Direct measurement of turbulent flows near a hydrothermal venting area for environmental impact assessment of deep sea mining

Understanding the dominant fluid mechanisms that affect particle behavior near the deep-sea bottom is important to assess the environmental impacts of submarine mining. In this study, time series measurements of turbulent flows near the sea floor were measured at hydrothermal fields in the Okinawa Trough to understand the dynamics of the bottom mixed layer (BML). The purpose of this study was to reveal deep-sea turbulent flow time series fluctuations using expendable vertical microstructure profiler (VMP-X), and to correlate them with in-situ environmental data obtained from the monitoring station installed on the sea floor. Turbulent flow measurements were carried out in the Kumejima west field (depth: 1400m) on June 24 and 29, 2017 and the Izena Hole (depth: 1600m) on January 16, 2018.

At the Kumejima west field, time series fluctuations of the turbulent energy dissipation rate (ɛ) data were successfully measured between the sea surface and the sea floor. The turbulent intensity (ɛ) near the sea bottom changed from 10^-10 to 10^-7 (W kg^-1) and the ɛ maximum was observed from a flood tide between ebb tides, and decreased to 10^-9 (W kg^-1) then in ebb tide on June 24. Moreover, the turbulent flow was relatively strong from the bottom to a height of around 80 m from the bottom, and water temperature rose 0.1 0.2 degrees Celsius. The measurement points were located to the northwest approximately 900m from the hydrothermal vent area. Deep-sea current data were successfully recovered for the layers at depths between the seafloor and 30 m above the bottom and a periodic fluctuation with an amplitude of approximately 10 cm/s for the east-west and north-south components was observed in synchronization with a variation in the water level. According to these environmental data and the complex bottom topography data, this characteristic water temperature fluctuation may occur as a result of hydrothermal venting (transport of the warm water or intrusion). Therefore, intensity of turbulence may also be affected by hydrothermal venting. However, to understand the dynamics of the BML, more continuous in-situ observational data is necessary. In a future study, we will collect and analyze more spatiotemporal turbulent flow data to understand the dynamics of the BML and improve the accuracy of the numerical model.