Enigmatic H2- and CH4-rich hydrothermal plumes at the ultramafic-hosted Lucky B site, 81°N on Lena Trough, Arctic Ocean

Tectonic uplift of mantle rocks along slow- and ultraslow-spreading mid-ocean ridges facilitates diverse styles of hydrothermal circulation. Here, we report on Lucky B, an ultramafic-hosted hydrothermal field on the ultraslow-spreading Lena Trough at 81°N in the ice-covered Arctic Ocean. At the seafloor we observed diffuse, metal-poor fluid discharge with abundant vent fauna alongside sites of massive sulfide deposits and hydrothermal chimneys, extending laterally over at least 1.9 km. The overlying water column exhibited two geochemically distinct plumes, the stronger of which showed strong redox and particle anomalies. We hence identify Lucky B as 'black smoker'-type system featuring distinct styles of venting from several major fluid sources. The strongest plume also contained high concentrations of dissolved hydrogen (H₂) and methane (CH₄), distinguishing Lucky B from other ultramafic-hosted systems that primarily emit serpentinization-derived H₂. Low H₂/CH₄ ratios and high CH₄ relative to dissolved Mn suggest an involvement of sediment in the subseafloor fluid-rock reactions. Our analysis of the plume microbiology revealed abundant chemoautotrophs that use primarily hydrothermal H₂ and sulfide as energy sources. Collectively, these findings reveal multifaceted hydrothermal venting at Lucky B, driven by geological and biogeochemical processes in the subseafloor and extending into the Arctic Ocean water column.