3:00 PM - 3:15 PM
[MIS20-05] Discharge of deeply rooted fluids from submarine mud volcanoes in the northern Ryukyu Trench slope
Keywords:pore water, hydrocarbon gas, mud volcano, Ryukyu Trench
The submarine mud volcano is a topographic feature formed by the upward and eruption of sediments with high pore fluid pressure, releasing fluids originating from deep subseafloor into seawater. Around off Japan, they are distributed in the Kumano Basin and off Tanegashima, and in recent years, it has been revealed that they are also distributed in the Hyuganada and off Kikai Island. In August 2023, a research cruise was conducted mainly the mud volcanoes in the Hyuganada and off Kikai Island. In this cruise, sediment core samples were collected from five mud volcanoes in Hyuganada, two in off Tanegashima, and five in off Kikai Island. In this study, we analyzed the chemical and isotopic composition of pore water and hydrocarbon gases in the sediments, and discussed the origin of fluids in the surface sediments of the submarine mud volcanoes in Hyuganada, off Tanegashima, and off Kikai Island.
The Cl concentration of the pore water decreased from the surface to the deeper layers in the sediment cores of all the mud volcanoes, indicating that low-Cl fluids are being supplied from deep subsruface to the surface. The delta-D and delta-18O of the pore water showed a trend for the delta-D value to decrease and the delta-18O value to increase with the decrease in Cl concentration. This trend indicates that water derived from the dehydration of clay minerals, with low Cl concentration, is being supplied from the deep subsurface. The decreasing trend of Cl concentration with depth varied among the mud volcanoes. Using a convection-diffusion model, the upward convection velocity was determined, and velocities are particularly large in the mud volcanoes off Kikai Island and the northern part of off Tanegashima.
The concentration of Li in the pore water increased with depth, and high Li concentrations were observed in the mud volcanoes with low Cl concentrations. The experienced temperature of fluids was estimated using the Li-Na geothermometer. The estimated temperatures of fluids in all areas were consistent within the of error range, with an average of 130 degC. By applying this experienced temperature and the geothermal gradient off Tanegashima, it was estimated that with high Li fluid is being supplied from 2.6 to 5.2 km below the seafloor. Additionally, the delta-7Li values in the mud volcanoes with high Li concentrations linearly decreased as the Cl/Li concentration ratio decreased, suggesting that the low delta-7Li values and high Li concentration fluid, typical of hydrothermal areas, is being supplied from deep subseafloor and mixed with seawater. The delta-7Li values of the mud volcanoes analyzed, regardless of the area (off Kikai Island, off Tanegashima, and Hyuganada), plotted along this mixing line, suggesting that the origin of Li in the fluids of these mud volcanoes is the similar.
The carbon isotopic compositions of methane in each mud volcano indicated that the methane originates from thermal decomposition of organic matters. Methane was formed at high temperatures (>80 degC) during the thermal decomposition of organic matter, and is supplied to the seafloor. This study estimated and discussed the origin of fluid in mud volcanoes across a wide area, from off Kikai Island to Hyuganada along the Ryukyu Trench. In all the mud volcanoes in the surveyed area, it was revealed that fluids are migratinf from the deep depths where methane formation though thermal decomposition, and clay mineral dehydration reactions occur.
The Cl concentration of the pore water decreased from the surface to the deeper layers in the sediment cores of all the mud volcanoes, indicating that low-Cl fluids are being supplied from deep subsruface to the surface. The delta-D and delta-18O of the pore water showed a trend for the delta-D value to decrease and the delta-18O value to increase with the decrease in Cl concentration. This trend indicates that water derived from the dehydration of clay minerals, with low Cl concentration, is being supplied from the deep subsurface. The decreasing trend of Cl concentration with depth varied among the mud volcanoes. Using a convection-diffusion model, the upward convection velocity was determined, and velocities are particularly large in the mud volcanoes off Kikai Island and the northern part of off Tanegashima.
The concentration of Li in the pore water increased with depth, and high Li concentrations were observed in the mud volcanoes with low Cl concentrations. The experienced temperature of fluids was estimated using the Li-Na geothermometer. The estimated temperatures of fluids in all areas were consistent within the of error range, with an average of 130 degC. By applying this experienced temperature and the geothermal gradient off Tanegashima, it was estimated that with high Li fluid is being supplied from 2.6 to 5.2 km below the seafloor. Additionally, the delta-7Li values in the mud volcanoes with high Li concentrations linearly decreased as the Cl/Li concentration ratio decreased, suggesting that the low delta-7Li values and high Li concentration fluid, typical of hydrothermal areas, is being supplied from deep subseafloor and mixed with seawater. The delta-7Li values of the mud volcanoes analyzed, regardless of the area (off Kikai Island, off Tanegashima, and Hyuganada), plotted along this mixing line, suggesting that the origin of Li in the fluids of these mud volcanoes is the similar.
The carbon isotopic compositions of methane in each mud volcano indicated that the methane originates from thermal decomposition of organic matters. Methane was formed at high temperatures (>80 degC) during the thermal decomposition of organic matter, and is supplied to the seafloor. This study estimated and discussed the origin of fluid in mud volcanoes across a wide area, from off Kikai Island to Hyuganada along the Ryukyu Trench. In all the mud volcanoes in the surveyed area, it was revealed that fluids are migratinf from the deep depths where methane formation though thermal decomposition, and clay mineral dehydration reactions occur.