulfide production through sulfate reduction (SR). δ 34 S H 2 S and Δ 33 S H 2 S data near the oxic–anoxic interface did not provide unequivocal evidence to support the important role of sulfur-intermediate disproportionation suggested by previous studies. Repeated observation of minimum δ 34 S SO 4 values near the interface suggests ‘readdition’ of 34S-depleted sulfate during sulfide oxidation. A slight increase in δ 34 S SO 4 values with depth extended over the water column may indicate a reservoir effect associated with removal of 34S-depleted sulfur during sulfide production through SR. Our δ 34 S H 2 S and Δ 33 S H 2 S data also do not show a clear role for sulfur-intermediate disproportionation in the deep anoxic water column. We interpret the large difference in δ 34S between sulfate and sulfide as reflecting fractionations during SR in the Cariaco deep waters that are larger than those generally observed in culturing studies.