Fig. 2
Synthesis figure summarizing water column and seafloor imaging results. (a) Deepest hydrothermal signals were recorded just above the seafloor, where NUI operated in ROV mode detected anomalies in ORP and CH4 near 81°21.9ʹN (Supplementary Fig. S1) and the OFOBS system revealed visual and sidescan sonar evidence of hydrothermally active seafloor, together with co-registered ORP anomalies, between 3,100–3,200 m depth (Figs. 2c and 4, Supplementary Figs. S4 and S5). In the overlying water column, NUI’s mapping and sensing surveys and our CTD casts intercepted two distinct non-buoyant plumes: a deep plume at > 3,000 m depth with weak ΔORP and a shallow plume at ~ 2,800–3,000 m with strong ΔORP and elevated optical backscatter values as well as high H2 and CH4 concentrations (Figs. 2b, 3 and 5, Supplementary Figs. S2 and S3). Hydrothermal signals from these CTD and NUI operations extended farther south as compared to our seafloor imaging surveys, where they converged with the locations at which massive sulfide material and water column hydrothermal anomalies had previously been observed (dotted lines representing track lines from prior expeditions29,30), highlighting both the wide lateral extent and the decade-long activity of the Lucky B hydrothermal field. Shown tracks are cropped to ≥ 2,000 m depth. Dashed track lines indicate positions of NUI and OFOBS directly above the seafloor. Horizontal scale bar represents 200 m at 81°22.2ʹN; note that the north–south distance between 81°21.6ʹN and 81°22.8ʹN is ~ 2,200 m. (b) Depth profile from a single upcast of CTD 061 (at ~ 13:51–13:56 UTC near 81°22.7ʹN, indicated in (a) as a thicker pink line), displaying coinciding negative ORP (30 s averages) and beam transmission anomalies diagnostic of a fresh, particle-laden plume. (c) Time-series MAPR record from station OFOBS 062 showing pronounced ΔORP (30 s averages) and positive temperature anomalies at ~ 20:20 UTC, coincident with images of abundant, highly localized fauna (Fig. 4f, e). Gray bars indicate OFOBS intersections with the shallow and deep plumes at the start and end of the deployment, respectively. Dashed line marks OFOBS towed few meter above the seafloor.
