10:15 AM - 10:30 AM
[MIS22-06] Origin of fluids in surface sediments of submarine mud volcanoes off Hyuganada, Tanegashima Island, and Kikaijima Island
Keywords:submarine mud volcano, Kikaijima Island, pore water, Hyuga-nada, Tanegashima Island
Mud volcano, which are distributed on land or on the seafloor along continental margins around the world, are topographic feature in which sediments with high pore pressure rise to the surface and erupt, releasing fluids from deep subsurface. In the sea off Japan, they are widely distributed in the Kumano Basin off the Kii Peninsula and off Tanegashima Island. Recent detailed topographic surveys have also revealed submarine mud volcanoes distributed in the areas off Hyuga-nada and Kikaijima Island. During the KH-23-4 cruise of R/V “Hakuho-Maru” in August 2023, sediments core samples were collected from mud volcanoes: HyMV5-1, HyMV9, HyMV23, HyMV24, and HyMV27 in Hyuga-nada, MV04 and TK12 off Tanegashima Island, and KSK0, KSK2, KSK3, KNK1, and KNK2 off Kikaijima Island. In this study, we investigated the origin of the fluids in these mud volcanoes using porewater chemistry. Porewater was extracted from sediment core samples obtained from the mud volcanoes. Cl- concentrations and stable isotopic compositions of hydrogen and oxygen (δD and δ18O) in the porewater were analyzed.
The Cl- concentrations in the pore water decreased with depth in all mud volcanoes, and at the deepest depths, Cl- concentrations were a few to 70% lower than those of seawater. The lower Cl- concentrations compared to seawater values (ca. 550 mM) indicate dilution by the addition of freshwater to the seawater-derived pore water. The decrease in Cl- concentration with depth suggests that the low Cl- rises from deeper depths to the seafloor surface. In particular, the Cl- concentrations of pore water at the bottom of the cores of KSK3 and KNK2 off Kikaijima Island were 70% and 75% lower (170 mM and 130 mM) than those of seawater. On the other hand, KSK0, KNK1, MV4, HyMV5-1, and HyMV24 showed concentrations several percent lower than seawater. The little change in Cl- in these mud volcanoes, indicates that these mud volcanoes are currently inactive due to the lack of water supply from the deep subsurface.
Except for the inactive mud volcanoes, the δD and δ18O values decreased and increased with decreasing Cl- concentrations, showing a strong correlation. This trend is a characteristic of dehydrated water form clay minerals. The smectite-illite reaction, a common dehydration reaction of clay minerals in marine sediments occur at 60-160°C. The depths at which the clay mineral dehydration occurred were estimated to be a few kilometers based on the known geothermal gradient (25-50 °C/km) at the area off Tanegashima Island.
There was a difference in the distribution of Cl- concentrations with depth for each mud volcano, can be explained by the difference in the advection velocity of the fluid, i.e., the activity of the mud volcano. The advection velocities estimated using the advective diffusion equation ranged from 0.01 to 5 cm/y. Although there were few relationships between estimated advection rate and each area, there are significant differences in the activity of each mud volcano off Kikaijima Island. The mud volcano off Hyuga-nada shows a higher average advection rate than the other volcanoes in the area. The advection rates of the mud volcanoes were larger in the following order: KSK2, KNK1 < TK12 < HyMV23 < KSK3 < HyMV27 < KNK2.
The Cl- concentrations in the pore water decreased with depth in all mud volcanoes, and at the deepest depths, Cl- concentrations were a few to 70% lower than those of seawater. The lower Cl- concentrations compared to seawater values (ca. 550 mM) indicate dilution by the addition of freshwater to the seawater-derived pore water. The decrease in Cl- concentration with depth suggests that the low Cl- rises from deeper depths to the seafloor surface. In particular, the Cl- concentrations of pore water at the bottom of the cores of KSK3 and KNK2 off Kikaijima Island were 70% and 75% lower (170 mM and 130 mM) than those of seawater. On the other hand, KSK0, KNK1, MV4, HyMV5-1, and HyMV24 showed concentrations several percent lower than seawater. The little change in Cl- in these mud volcanoes, indicates that these mud volcanoes are currently inactive due to the lack of water supply from the deep subsurface.
Except for the inactive mud volcanoes, the δD and δ18O values decreased and increased with decreasing Cl- concentrations, showing a strong correlation. This trend is a characteristic of dehydrated water form clay minerals. The smectite-illite reaction, a common dehydration reaction of clay minerals in marine sediments occur at 60-160°C. The depths at which the clay mineral dehydration occurred were estimated to be a few kilometers based on the known geothermal gradient (25-50 °C/km) at the area off Tanegashima Island.
There was a difference in the distribution of Cl- concentrations with depth for each mud volcano, can be explained by the difference in the advection velocity of the fluid, i.e., the activity of the mud volcano. The advection velocities estimated using the advective diffusion equation ranged from 0.01 to 5 cm/y. Although there were few relationships between estimated advection rate and each area, there are significant differences in the activity of each mud volcano off Kikaijima Island. The mud volcano off Hyuga-nada shows a higher average advection rate than the other volcanoes in the area. The advection rates of the mud volcanoes were larger in the following order: KSK2, KNK1 < TK12 < HyMV23 < KSK3 < HyMV27 < KNK2.