The Jøtul field revisited: High carbon and hydrogen fluxes from a sediment‑hosted hydrothermal vent site in the Knipovic Ridge

We report on the composition of gas‑tight hydrothermal vent fluid samples from the Jøtul field at the ultraslow spreading Knipovic ridge, collected during the R/V MARIA S. MERIAN expedition MSM131 in September 2024. The sampled fluids exhibit high pH values and total alkalinities. Elevated methane concentrations–exceeding those at the sediment-hosted Guaymas Basin vent site–suggest fluid-sediment interaction and thermal decomposition of organic matter derived from continental sediments. These fluids also contain high hydrogen concentrations (>14 mM), which surpass typical values for sediment-hosted hydrothermal vent fluids. The elevated hydrogen levels are accompanied by low H₂S concentrations (< 2.5 mM), which might point to a heazlewoodite-pentlandite mineral assemblage controlling the concentrations of these compounds. We suggest that the hydrothermal vent fluids at the Jøtul field acquire their distinct chemical signatures through a combination of fluid‑sediment interactions in the recharge and discharge zones, along with fluid rock interactions governed by ultramafic rocks in the high‑temperature reaction zone. This combination of subsurface conditions produces vent fluids that are metal‑poor but enriched in carbon and hydrogen. The high methane concentrations measured in the Jøtul field highlight hydrothermal fluid‑sediment interactions as a yet underestimated source of carbon emissions into the ocean.