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Volume 14, issue 4 | Copyright
Ocean Sci., 14, 575-588, 2018
https://doi.org/10.5194/os-14-575-2018
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 13 Jul 2018

Research article | 13 Jul 2018

Hadal water biogeochemistry over the Izu–Ogasawara Trench observed with a full-depth CTD-CMS

Shinsuke Kawagucci1,2,3,*, Akiko Makabe2,3,*, Taketoshi Kodama4, Yohei Matsui2,3, Chisato Yoshikawa2,5, Etsuro Ono6,a, Masahide Wakita7, Takuro Nunoura3,8, Hiroshi Uchida6, and Taichi Yokokawa3,8 Shinsuke Kawagucci et al.
  • 1Department of Subsurface Geobiological Analysis and Research (D-SUGAR), Japan Agency for Marine-Earth Science and Technology (JAMSTEC), 2-15 Natsushima-cho, Yokosuka 237-0061, Japan
  • 2Research and Development Center for Submarine Resources (SRRP), Japan Agency for Marine-Earth Science and Technology (JAMSTEC), 2-15 Natsushima-cho, Yokosuka 237-0061, Japan
  • 3Project Team for Development of New-Generation Research Protocol for submarine resources, Japan Agency for Marine-Earth Science and Technology (JAMSTEC), 2-15 Natsushima-cho, Yokosuka 237-0061, Japan
  • 4Japan Sea National Fisheries Research Institute, Japan Fisheries Research and Education Agency, Niigata, 5939-22 Suido-cho, Niigata 951-8121, Japan
  • 5Department of Biogeochemistry, Japan Agency for Marine-Earth Science and Technology (JAMSTEC), 2-15 Natsushima-cho, Yokosuka 237-0061, Japan
  • 6Research and Development Center for Global Change (RCGC), Japan Agency for Marine-Earth Science and Technology (JAMSTEC), 2-15 Natsushima-cho, Yokosuka 237-0061, Japan
  • 7Mutsu Institute for Oceanography, Japan Agency for Marine-Earth Science and Technology (JAMSTEC), 690 Kitasekine, Mutsu 035-0022, Japan
  • 8Research and Development Center for Marine Biosciences, Japan Agency for Marine-Earth Science and Technology (JAMSTEC), 2-15 Natsushima-cho, Yokosuka 237-0061, Japan
  • aPresent address: Japan Meteorological Agency, 1-3-4 Otemachi, Tokyo 100-8122, Japan
  • *These authors contributed equally to this work.

Abstract. Full-depth profiles of hydrographic and geochemical properties at the Izu–Ogasawara Trench were observed for the first time using a CTD-CMS (conductivity–temperature–depth profiler with carousel multiple sampling) system. Additionally, comparative samplings were done at the northern Mariana Trench using the same methods. A well-mixed hydrographic structure below 7000m was observed within the Izu–Ogasawara Trench. Seawater samples collected from this well-mixed hadal layer exhibited constant concentrations of nitrate, phosphate, silicate, and nitrous oxide as well as constant nitrogen and oxygen isotopic compositions of nitrate and nitrous oxide. These results agree well with previous observations of the Izu–Ogasawara hadal waters and deep-sea water surrounding the Izu–Ogasawara Trench. In turn, methane concentrations and isotopic compositions indicated spatial heterogeneity within the well-mixed hadal water mass, strongly suggesting a local methane source within the trench, in addition to the background methane originating from the general deep-sea bottom water. Sedimentary compound releases, associated with sediment re-suspensions, are considered to be the most likely mechanism for generating this significant CH4 anomaly.

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A full-depth-rated conductivity–temperature–depth profiler with carousel~multiple sampling (CTD-CMS) system on the research vessel R/V Kaimei can illuminate the last frontier, i.e., the hadal zone, an oceanographic province deeper than 7000 m. A series of biogeochemical observations of hadal seawater at the Izu–Ogasawara and Mariana trenches revealed basically well-mixed hydrographic properties and a heterogeneous distribution of likely sediment-derived compounds.
A full-depth-rated conductivity–temperature–depth profiler with carousel~multiple sampling...
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