Effects of microbial alteration of oceanic crust on sulfur cycling in hydrothermal systems

The accumulation of continental-derived sediment along mid-ocean ridges can influence sub-seafloor hydrothermal fluid circulation, including fluid composition, and the composition of associated mineral accumulations at the seafloor. The intermediate spreading-rate Juan de Fuca Ridge, off the west coast of North America, hosts both sedimented and sediment-free hydrothermal systems (Middle Valley and Axial Volcano, respectively), as well a hydrothermal system which occurs at the outer extent of continental-derived turbiditic sediment accumulation (Endeavour Hydrothermal Vent Field). The seafloor at Endeavour is mostly sediment-free and consists of basaltic lava flows. However, previously reported hydrothermal vent fluid compositions suggest the presence of buried sediment. Here, we compare S isotope ratios (³³S/³²S and ³⁴S/³²S) of hydrothermal chimney and talus samples, as well as sediments, from these three varyingly sedimented hydrothermal sites on the Juan de Fuca Ridge to S isotope ratios from hydrothermal deposits along the fast spreading archetypical East Pacific Rise in order to investigate the geological controls and microbial influence on hydrothermal S cycling. Using a combined isotopic mixing and fractionation model in Δ³³S and δ³⁴S space, we demonstrate that reduced S within sediments does not provide a significant contribution to the S budget of hydrothermal systems in sedimented environments on the Juan de Fuca Ridge. Instead, our data indicate that variations in S isotope compositions within and between vent fields reflects different degrees of sub-seafloor microbial crustal alteration and kinetic fractionations associated with reduction of seawater sulfate. The degree of microbially induced S isotope fractionation can be linked to intensity of crustal alteration and therefore age of the hydrothermal system. Our results illustrate the added value of a multiple S isotope approach to investigating S cycling in hydrothermal systems, and that the influence of microbial activity on hydrothermal systems extends well below the seafloor.