Flow and hydrography at a hydrothermally-active axial valley in the northern Juan de Fuca Ridge.

High-resolution transects of flow and hydrography over a 3-km hydrothermally-active portion of Endeavour Segment (Juan de Fuca Ridge) are presented. The measurements span the bottom 300 m, including the 100- m deep axial valley. The hydrographic sections reveal background gradients in temperature, salinity and density with colder, saltier, and denser water to the south and east of the axial valley. Coincident with the along-valley density gradient is a bottom-intensified, northward along-valley current. The vertical shear of this current scales with the thermal wind shear derived from the cross-valley density gradient, and the flow is aligned with the baroclinic pressure gradient associated with the along-valley density gradient. Both the density gradient and along-valley flow are observed in repeated sections taken at different times. In order to maintain a steady state momentum balance necessary for the persistence of the along-valley current on long timescales, the baroclinic pressure gradient force could be balanced by vertical friction or advection of momentum, or lateral friction along the valley sidewalls. Hydrothermal plumes in their rising, laterally spreading, and equilibrated stages are also observed in the sections. Rising plumes are evidenced by density inversions, convergent flow surrounding their cores, and cyclonic vorticity of magnitude 10 times the Coriolis parameter. From the limited number of rising plumes that were detected in the towyo surveys over each vent field, it appears that plume salinity and buoyancy anomalies vary from north to south, as plumes over the northern High Rise field are fresher and more buoyant than those over the Main Endeavour field to the south. Considerable temporal and spatial variability in the maximum vertical extent of the plumes is evident in repeated sections, and is likely caused by tidal advection and the inherent intermittency of the plumes themselves. Near the level of neutral buoyancy, thin layers of thermohaline intrusions with water mass properties characteristic of hydrothermal effluent are seen to emanate from regions of rising plumes. On a particular cross-valley section, divergent flow is found on one of these layers, suggestive of active lateral spreading. Models for hydrothermal plumes predict that during their equilibrated stage, this lateral spreading is arrested by rotation and an anticyclonic plume vortex filled with hydrothermal effluent forms. A bolus of hydrothermal effluent of 1.5 km in diameter suggestive of a plume vortex is observed on one of the sections. Whether this feature is truly a plume vortex is difficult to discern from velocity measurements due to the strong tidal motions that could have easily masked the relatively weak anticyclonic circulation thought to characterize a vortex of the same dimensions as the bolus.