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Estimates of the heat output of hydrothermal vents, identified along the Endeavor and Southern segments of the Juan de Fuca Ridge, are used to evaluate the total heat flux associated with hydrothermal circulation for the ridge segment. A 50‐m array carried by DSV Alvin sampled the temperature and vertical velocity structure of hydrothermal plumes from individual vents. These measurements are used to estimate the thermal flux associated with such plumes. The maximum heat flux calculated for a single vent is 50 MW (1 MW = 1×106 W). The median heat flux per vent is 9 MW and 3 MW, respectively, for the Endeavour Segment (18 vents) and Southern Segment (18 vents). The estimates for any given vent may vary over an order of magnitude. This uncertainty is due mainly to the difficulty of locating the centerline of the plume relative to the point of measurement, although the uncertainties in the constants for the appropriate equations based on laboratory experiments also contribute significantly to the net error. For the Endeavor Segment, the minimum total geothermal heat flux due to hydrothermal circulation exceeds 70 MW. The minimum estimate for the Southern Segment is 16 MW. The maximum estimate is probably closer to the total heat flux from high‐temperature venting (239 MW and 66 MW respectively). High‐temperature hydrothermal venting accounts for only a small fraction of the heat available according to steady state predictions of conductive heat flux; other hydrothermal phenomena (e.g., diffuse flow) probably account for a greater proportion of the total hydrothermal heat flux.

[1] We measure expansion rate and bending in a 23-hour time series of acoustic images of the lower 25 m section of a buoyant hydrothermal plume rising from Grotto vent in the Main Endeavour Field, Juan de Fuca Ridge. We then calculate entrainment coefficient, the constant of proportionality relating mean inflow velocity at the plume edge to maximum mean upward velocity within the plume. The plume section alternately bends southwest at relatively high inclinations (37°) and northeast at lower inclinations at irregular intervals twice during this time period, apparently driven by current reversals in the mixed semi-diurnal tidal cycle. The measured expansion rates (0.11 -0.25 m/m) and calculated entrainment coefficients (0.07 -0.18) are directly proportional to the degree of bending (R 2 = 0.75). The loss of buoyancy flux related to enhanced mixing in a stratified environment during bending may contribute to reduction of potential rise height consistent with predicted ($400 m) and measured (300 -350 m) plume tops. Citation: Rona, P. A.,

[2] We present a 26 day time series (October 2010) of physical properties (volume flux, flow velocity, expansion rate) of a vigorous deep-sea hydrothermal plume measured using our Cabled Observatory Vent Imaging Sonar (COVIS), which is connected to the Northeast Pacific Time Series Underwater Experiment Canada Cabled Observatory at the Main Endeavour Field on the Juan de Fuca Ridge. COVIS quantitatively monitors the initial buoyant rise of the plume from $5 m to $15 m above the vents. The time series exhibits temporal variations of the plume vertical volume flux (1:93 À 5:09 m 3 =s ), centerline vertical velocity component (0:11 À 0:24 m=s ) and expansion rate (0:082 À 0:21 m=m ); these variations have major spectral peaks at semidiurnal ($2 cycle/day) and inertial oscillation ($1:5 cycle/day) frequencies. The plume expansion rate (average $0:14 m=m ) is inversely proportional to the plume centerline vertical velocity component (coefficient of determination R 2 $ 0:5). This inverse proportionality, as well as the semidiurnal frequency, indicates interaction between the plume and ambient ocean currents consistent with an entrainment of ambient seawater that increases with the magnitude of ambient currents. The inertial oscillations observed in the time series provide evidence for the influence of surface storms on the dynamics of hydrothermal plumes.

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