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• Importance of pumiceous sediment in influencing chemistry of hydrothermal fluids in Okinawa Trough: insights from boron isotopic ratios

  Tomohiro Toki, Naoya Ebina, Yuki Mitsutome, Ke-Han Song, Ryuichi Shinjo, Junichiro Ishibashi, Shinsuke Kawagucci

  Progress in Earth and Planetary Science ( Springer Nature )  12 ( 1 ) 115   2025.12 [ Peer Review Accepted ]

  Type of publication: Research paper (scientific journal)

  　View Summary

  沖縄トラフにおける海底熱水に対する堆積物の関与について，これまでにも数多くの研究がなされてきている。しかし，その定量的な議論については，いまだに極めて限定的である。そこで，本研究では南部沖縄トラフ及び中部沖縄トラフから複数の海底熱水を採取し，ホウ素濃度及び同位体比を明らかにした。合わせて，周辺堆積物も採取し，それらのホウ素濃度及び同位体比も明らかにした。その結果，熱水中のホウ素濃度は，世界最高の値を示し，一方，ホウ素同位体比は，世界最低の値を示した。さらに，複数の熱水系から得られた熱水のホウ素濃度及び同位体比は，極めて大きな多様性を示し，多様な固相と反応していることが示唆された。一方，堆積物中のホウ素については，軽石以外の値は世界中で報告されているケイ酸塩堆積物の値と一致したが，一方，軽石については沖縄トラフの火山岩と同等の値を示した。中でも，伊平屋北海丘における熱水中のホウ素濃度及び同位体比は，軽石の影響なしには説明できないことが示され，一方，鳩間海丘の熱水中のホウ素については，火山岩と海水との反応だけで説明することが可能であった。一方，第四与那国海丘の海底熱水中のホウ素については，火山岩あるいは堆積物のいずれかと海水との反応だけでは説明が難しく，火山岩及び堆積物のいずれとも反応している必要があることが明らかとなった。伊是名海穴JADEサイトのホウ素については，複数のエンドメンバーがあることが明らかとなり，このことは噴出孔周辺の固相の不均一性を示している可能性がある。

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• An improved narrower-diameter EA/IRMS method with a built-in desulfurization reactor: optimization for nitrogen and carbon measurement without sulfur matrix interference

  Matsui, Y; Fujisaki, W; Aze, T; Miyajima, T; Kawagucci, S; Tasumi, E; Yamamoto, M; Takano, Y; Toki, T

  PROGRESS IN EARTH AND PLANETARY SCIENCE ( Progress in Earth and Planetary Science )  12 ( 1 )   2025.12 [ Peer Review Accepted ]

  Type of publication: Research paper (scientific journal)

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• Estimation of bubbled oxygen flux from a coral reef community

  Tomohiro Toki, Rie Matayoshi, Masataka Yamauchi, Seiichi Koizumi, Hiroyuki Fujimura

  Galaxea, Journal of Coral Reef Studies ( 日本サンゴ礁学会 )  27 ( 1 ) 47 - 61   2025.07 [ Peer Review Accepted ]

  Type of publication: Research paper (scientific journal)

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  We collected bubbles from the seafloor of coral reef areas to determine their quantity and chemical composition. The chemical composition was high in oxygen and low in carbon dioxide compared to that of the atmosphere. Oxygen fluxes to the atmosphere from coral reef waters are generally determined considering only dissolved oxygen in seawater, but accounting for bubbled oxygen increased such fluxes by about 70%. At the same time, the primary production rate in coral reefs is estimated only from the dissolved oxygen concentration in seawater, but accounting for bubbled oxygen increased it by about 20–30%. The composition of bubbled gases suggests that approximately 20% of the oxygen present may be derived from photosynthesis in the reef area. Global quantitative estimation of bubbled oxygen fluxes in tropical and subtropical coral reef waters remains critical for future research.

