Geochemical characteristics of hydrothermal fluids at Hatoma Knoll in the southern Okinawa Trough

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  • Toki Tomohiro
    Department of Chemistry, Biology and Marine Science, Faculty of Science, University of the Ryukyus
  • Itoh Michihiro
    Department of Chemistry, Biology and Marine Science, Faculty of Science, University of the Ryukyus
  • Iwata Daigo
    Department of Chemistry, Biology and Marine Science, Faculty of Science, University of the Ryukyus
  • Ohshima Shogo
    Department of Chemistry, Biology and Marine Science, Faculty of Science, University of the Ryukyus
  • Shinjo Ryuichi
    Department of Physics and Earth Sciences, Faculty of Science, University of the Ryukyus
  • Ishibashi Jun-ichiro
    Department of Earth and Planetary Sciences, Graduate School of Sciences, Kyushu University
  • Tsunogai Urumu
    Earth and Planetary System Science, Faculty of Science, Hokkaido University
  • Takahata Naoto
    Marine Analytical Chemistry, Department of Chemical Oceanography, Atmosphere and Ocean Research Institute, The University of Tokyo
  • Sano Yuji
    Marine Analytical Chemistry, Department of Chemical Oceanography, Atmosphere and Ocean Research Institute, The University of Tokyo
  • Yamanaka Toshiro
    Graduate School of Natural Science and Technology, Okayama University
  • Ijiri Akira
    Kochi Institute for Core Sample Research, Japan Agency for Marine-Earth Science and Technology (JAMSTEC)
  • Okabe Nobuaki
    Marine Inorganic Chemistry Group, Department of Chemical Oceanography, Atmosphere and Ocean Research Institute, The University of Tokyo
  • Gamo Toshitaka
    Marine Inorganic Chemistry Group, Department of Chemical Oceanography, Atmosphere and Ocean Research Institute, The University of Tokyo
  • Muramatsu Yasuyuki
    Department of Chemistry, Gakushuin University
  • Ueno Yuichiro
    Precambrian Ecosystem Laboratory (PEL), Japan Agency for Marine-Earth Science and Technology (JAMSTEC) Global Edge Institute, Tokyo Institute of Technology
  • Kawagucci Shinsuke
    Precambrian Ecosystem Laboratory (PEL), Japan Agency for Marine-Earth Science and Technology (JAMSTEC) Subsurface Geobiology Advanced Research (SUGAR) Project, Japan Agency for Marine-Earth Science and Technology (JAMSTEC)
  • Takai Ken
    Precambrian Ecosystem Laboratory (PEL), Japan Agency for Marine-Earth Science and Technology (JAMSTEC) Subsurface Geobiology Advanced Research (SUGAR) Project, Japan Agency for Marine-Earth Science and Technology (JAMSTEC)

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<p>Chemical and isotopic compositions of hydrothermal fluids from Hatoma Knoll in the southern Okinawa Trough were investigated. The hydrothermal fluids were derived from a single pure hydrothermal fluid source, but they underwent phase separation beneath the seafloor prior to venting. Only vapor-like fluids vent at the Hatoma system, and the most active area is around the center of the crater, based on the location of the maximum temperature and the lowest Cl– concentrations. Compared with other hydrothermal systems in the world, at Hatoma the pH and alkalinity, as well as the B, NH4+, K, Li, CO2, and CH4 concentrations, were higher, and the Fe and Al concentrations were lower, suggesting that the characteristics of the Hatoma hydrothermal fluids are comparable to those of the other Okinawa Trough hydrothermal fluids. Helium isotope ratios were lower than those of sediment-starved hydrothermal systems, suggesting that 4He derived from the sediment is supplied to the hydrothermal fluids in Hatoma Knoll. The carbon isotope ratios of CO2 in the hydrothermal fluids indicate an influence of organic carbon decomposition. The carbon isotope ratios of CH4 in the hydrothermal fluids imply that most of the CH4 originated from microbial methane produced in a recharge zone of the hydrothermal system. Although sediment influences are a typical feature of Okinawan Trough hydrothermal fluids, the Hatoma hydrothermal system has the lowest carbon isotope ratios of CH4 among them, which suggests that Hatoma is the most highly influenced by the sediments in the recharge zone. Thus, the degree of the sediment influences has a variable in each hydrothermal field in the Okinawa Trough.</p>

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