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

Abstract

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?m²) 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 δ³⁴S 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 δ³⁴S_SO4 and δ¹⁸O_SO4 as expected of microbial sulfate reduction. Instead, pore water δ³⁴S_SO4 values decreased (from approximately 21‰ to 17‰) as temperature increased (up to 97.4°C) across each hydrothermal feature. We interpret the inverse relationship between temperature and δ³⁴S_SO4 as a mixing process between oxic seawater and ³⁴S-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 δ³⁴S_SO4, the oxygen isotope composition of sulfate tended to be ¹⁸O-enriched in low pH (<5), high temperature (>75°C) pore waters. The shift toward high δ¹⁸O_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 δ³⁴S value of H₂S discharged to the seafloor is consistent with equilibrium isotope exchange with subsurface anhydrite veins at a temperature of ~300°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.