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• Concentrations and isotope ratios of Mo and W in Okinawa Trough hydrothermal fluids: Novel probes for hydrothermal processes in a back-arc basin

  Kohei Matsuoka, Takano Shotaro, Kawagucci Shinsuke, Toki Tomohiro, Sohrin Yoshiki

  ＧＥＯＣＨＥＭＩＣＡＬ　ＪＯＵＲＮＡＬ ( 一般社団法人日本地球化学会 )  advpub ( 0 )   2025 [ Peer Review Accepted ]

  Type of publication: Research paper (scientific journal)

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  <p>The concentrations and isotope ratios of molybdenum (Mo) and tungsten (W) are expected to serve as proxies for paleoceanography. Although submarine hydrothermal activities are potentially major sources and sinks of Mo and W, data on these elements in hydrothermal fluids are scarce. In this study, we present the concentrations and isotope ratios of dissolved Mo and W in hydrothermal fluid samples collected from nine active sites—Higashi Ensei, Iheya North, JADE, Hakurei, Higashi Izena, Daisan Kume, Yokosuka, Futagoyama, and Hatoma Knoll—within the Okinawa Trough, which is a back-arc basin. The Mo concentrations ranged from 4.2–411 nmol/kg, <i>δ</i>^98/95Mo values ranged from –0.17 to 3.92‰, W concentrations ranged from 1.7–238 nmol/kg, and <i>δ</i>^186/184W values ranged from –0.03 to 1.08‰. We found that both <i>δ</i>^98/95 Mo and <i>δ</i>^186/184W were close to 0‰ when Mo and W were dissolved from rocks and sediments during hydrothermal circulation. The phase separation of the hydrothermal fluids concentrates Mo and W in the liquid phase. Sediments can be a significant source of W at high temperatures and Mo even at low temperatures. The scavenging of Mo by Fe sulfides reduces its concentrations and elevates <i>δ</i>^98/95Mo values during the emission of high-temperature hydrothermal fluids, while having an insignificant effect on W. The scavenging of W by Fe hydroxides reduces its concentrations and elevates <i>δ</i>^186/184W values during mixing of hydrothermal fluids and seawater, while having an insignificant effect on Mo. Therefore, the data on Mo and W are useful for investigating the processes that occur during hydrothermal circulation.</p>

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• Origin of helium in basement rocks and carbonate veins in Yonaguni Island

  Toki, T; Yasumura, K; Takahata, N; Miyajima, Y; Miyaki, H; Oohashi, K; Otsubo, M

  GEOCHEMICAL JOURNAL ( 一般社団法人日本地球化学会 )  58 ( 6 ) 293 - 303   2024 [ Peer Review Accepted ]

  Type of publication: Research paper (scientific journal)

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  <p>Minerals formed in high-temperature environments are distributed along faults on Yonaguni Island. Volcanic activity is thought to be a possible heat source. In this study, samples from the carbonate veins distributed around the basement rocks and faults of Yonaguni Island were melted in vacuum, and the concentrations and isotope ratios of the noble gases in the furnace were investigated. The observed isotope ratios suggest that the noble gases were dominated by crustal helium, with little influence from mantle helium. These results suggest that there was some kind of non-volcanic heat supply at Yonaguni Island (e.g., eruption of non-volcanic hot springs).</p>

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Books 【 display / non-display 】

Books 【 display / non-display 】

• Nature in the Ryukyu Archipelago

  Tomohiro Toki ( Part: Single Author ,  When do you research hydrothermal systems? Now! )

  Change and Coral Reef/Island Dynamics Faculty of Science, University of the Ryukyus  2015

  　View Summary

  沖縄トラフの海底熱水研究の最新トピックをわかりやすく英語で紹介。

• Deep-sea Life - Biological observations using research submersibles

  Jun-ichiro Ishibashi, Tomhiro Toki ( Part: Multiple Authorship )

  Tokai University Press  2008.02

Other Papers 【 display / non-display 】

Other Papers 【 display / non-display 】

• Chemistry on coral reefs: In memory of Dr. Tamotsu Oomori (1944–2022)

  Fujimura Hiroyuki, Toki Tomohiro

  Galaxea, Journal of Coral Reef Studies ( 日本サンゴ礁学会 )  27 ( 1 ) 30 - 32   2025.07

   

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• Contribution and mission of geochemistry to mud volcano study

  Tomohiro Toki

  Bulletin of the Faculty of Science, University of the Ryukyus ( Faculty of Science, University of the Ryukyus )  ( 108 ) 1 - 26   2020.03

   

