Hydrothermal fluid geochemistry at the Iheya North field in the mid-Okinawa Trough: Implication for origin of methane in subseafloor fluid circulation systems

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  • KAWAGUCCI SHINSUKE
    Ocean Research Institute (ORI), The University of Tokyo Precambrian Ecosystem Laboratory (PEL), Japan Agency for Marine-Earth Science and Technology (JAMSTEC)
  • CHIBA HITOSHI
    Department of Earth Science, Graduate School of Natural Science and Technology, Okayama University
  • ISHIBASHI JUN-ICHIRO
    Faculty of Sciences, Kyushu University
  • YAMANAKA TOSHIRO
    Department of Earth Science, Graduate School of Natural Science and Technology, Okayama University
  • TOKI TOMOHIRO
    Faculty of Science, University of the Ryukyus
  • MURAMATSU YASUYUKI
    Faculty of Science, Gakushuin University
  • UENO YUICHIRO
    Precambrian Ecosystem Laboratory (PEL), Japan Agency for Marine-Earth Science and Technology (JAMSTEC) Global Edge Institute, Tokyo Institute of Technology
  • MAKABE AKIKO
    Department of Environmental Science and Technology, Tokyo Institute of Technology
  • INOUE KAZUHIRO
    Department of Environmental Science and Technology, Tokyo Institute of Technology
  • YOSHIDA NAOHIRO
    Department of Environmental Science and Technology, Tokyo Institute of Technology
  • NAKAGAWA SATOSHI
    Subsurface Geobiology Advanced Research (SUGAR) Project, Japan Agency for Marine-Earth Science and Technology (JAMSTEC) Faculty of Fisheries Sciences, Hokkaido University
  • NUNOURA TAKURO
    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)
  • TAKAHATA NAOTO
    Ocean Research Institute (ORI), The University of Tokyo
  • SANO YUJI
    Ocean Research Institute (ORI), The University of Tokyo
  • NARITA TAKU
    Ocean Research Institute (ORI), The University of Tokyo
  • TERANISHI GENTA
    Ocean Research Institute (ORI), The University of Tokyo
  • OBATA HAJIME
    Ocean Research Institute (ORI), The University of Tokyo
  • GAMO TOSHITAKA
    Ocean Research Institute (ORI), The University of Tokyo

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Geochemical characteristics of hydrothermal fluids in the Iheya North hydrothermal field, mid-Okinawa Trough, was investigated. Twelve-years observation reveals temporal variation of vent fluid chemistry potentially controlled by temporally varying pattern of the phase-separation and -segregation, while the constant Element/Cl ratios among the periods and chimneys indicate the stable chemical composition of the source hydrothermal fluid prior to undergoing phase-separation. The high K contents in the estimated source fluid are typical in the arc-backarc hydrothermal systems due to the hydrothermal reaction with the K-enriched felsic rocks. The high I, B and NH4 contents and alkalinity are derived from decomposition of the sedimentary organic matters. Compositional and isotopic properties of gas species, CH4, H2, CO2, and C2H6, strongly suggest a dominance of biogenic CH4 associated with the sedimentary organic matter. Based on the carbon mass balance calculation and the multidisciplinary investigations of the Iheya North hydrothermal system since the discovery, we hypothesized that the microbial methanogenesis occurs not only within the Central Valley where hydrothermal vents exist, but also in the spatially abundant and widespread basin-filling sediments surrounding the Iheya North Knoll, and that the microbially produced CH4 is recharged together with the source fluid into the deep hydrothermal reaction zone. This “Microbial Methanogenesis at Recharge area in hydrothermal circulation” (MMR) model would be an implication for the generation and incorporation of hydrothermal fluid CH4 in the deep-sea hydrothermal systems but also for those of cold seep CH4 and for the presently uncertain hydrothermal fluid paths in the subseafloor environments. In the near future, the IODP drilling will be conducted in the Iheya North hydrothermal system, and give an excellent opportunity to testify our MMR model.

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