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  1. Article ; Online: Estimation of the depth of origin of fluids using noble gases in the surface sediments of submarine mud volcanoes off Tanegashima Island

    Yuki Mitsutome / Tomohiro Toki / Takanori Kagoshima / Yuji Sano / Yama Tomonaga / Akira Ijiri

    Scientific Reports, Vol 13, Iss 1, Pp 1-

    2023  Volume 12

    Abstract: Abstract The helium isotope ratio (3He/4He), concentration ratio of neon-20 to helium-4 (20Ne/4He), argon (Ar), krypton (Kr), and xenon (Xe) concentrations were measured in the porewater of surface sediments of several submarine mud volcanoes. From the ... ...

    Abstract Abstract The helium isotope ratio (3He/4He), concentration ratio of neon-20 to helium-4 (20Ne/4He), argon (Ar), krypton (Kr), and xenon (Xe) concentrations were measured in the porewater of surface sediments of several submarine mud volcanoes. From the 3He/4He values (0.18–0.93RA), the estimated He origin is almost 90% crustal He, with little contribution from mantle-derived He. The determined Ar, Kr, and Xe concentrations lie within the solubility equilibrium range expected for temperatures from 83 °C up to 230 °C and are consistent with the temperature range of the dehydration origin of clay minerals. Considering the geothermal gradient in the investigated region (25 °C/km), these gases are considered to have reached dissolution equilibrium at a depth of about 3.3 km to 9.2 km below the seafloor. As the depth of the plate boundary is 18 km below the seafloor, the noble gas signatures are likely to originate from the crust, not from the plate boundary. This is consistent with the results presented by the He isotope ratios.
    Keywords Medicine ; R ; Science ; Q
    Subject code 550
    Language English
    Publishing date 2023-04-01T00:00:00Z
    Publisher Nature Portfolio
    Document type Article ; Online
    Database BASE - Bielefeld Academic Search Engine (life sciences selection)

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  2. Article ; Online: Assessing the activity of mud volcanism using boron isotope ratios in pore water from surface sediments of mud volcanoes off Tanegashima (SW Japan)

    Yuki Mitsutome / Ko Agena / Tomohiro Toki / Ke-Han Song / Ryuichi Shinjo / Akira Ijiri

    Frontiers in Marine Science, Vol

    2023  Volume 10

    Abstract: Mud volcanoes can cause various geohazards, so it is very important to know their activity level and their distribution. Surface sediments were collected from four submarine mud volcanoes (MVs) off Tanegashima (SW Japan), namely, MV1, MV2, MV3, and MV14. ...

    Abstract Mud volcanoes can cause various geohazards, so it is very important to know their activity level and their distribution. Surface sediments were collected from four submarine mud volcanoes (MVs) off Tanegashima (SW Japan), namely, MV1, MV2, MV3, and MV14. We extracted pore water from the surface sediments and investigated its chemical and isotopic compositions. The sodium (Na) and chloride (Cl-) concentrations decreased and the boron (B) and lithium (Li) concentrations increased with increasing depth, suggesting that some fluids with lower Na and Cl- concentrations and higher B and Li concentrations than seawater were supplied upward from the deep sub-seafloor. The fluid advection velocities estimated from the pore-water profiles differed for each MV, and those of MV3 were the fastest (14 cm/yr) in this study. The estimated equilibrium temperature with clay minerals using Na and Li concentrations were 93-134°C, corresponding to the temperature of environments around 3.7 to 5.3 km below the seafloor. This indicates that these components originated from these depths and that the origin depth did not reach the plate boundary in this area. The B isotope ratio in the pore water was extremely high up to +57 ‰, suggesting that it was strongly affected by adsorption onto the surface of the sediments. A higher B isotope ratio (+57 ‰) was detected in MV3, which was considered to be more active, indicating that more B was adsorbed onto clay minerals supplied from deeper depths.
    Keywords mud volcano ; Tanegashima ; pore water ; boron isotope ; activity ; Science ; Q ; General. Including nature conservation ; geographical distribution ; QH1-199.5
    Subject code 550
    Language English
    Publishing date 2023-09-01T00:00:00Z
    Publisher Frontiers Media S.A.
    Document type Article ; Online
    Database BASE - Bielefeld Academic Search Engine (life sciences selection)

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  3. Article ; Online: Spring discharge mechanism along the southeast coast of Yonaguni Island in the southern Ryukyu forearc

    Tomohiro Toki / Hina Kataoka / Ryogo Takada / Shinji Nakaya / Syogo Oshima / Akira Ijiri

    Journal of Hydrology: Regional Studies, Vol 40, Iss , Pp 101051- (2022)

    2022  

    Abstract: Study region: Spring along the southeast coast of Yonaguni Island, located at the western end of the Ryukyu forearc. Study focus: The chemical composition of groundwater, including SF6 concentration, was investigated to clarify the mechanism of ... ...

