J. W. Lavelle

On the calculation of total heat, salt and tracer fluxes from ocean hydrothermal events

Abstract Total heat, salt, and other tracer masses released during a hydrothermal event are shown to be proportional to, but not necessarily equal to, volume integrals of resulting water column anomalies. Proportionality coefficients depend on anomaly definition, on background hydrographic and tracer profiles, on expansion coefficients of the equation of state at an appropriate pressure, and on tracer to heat anomaly ratios at the venting source. For Gorda Ridge event plumes, which are described in other papers of this issue, volume integrals of conventionally defined heat anomalies underestimate actual released heat by a factor of 2.4 if the discharge is not anomalously saline. Under certain combinations of hydrographic and source anomaly conditions, not unlike those found on the East Pacific Rise at 10°N, the apparent total heat released during an event can be deceivingly zero. This analysis also establishes a linear relationship between the ratio of tracer anomaly to heat anomaly at any point in the plume to the same ratio at the source. One consequence is that the ratio of anomalous 3 He to heat in Gorda Ridge event plumes is approximately 2.4 larger in the water column than it is at the source. Results are independent of the entrainment process involved in event plume formation, and they are shown to hold true even for background hydrographic profiles that do not vary linearly with depth.

On subinertial oscillations trapped by the Juan de Fuca Ridge, northeast Pacific

Spectra of currents measured along the 2100 m deep Juan de Fuca Ridge in the northeast Pacific have prominent tidal, inertial, and weather-band (3–7 day period) spectral peaks. The weather-band peak, in particular, has a number of interesting features. It is strongest in close lateral proximity to the ridge and strongest near ridge depth; intensification near the ridge is characteristic of trapped motion. Spectral peak intensities vary seasonally, with largest amplitudes occurring in autumn and winter; seasonal variation suggests that surface weather is forcing flow at depth. Together, trapping and seasonality indicate opportunistic amplification of oscillatory motion at the ridge. At all frequencies, forcing, topography, and stratification together shape current and hydrographic distributions near the ridge. Effects of those interactions for subinertial motion are quantified here using a primitive equation numerical model. Forcing period (1–10 days) and friction are the principal dependencies examined. Results show that flow over the Juan de Fuca Ridge can be amplified by factors of 3–4 for diurnal and up to a factor of 7 for weather-band frequencies. Amplification is ridge trapped, with effects extending many hundreds of meters upward and extending 5–10 km laterally for 5-day-period flow; the strength and area of amplification increase with increasing period over the 1- to 10-day-period band. Oscillatory weather-band flow leads to vertical velocities on the order of 0.3 cm s−1 on ridge flanks, which in turn cause periodic temperature (T) and salinity (S) variations with amplitudes of as much as 0.05°C and 0.01 practical salinity units, respectively. The vertical motion and consequent vertical displacement (>100 m) lead to isotherms that plunge below the crest alternately each side of the ridge, a distribution observed in conductivity-temperature-depth transect data. Near the ridge crest, cross-ridge baroclinic pressure gradients caused by cyclically plunging isopycnals are in near-geostrophic balance with the Coriolis force associated with along-ridge flow. Along-ridge current amplification is therefore closely tied to isopycnal movement up and down ridge flanks. Since weather-band oscillations lead to much larger cross-ridge baroclinic pressure gradients than do diurnal motions of the same amplitude, weather-band forcing causes greater along-ridge current amplification. Results vary strongly with forcing period and direction of forcing with respect to the ridge but depend only moderately on friction and turbulent mixing coefficients.

Acoustic measurements of vertical velocity and temperature fluctuations of a hydrothermal plume with comparisons to large eddy simulations

The acoustic scintillation method is used to study the hydrothermal plume of Dante within the Main Endeavour vent field (MEF) at the Endeavour segment of the Juan de Fuca Ridge. Forty days of vertical velocity and temperature fluctuations were obtained from the rising plume above the Dante edifice in an environment where the flow is dominated by strong (5cm/s) semi-diurnal tidal currents and a northerly mean residual current (3cm/s). These acoustic measurements provide a window on deep-sea hydrothermal plume dynamics in strong oscillatory cross flows. A large eddy simulation, parameterized with anisotropic mixing coefficients, taking into account ambient stratification and time-dependent background flows and calibrated by the acoustic measurements, yields insight into turbulent processes, entrainment, plume bending, rise height, and, inferentially, mound heat flux. The turbulent dissipation rates for kinetic energy (e) and thermal variance (eθ) is approximated by computing the Reynolds averaged sub-grid s...