Discovery of the “Lucky B” ultramafic-hosted hydrothermal field in the ultraslow-spreading Lena Trough, Arctic Ocean

Deep-sea hydrothermal systems are particularly difficult to locate and investigate in the ice-covered Arctic Ocean. Here we report findings of a system in the ultraslow-spreading Lena Trough at 81°22’N, from Expedition PS137 of the icebreaker R/V Polarstern in the summer of 2023. The site, named ‘Lucky B’, was first discovered after the recovery of massive sulfides in a dredge haul in 1999 (Snow et al., 2001) on the western flank of the Lucky Ridge, a 130 km-long ultramafic topographic high. We investigated the buoyant and non-buoyant parts of Lucky B’s hydrothermal plume making use of physical sensors and geochemical hydrothermal tracers. The non-buoyant plume was found at approx. 300–400 m above the seafloor and was characterized by high turbidity and pronounced anomalies in oxidation–reduction potential and temperature (up to ~80 mV and 0.02°C, respectively). In its buoyant part, the plume contained high dissolved H2 (~450 nmol/L) and CH4 (~250 nmol/L). Our seafloor observations, using deep-sea robotics, revealed widespread traces of hydrothermal activity at approx. 3,000–3,300 m water depth. These included hydrothermally discolored rocks and sediment and local outflow of clear hydrothermal fluids with abundant macrofauna. Guided by our discoveries, follow-on operations with the Norwegian icebreaker R/V Kronprins Haakon resulted in ROV dives to a large black smoker vent field with a number of several m-high chimneys. The visual appearance of the suggests vent fluid temperatures similar to those of other high-temperature ultramafic-influenced systems (e.g., 365°C at Rainbow; Charlou et al., 2002). Confirmation awaits comparison with ongoing analyses of hydrothermal plume samples (He isotope signatures, Fe and Mn concentrations, CH4:Mn ratios) and of sulfide chimneys dredged in 1999 (mineral assemblages). In the Arctic, Lucky B is the first hydrothermal system that has been traced to its seafloor source that is associated with ultramafic rocks outcropping at the seafloor. Additional work will be required to conceive its role on biogeochemical cycles in the ice-covered ocean.