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 (δ34SSO4δ34SSO4) and sulfide (δ34SH2Sδ34SH2S) were similar to trends observed in the Black Sea water column: δ34SH2Sδ34SH2S and δ34SSO4δ34SSO4 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 δ34SH2Sδ34SH2S 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). δ34SH2Sδ34SH2S and Δ33SH2SΔ33SH2S 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 δ34SSO4δ34SSO4 values near the interface suggests ‘readdition’ of 34S-depleted sulfate during sulfide oxidation. A slight increase in δ34SSO4δ34SSO4 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 δ34SH2Sδ34SH2S and Δ33SH2SΔ33SH2S data also do not show a clear role for sulfur-intermediate disproportionation in the deep anoxic water column. We interpret the large difference in δ34S between sulfate and sulfide as reflecting fractionations during SR in the Cariaco deep waters that are larger than those generally observed in culturing studies.