LONG-TERM SENSOR MONITORING EQUIPPED WITH THE KUROKO CULTIVATION APPARATUS ON THE DEEP-SEA ARTIFICIAL HYDROTHERMAL VENT AT THE MIDDLE OKINAWA TROUGH

The Iheya-North and Noho hydrothermal fields in the middle Okinawa Trough are active ongoing sulfide mineralization in the back-arc hydrothermal systems. From February to March 2016, Cruise CK16-01 by D/V Chikyu targeted both fields to comprehend sub-seafloor geological structure and polymetallic sulfide mineralization. In this cruise, we installed the Kuroko cultivation apparatus equipped with P/T sensors, flowmeter and load cell to monitor pressure, temperature and flow rate of hydrothermal fluid discharged from the artificial hydrothermal vent at the Iheya-North Knoll and Noho Site. During Cruise KR16-17 in January 2017, two cultivation cell parts were recovered by ROV Kaiko Mk-IV and R/V Kairei. In the present study, we report physical monitoring data of hydrothermal fluid obtained for more than 10 months at two deep-sea artificial hydrothermal vents.

At Hole C9017B of the Noho Site, fluid temperature was constant for 5 months from the beginning of the cell deployment. Then, temperature became decreaseing from 75 to 40 °C gradually. The day and monthly periodic oscillation of temperature whose amplitude was ca. 0.3- 0.5 °C induced by tidal wave was observed. Among such periodicity, timings of high temperature peaks were synchronous with those of low pressure peaks. Drilling operation at Hole C9017A, 10 m away from Hole C9017B during Cruise CK16-05 in November 2016 in order to settle another Kuroko cultivation apparatus, might have disturbed sub-seafloor hydrothermal paths and induced sudden temperature and pressure drops.

At Hole C9024A of the Iheya-North Knoll, pressure, temperature and flow rate also oscillated periodically with a tidal wave, and timings of the maximal and minimal points of flow rated coincided with those of temperature. The maximum temperature of hydrothermal fluid within the Kuroko cultivation cell part reached 308 °C, which is similar to the maximum temperature of 311 °C observed from the thermometer by ROV at the Iheya-North Knoll. The average flow rate of hydrothermal fluid was ca. 300 L/min and, 9 days later, flow rate became zero, suggesting the imperameable minerals sealed top of the cultivation apparatus. On the contrary, P/T sensors continuously functioned will, indicating that hydrothermal fluid flowed within/beneath cemented impermeable parts after flowmeter plugging.