Sub-seafloor sulfur cycling in a low-temperature barite field: A multi-proxy study from the Arctic Loki’s Castle vent field

The Loki's Castle vent field at the ultraslow-spreading Arctic Mid-Ocean Ridge (AMOR) hosts a low-temperature venting area, which is characterized by microbial mats and numerous up to 1 m tall barite chimneys. An actively-venting barite chimney yielded δ18OSO4 and δ34SSO4 values heavier than ambient seawater and sulfide-oxidizing bacteria in microbial mats identified microbial sulfate reduction and sulfide oxidation as the main processes. In order to investigate the chemical and microbial structure below the barite field, we obtained two gravity cores and present chemical composition (e.g., H2S, SO42−, NH4+, DIC) and stable isotope data for the pore fluids (δ34SSO4, δ13CDIC) together with stable isotope (δ13Corg, δ34S, ∆33S) and lipid biomarker data on bulk sediments. The gravity core more distant to the high-temperature vents shows seawater-like pore fluid profiles with only minor vent fluid contribution (<1%), whereas the bulk sediments yield negative δ34S and positive ∆33S values indicative of sulfate reduction. In contrast, the pore fluid data in close proximity to the high-temperature vents (5–9% vent fluid contribution) record distinct horizons showing sulfate depletion, which coincide with δ34SSO4 values that are higher than those for ambient seawater sulfate. The sediments in these horizons record negative δ34S and positive ∆33S values, indicating that both the pore fluids and the sediments are influenced by active sulfate reduction. We also detected a greater abundance of archaeal mono- and dialkyl tetraether lipids (GMGTs, GDGTs) and bacterial fatty acids in the sediments at actively venting sites, pointing to a more diverse microbial community. Moreover, a positive correlation observed between GMGT abundance and sulfur concentration in the sediments indicates that the availability of sulfur is crucial for the presence of GMGT-producing archaea. Our multi-proxy approach suggests that sulfate reduction in the sub-seafloor sediments of the Loki's Castle barite field is largely driven by microbial processes.