    Abstract Study region: Spring along the southeast coast of Yonaguni Island, located at the western end of the Ryukyu forearc. Study focus: The chemical composition of groundwater, including SF6 concentration, was investigated to clarify the mechanism of groundwater discharge around Sanninudai on the southeast coast of Yonaguni Island. New hydrological insights for the region: Accurate determination of groundwater flow paths is essential to the conservation and management of groundwater sources and their sustainable use. Other areas around the world may show similar complexity, requiring a review of past findings. This possibility indicates the need for re-examination of groundwater sources around the world. In particular, the development of novel analytical methods for chemical components may provide valuable information. The chemical composition of spring water on remote islands, where anthropogenic activities have little influence, should be investigated continuously to monitor the effects of sea-level rise caused by global warming.
    Keywords Yonaguni Island ; Spring water ; Discharging mechanism ; Seawater influence ; Apparent residence time ; Physical geography ; GB3-5030 ; Geology ; QE1-996.5
    Subject code 550
    Language English
    Publishing date 2022-04-01T00:00:00Z
    Publisher Elsevier
    Document type Article ; Online
    Database BASE - Bielefeld Academic Search Engine (life sciences selection)

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  4. Article ; Online: Low-Temperature Clay Mineral Dehydration Contributes to Porewater Dilution in Bering Sea Slope Subseafloor

    Akira Ijiri / Naotaka Tomioka / Shigeyuki Wakaki / Harue Masuda / Katsumi Shozugawa / Sunghan Kim / Boo-Keun Khim / Masafumi Murayama / Motoyuki Matsuo / Fumio Inagaki

    Frontiers in Earth Science, Vol

    2018  Volume 6

    Abstract: Widespread diagenesis of clay minerals occurs in deeply buried marine sediments under high-temperature and high-pressure conditions. For example, the smectite-to-illite (S-I) transformation has been often observed in sediments at in situ temperatures ... ...

    Abstract Widespread diagenesis of clay minerals occurs in deeply buried marine sediments under high-temperature and high-pressure conditions. For example, the smectite-to-illite (S-I) transformation has been often observed in sediments at in situ temperatures above ~60°C. However, it remains largely unknown whether such diagenetic processes naturally occur in relatively shallow and low-temperature sediments and, if so, what the consequences are of any related chemical reactions to the geochemical characteristics in the deep biosphere. We evaluated the possibility of naturally occurring S-I transformation at temperatures below 40°C in continental slope sediments of the Bering Sea by examining porewater chemistry, clay mineralogy, and chemical composition of clay minerals measured to ~800 m beneath the seafloor (mbsf) in core samples acquired during Integrated Ocean Drilling Program Expedition 323. In porewater from these cores, chloride concentrations decreased with increasing depth from 560 mM near the seafloor to 500 mM at ~800 mbsf; δ18O increased from 0 to 1.5‰; and δD decreased from −1 to −9‰. These trends are consistent with the addition of water derived from S-I transformation. The discrete low Cl− spikes observed between ~200 and ~450 mbsf could be attributed to the dissociation of methane hydrate. X-ray diffraction analysis of the clay-size fraction (<2 μm) showed an increase of illite content in the I/S mixed layer with increasing depth to 150 mbsf. This increase may imply the occurrence of S-I transformation. The decrease of Fe3+/Fe2+ in the clay-size fraction with increasing depth strongly suggests microbial reduction of Fe(III) in clay minerals with burial, which also has the potential to promote the S-I transformation. Our results imply the significant ecological roles on the diagenesis of siliciclastic clay minerals underlying the high-productivity surface seawater at continental margins.
    Keywords smectite-to-illite transformation ; porewater chemistry ; clay mineralogy ; iron reduction ; Bering Sea Slope ; Science ; Q
    Subject code 550
    Language English
    Publishing date 2018-04-01T00:00:00Z
    Publisher Frontiers Media S.A.
    Document type Article ; Online
    Database BASE - Bielefeld Academic Search Engine (life sciences selection)

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  5. Article ; Online: Cultivable microbial community in 2-km-deep, 20-million-year-old subseafloor coalbeds through ~1000 days anaerobic bioreactor cultivation

    Hiroyuki Imachi / Eiji Tasumi / Yoshihiro Takaki / Tatsuhiko Hoshino / Florence Schubotz / Shuchai Gan / Tzu-Hsuan Tu / Yumi Saito / Yuko Yamanaka / Akira Ijiri / Yohei Matsui / Masayuki Miyazaki / Yuki Morono / Ken Takai / Kai-Uwe Hinrichs / Fumio Inagaki

    Scientific Reports, Vol 9, Iss 1, Pp 1-

    2019  Volume 16

    Abstract: Abstract Recent explorations of scientific ocean drilling have revealed the presence of microbial communities persisting in sediments down to ~2.5 km below the ocean floor. However, our knowledge of these microbial populations in the deep subseafloor ... ...

    Abstract Abstract Recent explorations of scientific ocean drilling have revealed the presence of microbial communities persisting in sediments down to ~2.5 km below the ocean floor. However, our knowledge of these microbial populations in the deep subseafloor sedimentary biosphere remains limited. Here, we present a cultivation experiment of 2-km-deep subseafloor microbial communities in 20-million-year-old lignite coalbeds using a continuous-flow bioreactor operating at 40 °C for 1029 days with lignite particles as the major energy source. Chemical monitoring of effluent samples via fluorescence emission-excitation matrices spectroscopy and stable isotope analyses traced the transformation of coalbed-derived organic matter in the dissolved phase. Hereby, the production of acetate and 13C-depleted methane together with the increase and transformation of high molecular weight humics point to an active lignite-degrading methanogenic community present within the bioreactor. Electron microscopy revealed abundant microbial cells growing on the surface of lignite particles. Small subunit rRNA gene sequence analysis revealed that diverse microorganisms grew in the bioreactor (e.g., phyla Proteobacteria, Firmicutes, Chloroflexi, Actinobacteria, Bacteroidetes, Spirochaetes, Tenericutes, Ignavibacteriae, and SBR1093). These results indicate that activation and adaptive growth of 2-km-deep microbes was successfully accomplished using a continuous-flow bioreactor, which lays the groundwork to explore networks of microbial communities of the deep biosphere and their physiologies.
    Keywords Medicine ; R ; Science ; Q
    Subject code 551
    Language English
    Publishing date 2019-02-01T00:00:00Z
    Publisher Nature Publishing Group
    Document type Article ; Online
    Database BASE - Bielefeld Academic Search Engine (life sciences selection)

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