The Jøtul hydrothermal field: High H2 production in sediment-hosted hydrothermal vent fluids at an ultraslow spreading mid-ocean ridge

We present results from our seagoing mission MSM131 aboard R/V Maria S. Merian to the Jøtul hydrothermal field, located on the ultraslow-spreading northern Knipovich Ridge. In autumn 2024, we conducted water column surveys, geological investigations, and collected hydrothermal vent fluids using isobaric gas-tight samplers. The Jøtul hydrothermal field lies near the continental shelf of Svalbard, where clastic sediments are deposited within the rift valley. The hydrothermal vent fluids are metal-poor, alkaline and exhibit the highest methane concentrations ever recorded at mid-ocean ridge hydrothermal vent sites, all indicating the importance of fluid-sediment interactions in the subsurface of this vent site. Unexpectedly, the vent fluids also contain hydrogen (H₂) concentrations that are substantially higher than those found in other sediment-hosted vent systems. Together with low hydrogen sulfide (H₂S) concentrations, this composition is typically associated with fluids that have interacted with ultramafic rocks. The presence of altered ultramafic rocks on the seafloor near the Jøtul field suggests that such interaction may have contributed to the fluid composition. However, our thermodynamic calculations indicate the high H₂/H₂S ratios are not necessarily related to ultramafic rock alteration, but rather may arise from thermal degradation of organic matter, followed by abiotic oxidation of methane at reaction-zone temperatures around 400°C. While high H₂ concentrations are often considered indicative of fluid-rock reactions with ultramafic rocks, our findings demonstrate that high-pressure, high-temperature fluid sediment interactions can also be a significant source of H₂ emissions into the ocean.