Abstract
Submarine mud volcanoes, which are releasing fluids originated in deep subsurface. Around Japan, they are found extensively in the Kumano Basin off the Kii Peninsula and off Tanegashima Island. Recent detailed topographic surveys have also identified submarine mud volcanoes distributed in areas Hyuga-nada and off Kikaijima Island. During the KH-23-4 cruise of R/V "Hakuho-Maru" in August 2023, sediments core samples were collected from these mud volcanoes. We investigated the origin of the fluids in these mud volcanoes using porewater chemistry, including Cl-, Li, and Na concentrations as well as stable isotopic compositions of hydrogen and oxygen in the porewater. The Cl- concentrations decreased with depth in all mud volcanoes. Specifically, the Cl- concentrations in a mud volcano off Kikaijima were 70% lower than those of seawater. This decrease suggests that the low Cl- fluids migrate from deeper depths to the seafloor surface. The stable isotopic compositions of hydrogen and oxygen decreased and increased, respectively, with decreasing Cl- concentrations at all active mud volcanoes, indicating a characteristic of water derived from clay mineral dehydration. The Li concentrations increased with depth, especially in mud volcanoes with low Cl- concentrations. The experienced temperatures of fluid estimated using the Na/Li geothermometer were approximately 140 degrees Celsius for all mud volcanoes, suggesting that the fluids derived from the clay mineral dehydration is supplied from several kilometers below seafloor.
