Periodic behaviors of deep-ocean flow and turbulent mixing in hydrothermal field on the Okinawa Trough, Japan

In this study, the current fluctuations were measured at the hydrothermal fields in the Okinawa Trough (Hatoma Knoll, Irabu Knoll, and Iheya North field) to characterize the fluid environment. The purpose of this study was to reveal the fluctuations of the deep-sea currents using an acoustic Doppler current profiler (ADCP), and to correlate them with the development of a bottom mixed layer as predicted by a numerical model. An ADCP (Teledyne RD Instruments Workhorse Sentinel, 300 kHz) was installed on the seabed at the Hatoma Knoll (depth about 1500 m), Irabu Knoll (depth about 2000 m), and Iheya North field (depth about 1000 m). For each observation point, the deep-sea current data were successfully recovered in the layer at depths between the seafloor and 40–60 m above the seafloor. Notably, the measurement results obtained at the Hatoma Knoll and Iheya North field showed a remarkably interesting fluctuation. During a time period of approximately 3 h, the range in the depths at which the currents were successfully recorded increased in a pulse form, up to around the upper layer at 120 m, allowing measurements in almost all of the layers. The time period of this pulse corresponded to the shift from an ebb tide to high tide. With the exception of these pulses, the currents could only be recorded from the seafloor to about a 40 m layer. This result indicated that a suitable environment for ADCP measurements was formed only within this range. To investigate the cause, a numerical model experiment was performed for the bottom turbulence and subsequent occurrence of vertical mixing of the suspended particulate matter in the Okinawa Trough using a large-eddy simulation (LES) model. The simulated results suggested that the semidiurnal tide periodically enhances the bottom turbulence at intervals of approximately 6 h, with the result that the bottom mixed layer thickness increases to a maximum of approximately 40–60 m. It was concluded that a suitable environment for ADCP measurements near the deep seafloor was formed mainly by the bottom mixed layer corresponding to the tide.