| Summary: | 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 activelyventing barite chimney yielded delta O-18(SO4) and delta S-34(SO4) 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 (delta S-34(SO4), delta C-13(DIC)) together with stable isotope (delta C-13(org), delta S-34, Delta S-33) 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 delta S-34 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 delta S-34(SO4) values that are higher than those for ambient seawater sulfate. The sediments in these horizons record negative delta S-34 and positive Delta S-33 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 ...
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