ABSTRACT

We conducted four dives using the Urashima autonomous underwater vehicle (AUV) over three hydrothermal vent fields (Snail, Archean, and Pika) within the South Mariana Trough in July 2009, and measured the echo intensity (EI) with an acoustic Doppler current profiler (ADCP) mounted on the AUV. The ADCP data produced 3D structures of the EI over the vents, which clearly showed higher EI anomalies over the Pika and Archean vents. The characteristics of the higher EI anomalies were carefully examined and described in terms of their relationships with turbidity that was simultaneously observed with a single point turbidity sensor. It is plausible that the higher EI anomaly indicates the thickness of the plumes at some locations (i.e., 230 m thickness at the Pika site, 140 m at the Archean, and 160 m at the Snail) where both the turbidity and EI have a signal. The results also explain the long-term southwestward advection of the higher anomaly water by the local mean current.