Four new hydrothermal vent fields were discovered on the slow spreading Central Indian Ridge (8–12°S; Segments 1–3), all located off‐axis on abyssal hill structures or Ocean Core Complexes (OCCs). Each site was characterized using seafloor observation (towed camera system), plume chemistry (Fe, Mn, and CH4; Conductivity, Temperature, and Depth sensor [CTD]/Miniature Autonomous Plume Recorder [MAPR]), and rock sampling (TVgrab/dredges). Different styles of venting on each segment reflect different geological settings, rock types, likely heat sources, and fluid pathways. The segment 1 field was located on the western flank of the axial valley at the base of OCC‐1‐1. High‐temperature venting was inferred from plume characteristics and extensive seafloor sulfide mineralization, but only diffuse venting was observed. This site appears to be a magmatic‐influenced basaltic‐hosted system despite its off‐axis location. Two low‐temperature diffusely venting sites were located on abyssal hills 6 and 9 km off‐axis on Segment 2. Plume particle, metal, and CH4 concentrations were all very low, suggesting dilution of hydrothermal fluids by intrusion of seawater into the highly permeable flank area fault zone. The “Onnuri Vent Field” (OVF), located at the summit of OCC‐3‐2, vented clear, low‐temperature fluids supporting abundant vent organisms (21 macrofaunal taxa). The plume particle signal was low to absent, but strong ORP anomalies correlated with high CH4 and low metal concentrations. Sulfide mineralization was present, which suggests both serpentinization and magmatic/lithospheric influence on fluid composition. The detachment fault is the likely pathway for hydrothermal fluid circulation at this off‐axis location. These new vent field discoveries, especially the OVF, contribute valuable information toward understanding Indian Ocean hydrothermal systems and their ecology/biogeography.

The regional distribution of hydrothermal vent activity in the northeastern (NE) Lau Basin was recently reported by the Ridge 2000 program; however, active venting sources have yet to be located. Here, we report geological and hydrological evidence that indicates the presence of three active hydrothermal venting sources in the NE Lau Basin: the Fonualei Rift and Spreading Center (FRSC), the Northeast Lau Spreading Center (NELSC), and an off-axis caldera (MTJ-1). These examples of hydrothermal activity were recognized by the appearance of hydrothermal plume signals in the water column, including anomalies in light-transmission, methane, adenosine 5′-triphosphate (ATP), and trace metal concentrations (TDMn and TDFe). Three active venting sources were identified by the observation of possible buoyant plumes during conductivity-temperature-depth (CTD) tow-yo surveys and by the recovery of hydrothermal precipitates (chimneys and altered rocks). The strongest light-transmission anomaly, an order of magnitude greater than those at other sites, was observed at the central cone of the MTJ-1 caldera. The recovery of eruption debris at a central volcanic cone, including molten sulfur, volcanic ash, and lapilli, indicates an active volcanic eruption and hydrothermal venting at the MTJ-1 caldera. Our results suggest extensive and various hydrothermal activity in the NE Lau Basin, thereby providing valuable insight into hydrothermal and volcanic processes in back-arc environments.

Historical Russian data provide indication of winter convection reaching down to about 1000 m depth near Vladivostok. However, this kind of convection does not occur every winter. Further data analysis suggests that the location of convection is driven offshore by the coastal buoyant water, which otherwise would be the coastal area. The coastal buoyant water is mostly cold fresh water but occasionally warm coastal water in the south. Due to the large extent of fresh coastal water in the northern part of the basin, the convection does not happen in this area despite the most intense surface cooling.

The complex geology and expansive axial valleys typical of slow-spreading ridges makes evaluating their hydrothermal activity a challenge. This challenge has gone largely unmet, as the most undersampled MOR type for hydrothermal activity is slow spreading (20-55 mm/yr). Here we report the first systematic hydrothermal plume survey conducted on the Central Indian Ridge (CIR, 8 S-17 S), the most extensive such survey yet conducted on a slow-spreading ridge. Using a combined CTD/Miniature Autonomous Plume Recorder (MAPR) package, we used 118 vertical casts along seven segments of the CIR ( 700 km of ridge length) to estimate the frequency of hydrothermal activity. Evidence for hydrothermal activity (particle and methane plumes) was found on each of the seven spreading segments, with most plumes found between 3000 and 3500 m, generally <1000 m above bottom. We most commonly found plumes on asymmetric ridge sections where ultramafic massifs formed along one ridge flank near ridge-transform intersections or nontransform offsets. The estimated plume incidence (p h ) for axial and wall casts (p h 50.30, 35 of 118 casts) is consistent with the existing global trend, indicating that the long-term magmatic budget on the CIR is the primary control on the spatial frequency of hydrothermal venting. Our results show that the tectonic fabric of the CIR strongly determines where hydrothermal venting is expressed, and that using only near-axial sampling might underestimate hydrothermal activity along slow-spreading and ultraslowspreading ridges. Serpentinization is a minor contributor to the plume inventory, based on 15 profiles with methane anomalies only, predominantly at depths above the local valley walls.

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