Fractionation of Sulfur Isotopes in the Cariaco Basin

The detailed distribution of sulfur isotopes was studied in the water column of the Cariaco Basin, with high depth resolution across the redox transition zone, in Nov. 2007 and May 2008 as part of the on-going CARIACO (CArbon Retention In A Colored Ocean) time series project. Long term monitoring (1995 to present) of the vertical position of the chemocline (200-350 m) and thickness of the suboxic zone (0-50 m) reveals highly dynamic spatial and temporal variability tied to the influx of oxygenated water into the restricted basin. The link between the basin hydrography and chemoautotrophic activity is potentially expressed in chemical speciation and composition of sulfur compounds cycled within the chemocline. For Nov. 2007 cruise, we measured the δ34S of total dissolved sulfide (δ34SH2S) and sulfate (δ34SSO4), and the δ18O of sulfate (δ18OSO4) from water samples. In close agreement with results from the Black Sea, δ34SH2S, δ34SSO4 and δ18OSO4 were relatively constant in the deep anoxic water, with the δ34SH2S depleted in 34S relative to δ34SSO4 by roughly 50 permil. However, near the oxic-anoxic interface, δ34SH2S was 3 permil higher than that in the deep water due to sulfide oxidation and/or in situ sulfide production. In May 2008, we collected water samples to measure three sulfur isotopes (33S, 34S and 36S) in sulfate, sulfide and elemental sulfur to explore the contribution of biological processes (sulfate reduction, sulfur intermediates disproportionation, and chemoautotrophic sulfur oxidation) to the isotopic composition of sulfur compounds in the basin. Also during the same cruise, an oxygen intrusion event was observed and we will present sulfur isotope data associated with that event.