Basic Layer Structure Research Articles

Acoustic Doppler current profiler observations of a mid‐ocean ridge hydrothermal plume

On September 10, 1987, an oceanic profiling package consisting of a Guildline conductivity‐temperature‐depth probe (CTD), SeaTec transmissometer and RD Instruments 150‐kHz acoustic Doppler current profiler (ADCP) was used to investigate the hydrothermal plume emanating from vent sites located at 2200‐m depth on the Endeavour segment of Juan de Fuca Ridge in the northeast Pacific Ocean (47°58′N; 129°06′W). The CTD‐transmissometer profiles indicate that the plume immediately downstream of the vent site consisted of two 50‐m‐thick particle‐laden effluent layers separated by a comparably thick zone of relatively particle‐free ambient fluid. The lower 50 m of the water column appeared to be dominated by smaller concentrations of resuspended particles within a benthic boundary layer. Maximum temperature, salinity, and light attenuation anomalies within the main core of the plume at 1970‐m depth were approximately +0.04°C, +0.01 ppt, and +0.05 m−1, respectively. The ADCP measurements represent the first attempt to acoustically map the current shear and backscatter amplitude associated with a mid‐ocean ridge hydrothermal plume. Repeated profiles by the ADCP revealed that the top of the plume was a region of enhanced current shear with vertical changes in speed of the order of 0.05 m s−1 over depths of approximately 100 m. The velocity measurements also indicate that the current reversed from northwest to southwest over the 1‐hour observation period, consistent with the predicted flow derived from moored current meters recovered immediately prior to the profiler survey. Contrary to expectation, acoustic backscatter signals within the particle‐laden layers of the plume were lower than background reference levels, while echo return amplitudes from the particle‐free layer were close to background levels. Time series plots of the amplitude data indicate that the basic layered structure of the plume is disrupted by billowlike features with vertical and horizontal wavelengths of 40 to 60 m and 20 to 40 m, respectively. We suggest that the weakened acoustic backscatter signals from the effluent layers is linked to a reduction in the numbers of zooplankton relative to the ambient waters.