Sulfur and oxygen isotope insights into sulfur cycling in shallow-sea hydrothermal vents, Milos, Greece

Shallow-sea (5 m depth) hydrothermal venting off Milos Island provides an ideal opportunity to target transitions between igneous abiogenic sulfide inputs and biogenic sulfide production during microbial sulfate reduction. Seafloor vent features include large (>1 m(2)) white patches containing hydrothermal minerals (elemental sulfur and orange/yellow patches of arsenic-sulfides) and cells of sulfur oxidizing and reducing microorganisms. Sulfide-sensitive film deployed in the vent and non-vent sediments captured strong geochemical spatial patterns that varied from advective to diffusive sulfide transport from the subsurface. Despite clear visual evidence for the close association of vent organisms and hydrothermalism, the sulfur and oxygen isotope composition of pore fluids did not permit delineation of a biotic signal separate from an abiotic signal. Hydrogen sulfide (H2S) in the free gas had uniform delta S-34 values (2.5 +/- 0.28%, n = 4) that were nearly identical to pore water H2S (2.7 +/- 0.36%, n = 21). In pore water sulfate, there were no paired increases in delta S-34(SO4) and delta O-18(SO4) as expected of microbial sulfate reduction. Instead, pore water delta S-34(SO4) values decreased (from approximately 21 parts per thousand to 17 parts per thousand) as temperature increased (up to 97.4 degrees C) across each hydrothermal feature. We interpret the inverse relationship between temperature and delta S-34(SO4) as a mixing process between oxic seawater and S-34-depleted hydrothermal inputs that are oxidized during seawater entrainment. An isotope mass balance model suggests secondary sulfate from sulfide oxidation provides at least 15% of the bulk sulfate pool. Coincident with this trend in delta S-34(SO4), the oxygen isotope composition of sulfate tended to be O-18-enriched in low pH (<5), high temperature (>75 degrees C) pore waters. The shift toward high delta O-18(SO4) is consistent with equilibrium isotope exchange under acidic and high temperature conditions. The source of H2S contained in hydrothermal fluids could not be determined with the present dataset; however, the end-member delta S-34 value of H2S discharged to the seafloor is consistent with equilibrium isotope exchange with subsurface anhydrite veins at a temperature of similar to 300 degrees C. Any biological sulfur cycling within these hydrothermal systems is masked by abiotic chemical reactions driven by mixing between low-sulfate, H2S-rich hydrothermal fluids and oxic, sulfate-rich seawater.