Microbial sulfate reduction within the Iheya North subseafloor hydrothermal system constrained by quadruple sulfur isotopes

Abstract

Subseafloor hydrothermal systems may host spatially extended and numerically abundant microbial communities sustained by sulfate reduction as one of the important terminal electron accepting metabolisms. In order to estimate microbial sulfate reduction in a subseafloor hydrothermal regime, we analyzed sulfur isotopes (S32/S33/S34/S36) of pore-water sulfate and mineralized sulfide in the upper 100 m of sedimentary sequences at the Iheya North hydrothermal field in the Okinawa Trough recovered in Integrated Ocean Drilling Program Expedition 331 (IODP Exp 331). On the basis of the pore water chemistry and temperature profiles, the subseafloor environment is divided into three hydrogeologic units. In the topmost Unit-1, relatively fresh seawater is recharged, and the bottommost Unit-3 is characterized by predominance of endmember-like high-temperature hydrothermal fluid (>300 °C) underlying the impermeable cap rock layers. Intermediate Unit-2 is subject to mixing between the hydrothermal fluid and seawater. The δ34S values of sulfate in the Unit-2 mixing zone were found to be more 34S-enriched than the values expected from simple mixing model of seawater sulfate in the Unit-1 with the hydrothermal fluid in the Unit-3. The observed SSO434-enrichment and sulfate concentration [SO2−4]-depletion suggest sulfate reduction is taking place below the seafloor. Based on our model calculation, the isotope discrimination (ε34) is estimated to be −21‰. This large isotope discrimination together with slight Δ33S′ enrichment and Δ36S′ depletion reveals that sulfate reduction is caused by microbial processes but not by thermochemical processes. In addition, our numerical simulation points out that sulfate may be reduced prior to presently undergoing mixing with high-temperature fluid, probably within the seawater recharge zone. Despite the abundant input of hydrothermal H2S, mineralized sulfide below 10 m seafloor (mbsf) shows characteristic sulfur isotopic signatures that support the significant contribution of subseafloor microbial sulfate reduction, potentially corresponding to approximately 20% of the total sulfide mineral formation. Active microbial sulfate reduction below the seafloor may be promoted by significant input of seawater to the habitats through the vigorous hydrothermal circulation in vicinity of Iheya North field.