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[MIS18-18] Tidally-modulated temperature observed at a drillsite in the Noho hydrothermal field in the mid-Okinawa Trough
Keywords:Okinawa Trough hydrothermal activity, Tidal modulation, Kuroko cultivation system
In the Okinawa Trough, various active hydrothermal vents have been found and explored. One of them is the Noho hydrothermal field located on the south side of the Iheya Minor Ridge, mid-Okinawa Trough. From March to December 2016, a black-ore cultitivation system was installed at the top of the drilled Hole C9017B under the Cross-ministerial Strategic Innovation Promotion Program (SIP) funded by CSTI of the Cabinet Office of Japan. Two pairs of temperature and pressure gauges, as well as flowmeter, loca cell and their loggees, were installed within the system, and continuous data were obtained.
Hole C9017B was drilled up to 50 m, after which the screened casing was installed down to ~20 m. The black-ore system directly attached to the hole consists of a 2” inner diameter pipe extending from the hole and a cultivation tank (called Cell) attached to the top of the pipe. T/P gauges were installed inside of the pipe and the Cell, and data were obtained at 2-minute intervals. Here we describe their fluctuations and discuss possible causes.
We found that the temperature and pressure of the pipes and cells fluctuated in accordance with the tidal (M2) period. In the beginning, the average temperature was 75-76 degC, and the half-amplitude of tidal modulation was about 0.3 degC. Since their modulation started immediately after the installation to Hole C9017B, it is probably attributed to the tidal modulation at the hydrothermal reservoir beneath the hole, rather than to the variation in the ambient water column. Although the resolution was insufficient, the pressure data also recorded the tidal modulation that is in good agreement, both amplitude and phase, with the theoretical tide, predicted by the ‘NAOTIDE' tide prediction program (Matsumoto et al., 2000). It indicates that the pressure in the cultivation system corresponds to the ocean tide.
The phase of the temperature fluctuation of both the pipe and Cell is delayed by about 150 deg from the pressure estimate by NAOTIDE. The possible mechanisms of such fluctuation may be the variation of ambient seawater (periodic movement in response to the tide), or the change in flow velocity in the cultivation system leading to the variation in dynamic pressure change and the conductive cooling efficiency. However, we assess that neither can sufficiently explain the observed temperature fluctuation.
Therefore, we propose that the temperature and pressure of venting fluid are actually modulated by the oceanic tide. Jupp & Schultz (2004) proposed a one-dimensional poroelastic and heat conduction model, by which the temperature, pressure, and velocity of the venting fluid are affected by tides. Based on this, we assess that the phase delay of temperature by 150 deg requires a high permeability in the formation, a deeper tidal penetration depth, and the larger Darcy velocity.
Another possibility is that cracks fluctuate with changes in tidal stress, causing the change in permeability. Since the compressibility of water is larger than that of solids, tides will open and close underground cracks. We propose the most probable model by further analyses.
As an aperiodic change, the measured temperatures gradually decreased (to 30-40 °C) in August, accompanied by the amplitude increase in tidal modulation. It is probably caused by the clogging of pipes in the cultivation system due to the hydrothermal precipitation.
Hole C9017B was drilled up to 50 m, after which the screened casing was installed down to ~20 m. The black-ore system directly attached to the hole consists of a 2” inner diameter pipe extending from the hole and a cultivation tank (called Cell) attached to the top of the pipe. T/P gauges were installed inside of the pipe and the Cell, and data were obtained at 2-minute intervals. Here we describe their fluctuations and discuss possible causes.
We found that the temperature and pressure of the pipes and cells fluctuated in accordance with the tidal (M2) period. In the beginning, the average temperature was 75-76 degC, and the half-amplitude of tidal modulation was about 0.3 degC. Since their modulation started immediately after the installation to Hole C9017B, it is probably attributed to the tidal modulation at the hydrothermal reservoir beneath the hole, rather than to the variation in the ambient water column. Although the resolution was insufficient, the pressure data also recorded the tidal modulation that is in good agreement, both amplitude and phase, with the theoretical tide, predicted by the ‘NAOTIDE' tide prediction program (Matsumoto et al., 2000). It indicates that the pressure in the cultivation system corresponds to the ocean tide.
The phase of the temperature fluctuation of both the pipe and Cell is delayed by about 150 deg from the pressure estimate by NAOTIDE. The possible mechanisms of such fluctuation may be the variation of ambient seawater (periodic movement in response to the tide), or the change in flow velocity in the cultivation system leading to the variation in dynamic pressure change and the conductive cooling efficiency. However, we assess that neither can sufficiently explain the observed temperature fluctuation.
Therefore, we propose that the temperature and pressure of venting fluid are actually modulated by the oceanic tide. Jupp & Schultz (2004) proposed a one-dimensional poroelastic and heat conduction model, by which the temperature, pressure, and velocity of the venting fluid are affected by tides. Based on this, we assess that the phase delay of temperature by 150 deg requires a high permeability in the formation, a deeper tidal penetration depth, and the larger Darcy velocity.
Another possibility is that cracks fluctuate with changes in tidal stress, causing the change in permeability. Since the compressibility of water is larger than that of solids, tides will open and close underground cracks. We propose the most probable model by further analyses.
As an aperiodic change, the measured temperatures gradually decreased (to 30-40 °C) in August, accompanied by the amplitude increase in tidal modulation. It is probably caused by the clogging of pipes in the cultivation system due to the hydrothermal precipitation.