During the August 2000 Flow Mow program we examined how Northeast Pacific hydrography is altered by hyrothermal plumes above the Endeavour Segment of the Juan de Fuca Ridge. Our goal was to define a background hydrography that would allow meaningful calculation of thermal anomalies and associated heat fluxes from measurements made above the Main Endeavour Field (MEF). We used a standard instrument package that measured temperature, conductivity, pressure, light transmission, optical backscatter, and altitude, but also attached redox potential sensors and Niskin bottles to characterize fluid chemistry. Additionally, we separated our two sets of temperature and conductivity sensors vertically on the instrument and found that the difference in potential density measurements provided a robust indicator of fluid instabilities.
We completed 40 navigated, small-scale surveys over the Segment. At 2 stations, north and south of the MEF, we monitored the effects of tidal currents on hydrothermal plume distributions by vertically cycling the package between 5 and 500m above the seafloor throughout multiple tidal cycles. We amended our spatial coverage with both a 24-hr vertically oscillating tow around the perimeter of the MEF and multiple horizontal tows directly over known MEF sources at 3 distinct, constant depths: 50, 80, and 300m above the axial valley floor.
Potential density profiles from the MEF vicinity suggest that a mixed-layer is a common near-bottom feature within the axial valley. The layer is typically about 50m thick and is defined by vertical profiles of potential temperature and density. On a regional scale, we also observed a weakening of the Northeast Pacific deep ocean stratification within about 300m of the bottom.
Both the vertical stations and the perimeter survey indicate that the neutrally buoyant plume above the MEF has a highly variable distribution. During our field program we observed equilibrated effluent clouds offset from known MEF sources by deep sea currents. Generally, the effluent (defined as anomalously warm, occlusive, turbulent, and reactive) appears to be advected to the southwest as predicted by 1995 current meter time series. The equilibrated plumes are intermixed with a complex assemblage of buoyant, chemically-active plumes that we intercepted over both focussed sources in the MEF and a diffuse flow site called Quebec.
Current measurements were made throughout the hydrographic survey by a nearby mooring with meters at about 50, 80, 110, 140, and 200m above the seafloor. We interpret our hydrographic results with attention to the current data and explore the calculation of heat fluxes with the combined data sets.