11:15 AM - 11:30 AM
[SCG56-03] Local variations in heat transport through sediment in an area with fluid discharge in Lake Biwa
Keywords:Lake Biwa, fluid discharge, heat transport, long-term temperature monitoring, heat flow
Fluid discharge including emission of gas bubbles has been observed around the deepest part of Lake Biwa through visual surveys with AUV and ROV and acoustic imaging with echo sounders (Kumagai et al., 2021). These observations indicate the existence of upward flow of pore water and gas through bottom sediments, which should affect heat transport process and temperature distribution in sediments.
We conducted long-term monitoring of temperature profile in bottom sediment for investigation of the heat transport process in the vicinity of fluid vents with prominent activity, around 35°20'N, 135°06'E. A 2-m long probe, along which six small temperature recorders were mounted, was penetrated into bottom sediment and kept for about six months or longer. We made two monitoring experiments using the same probe: (1) from October 5, 2022 to March 25, 2023, (2) from October 4, 2023 to July 17, 2024. In the second experiment, another temperature recorder was installed just above the probe for monitoring bottom water temperature. Although the locations of the probe on the lake bottom could not be precisely determined, the two sites are supposed to be within 50 m from each other. The water depth is about 100 m.
In both experiments, we observed significant temporal variations of temperature distribution in surface sediment due to downward propagation of bottom water temperature variation. Analysis of the temperature variations at different depths revealed that heat transport process through sediment is much different between the two sites. The temperature variations at the second experiment site are well explained with conductive heat transport only. Supposing that thermal property of sediment is uniform, the thermal diffusivity is estimated to be about 2.5 × 10-7 m2/s, an appropriate value for soft sediment. It means that vertical fluid flow through sediment is insignificant, even if it exists. The undisturbed temperature gradient is around 160 mK/m. In contrast, the temperature records at the first site cannot be reproduced with conductive heat transport. Apparent thermal diffusivity calculated from temperature variations of neighboring sensors is extraordinarily high, suggesting that downward pore fluid flow contributes to heat transport at this site.
We also conducted closely-spaced measurements of sediment temperature in the fluid venting area where the two monitoring sites are located. We used a 3-m long probe with 12 to 16 temperature sensors and obtained temperature profiles at the moment of measurement. Although the measured profiles are severely disturbed by bottom water temperature variation, extremely high temperature gradient (e.g., over 250 mK/m) can still be recognized. We found that such high heat flow sites are concentrated in a small area with a diameter of about 200 m and the two monitoring sites are inside of this area. A local, extreme heat flow anomaly and contrasting heat transport process at the monitoring sites indicate that thermal structure and fluid flow regime around active fluid vents on the Lake Biwa bottom are highly variable and complicated.
We conducted long-term monitoring of temperature profile in bottom sediment for investigation of the heat transport process in the vicinity of fluid vents with prominent activity, around 35°20'N, 135°06'E. A 2-m long probe, along which six small temperature recorders were mounted, was penetrated into bottom sediment and kept for about six months or longer. We made two monitoring experiments using the same probe: (1) from October 5, 2022 to March 25, 2023, (2) from October 4, 2023 to July 17, 2024. In the second experiment, another temperature recorder was installed just above the probe for monitoring bottom water temperature. Although the locations of the probe on the lake bottom could not be precisely determined, the two sites are supposed to be within 50 m from each other. The water depth is about 100 m.
In both experiments, we observed significant temporal variations of temperature distribution in surface sediment due to downward propagation of bottom water temperature variation. Analysis of the temperature variations at different depths revealed that heat transport process through sediment is much different between the two sites. The temperature variations at the second experiment site are well explained with conductive heat transport only. Supposing that thermal property of sediment is uniform, the thermal diffusivity is estimated to be about 2.5 × 10-7 m2/s, an appropriate value for soft sediment. It means that vertical fluid flow through sediment is insignificant, even if it exists. The undisturbed temperature gradient is around 160 mK/m. In contrast, the temperature records at the first site cannot be reproduced with conductive heat transport. Apparent thermal diffusivity calculated from temperature variations of neighboring sensors is extraordinarily high, suggesting that downward pore fluid flow contributes to heat transport at this site.
We also conducted closely-spaced measurements of sediment temperature in the fluid venting area where the two monitoring sites are located. We used a 3-m long probe with 12 to 16 temperature sensors and obtained temperature profiles at the moment of measurement. Although the measured profiles are severely disturbed by bottom water temperature variation, extremely high temperature gradient (e.g., over 250 mK/m) can still be recognized. We found that such high heat flow sites are concentrated in a small area with a diameter of about 200 m and the two monitoring sites are inside of this area. A local, extreme heat flow anomaly and contrasting heat transport process at the monitoring sites indicate that thermal structure and fluid flow regime around active fluid vents on the Lake Biwa bottom are highly variable and complicated.