High H2 production in sediment-hosted hydrothermal fluids at an ultraslow spreading mid-ocean ridge

Abstract Hydrothermal systems at ultraslow-spreading mid-ocean ridges remain poorly characterized, particularly where sedimentary and ultramafic influences intersect. Here we present geochemical analyses of vent fluids collected in 2024 from the Jøtul hydrothermal field on the northern Knipovich Ridge. Major element concentrations, dissolved gases, and thermodynamic modeling are used to investigate fluid-rock interactions. The fluids exhibit exceptionally high CH 4 concentrations, that exceed those at the Guaymas Basin, and display characteristics typical of sediment-hosted hydrothermal systems, indicating thermal decomposition of organic matter in clastic sediments. In contrast, high H 2 (>15 mM) and low H 2 S concentrations are more typical of ultramafic-hosted fluids, while geological evidence indicates that the vent field lies atop a detachment fault. Thermodynamic modeling suggests that these high H 2 /H 2 S ratios may result solely from degradation of organic matter followed by abiotic CH 4 oxidation at ~400 °C, rather than from reactions with ultramafic rocks. These results expand the known diversity of sediment-hosted vent fluid compositions and highlight fluid-sediment interaction as an underestimated source of carbon and hydrogen.