Ultramafic-influenced submarine venting on basaltic seafloor at the Polaris site, 87°N, Gakkel Ridge

• We have tracked Earth's northernmost hydrothermal system to its seafloor source. • Polaris discharges metal-poor, H 2 - and CH 4 -enriched fluids into the Arctic Ocean. • Polaris extends the known diversity of ultramafic-influenced hydrothermal systems. • Hybrid mafic–ultramafic vent sites may be common along ultraslow-spreading ridges. • Our study challenges the ability of established approaches to vent exploration. The nature of deep-sea hydrothermal systems is commonly inferred from physicochemical plume characteristics and seafloor observations, as was the case for the ‘Polaris’ site on the ultraslow-spreading Gakkel Ridge, Earth's northernmost hydrothermal system. Initial reports showing temperature and turbidity anomalies in its hydrothermal plume combined with its location on a neovolcanic axial seamount suggested a volcanically-hosted ‘black smoker’-type system. That interpretation, however, is inconsistent with our more complete data set derived from extensive water column sampling and seafloor surveys. The buoyant plume exhibits minor turbidity anomalies and low metal concentrations (dissolved Mn ≤ 3.1 nM), but contains substantial concentrations of H 2 (275 nM) and 13 C-enriched CH 4 (365 nM, δ 13 C = –13.2). Instead of a ‘black smoker’ vent field, we observed small-scale chimney structures at the seafloor. Together, these data imply intermediate-temperature reaction of hydrothermal fluids with ultramafic rock in the subseafloor before discharge through pillow basalt outcrops at the seafloor. Our study challenges the ability of established approaches to vent exploration, reliant exclusively on in situ sensing to reveal the full geodiversity of subseafloor hydrothermal venting. Ultramafic-influenced systems, releasing H 2 and CH 4 into the ocean, may be a recurring feature along the entire 25% of the global ridge system that is ultraslow-spreading.