[SCG67-P04] Long-term monitoring of vertical temperature gradient of seawater on the deep seafloor
Keywords:precise thermometer, vertical temperature gradient
In a trial experiment conducted in the ultra-deep (7,202 m) seafloor at the Japan Trench, five miniature temperature loggers (RBR-TR1050) were attached to the outside of a 17-inch pressure vessel, two on the top, one on the middle and two at the bottom of the vessel casing. The data allowed us to confirm long-term stability of the temperature measurements owing to the extremely stable environment with temperature variations less than 0.005 ºC. Differences among the observed temperatures, 0.005 to 0.013 ºC, were almost constant suggesting that sensor drifts would be negligible. Correlation between the amounts of temperature difference and the distances across the sensors were found and we interpret the measured differences represent actual spatial differences in the temperature field around the seafloor instrument. We also made two observations using a pair of the temperature loggers attached to the top and bottom of the instruments equipped with the vessels of the identical size to the trial experiment. In the observations made at a site on the outer slope of the Japan Trench (5,465 m) and on the inner slope of HSZ (2,166 m), the variations of temperatures were more than 0.1 ºC, much larger than those in the ultra-deep environment, indicating larger disturbances in the shallower water. The amounts of the temperature differences are similar to those at the stable seafloor but show evident temporal variations, indicating that the temperature gradients are also disturbed in these sites. Interestingly, long-term variation in the vertical temperature gradient was recognized in HSZ, although it is difficult to give physical explanations at this moment.