Effect of Current Variability on Temperatures Measured in a Non-buoyant Hydrothermal Plume


Wetzler, M.A., and J.W. Lavelle
Marine Geophysical Researches, 20, 505-516, 1998.


Abstract:



Temperature and currents were measured just downstream of the prodigiously discharging Pipe Organ vent field on the Juan de Fuca Ridge to look at the temporal variability of the hydrothermal signal passing a single site. Temperatures (theta) at three depths were sampled every 15 s while current speed and direction at a single depth was sampled hourly over a 4 day interval in September 1997 from a taut-wire mooring located ~100 m south of Pipe Organ. The high temporal variability of temperature, and hen ce inferred heat flux, is due in large part to current variations. An analytic model of heat transport and the measured currents were used to extrapolate the measured data in to two-dimensional time-dependent fields. The effort first required matching the modeled temperature time series to the measured series at the mooring site. This extrapolation of the measurements to two dimensions with model, when animated, demonstrates the pooling of effluent over the vent at various times, and streaming of ef fluent as a very narrow plume away from the vent at others, and spawning of boluses of heated fluid and their transport away from the vent region when a pooling period is followed by a streaming period. The model temperature fields also allows the comparison of methods used in calculating heat flux via moored instruments and CTD's; heat flux estimates are shown to vary widely depending upon analysis assumptions. When the measured theta time series showed peaks that could not be accounted for by discharge Pipe Organ alone, time reversed trajectories of currents were used to suggest the location of other contributory vent fields. A contribution to the measured theta at the mooring site by significant heat flux at or near Cavern vent field, some 2 km upstream, was thus indicated.