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

Abstract

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 H₂ (275 nM) and ¹³C-enriched CH₄ (365 nM, δ¹³C = –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 H₂ and CH₄ into the ocean, may be a recurring feature along the entire 25% of the global ridge system that is ultraslow-spreading.