Hydrography and flow in the axial valley of the Endeavour segment: implications for larval dispersal

Abstract

The axial valley of the Endeavour Segment (Juan de Fuca Ridge) is a region of intense hydrothermal activity that supports an exotic chemosynthetic ecosystem. Previous studies of circulation in the region have mostly focused on levels above the valley and near the rise height of high-temperature (>300°C) effluent from focused sources to obtain an integrated measure of heat and chemical fluxes into the deep ocean. However, it is thought that a large portion of hydrothermal heating is attributable to lower-temperature (5--50°C) buoyant fluid that emanates from diffuse hydrothermal sources and equilibrates at levels within the valley. Additionally, the largest concentrations of vent organisms and their larvae are found within the bottom meters of the axial valley.The intent of this thesis is to characterize the flow and hydrography from the level of neutral buoyancy of the high temperature plumes, down through the axial valley and into the bottom boundary layer (BBL) and contribute to the understanding of transport processes that pray be involved in the near-bottom dispersal of larvae or diffuse hydrothermal effluent. This is done primarily through analysis of several observational data sets consisting of point measurements of flow, temperature, and turbulent heat and momentum fluxes; timeseries of velocity profiles within the valley; and high resolution cross/along valley sections of flow and hydrography that extend up to the level of neutral buoyancy.Flow-topography interaction along with convective hydrothermal plumes yield a very dynamic environment. Within the BBL, currents are spatially heterogeneous and turbulence is generated by a combination of shear and buoyancy that depends on the phase of the tide and the relative distance from buoyancy sources. Above the BBL, within the valley, and at a site away from hydrothermal vents, subinertial flows are intensified and re-aligned along valley towards the bottom. This is consistent with solutions from a quasianalytic model for subinertial motions, which shows attenuation of cross-valley flow with depth. Beneath the level of neutral buoyancy, sections of hydrography and velocity reveal structures related to rising high-temperature plumes superposed on across and along valley density gradients that are thought to play a role in driving along-valley currents.