• Simple method of collecting pyrolysis gas from bagasse

  Ryo Itou, Keiko Nakasone, Tomohiro Toki

  Bulletin of the Faculty of Science, University of the Ryukyus ( Faculty of Science, University of the Ryukyus )  ( 102 ) 1 - 11   2016.09

   

• Community structure of a microbial mat from the methane seep off Yagaji Island

  Tomohiro Toki, Hisako Hirayama, Syogo Oshima

  Bulletin of the Faculty of Science, University of the Ryukyus ( Faculty of Science, University of the Ryukyus )  ( 100 ) 5 - 12   2015.09

   

• Geochemical and geological investigations in the area around the southern Ryukyu islands - Report on RN10 cruise by T/S Nagasaki-maru -

  Tomohiro Toki, Masahide Furukawa, Hiroshi Yoshimura, Youji Kanda, Dae-Yul Baek, Kazuhiko Fujita, and Hiroyuki Fujimura

  Bulletin of the Faculty of Science, University of the Ryukyus ( Faculty of Science, University of the Ryukyus )  ( 91 ) 9 - 19   2011.03

   

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Presentations 【 display / non-display 】

Presentations 【 display / non-display 】

• Structural characteristics of the Upper Prism in the Japan Trench suggested by physical and magnetic properties

  Nana Kamiya, Yohei Hamada, Hanaya Okuda, Keisuke Nakamoto, Taizo Uchida, Hinako Hosono, Yoshitaka Hashimoto, Tomohiro Toki, Yusuke Kubo

  AGU Fall Meeting 2025  (New Orleans, LA & Online Everywhere)  2025.12  -  2025.12 

• Origin of helium in surface sediments in the continental slope area of the East China Sea

  Tomohiro Toki, Ryusei Yonamine, Takanori Kagoshima, Naoto Takahata

  Annual Meeting of the Geological Society of Japan  2025.09  -  2025.09 

• Indigenous traditions and water governance: The case of the Erfeng irrigation canal

  Miyuki Shimabukuro, Tomohiro Toki, Yoshiaki Kubo, Hitoshi Shimabukuro

  The 2nd International Sociohydrology Conference  (東京大学・本郷キャンパス)  2025.07  -  2025.07 

• Origin of lithium in pore fluids near offshore fault zone, northern Noto Peninsula, two months after the 2024 Noto Peninsula earthquake (M7.6) during the R/V Hakuho-maru cruise

  Zahra Zandvakili, Yoshiro Nishio, Tomohiro Toki, Takanori Kagoshima, Jin-Oh Park

  日本地球惑星科学連合2025年大会  (幕張メッセ)  2025.05  -  2025.05 

• Distribution of dissolved methane in seawater from the East China Sea to the Ryukyu forearc

  TOKI, Tomohiro

  Annual Meeting of the Geological Society of Japan  2024.09  -  2024.09 

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Grant-in-Aid for Scientific Research 【 display / non-display 】

Grant-in-Aid for Scientific Research 【 display / non-display 】

• Can seamount subduction weaken locking of megathrust earthquake? Test at 2 seamounts in Nankai Trough

  Grant-in-Aid for Scientific Research(S)

  Project Year: 2024.04  -  2029.03 

  Direct: 152,900,000 (YEN)  Overheads: 198,770,000 (YEN)  Total: 45,870,000 (YEN)

• Geosphere-hydrosphere-biosphere connected by submarine mud volcanism: Diffusion processes of deep life and carbon, and their effects on the ocean

  Grant-in-Aid for Scientific Research(A)

  Project Year: 2024.04  -  2029.03 

  Direct: 36,800,000 (YEN)  Overheads: 47,840,000 (YEN)  Total: 11,040,000 (YEN)

• Grant-in-Aid for Scientific Research(C)

  Project Year: 2023.04  -  2027.03 

  Direct: 3,600,000 (YEN)  Overheads: 4,680,000 (YEN)  Total: 1,080,000 (YEN)

• Fluid flow in the uppermost part of the incoming plate seaward of the trench: Regional variations and influence on processes around the plate interface

  Grant-in-Aid for Scientific Research(B)

  Project Year: 2023.04  -  2026.03 

  Direct: 14,400,000 (YEN)  Overheads: 18,720,000 (YEN)  Total: 4,320,000 (YEN)

• Construction of a model for the diffusion and circulation of deep subsurface life and carbon to the ocean via submarine mud volcanism

  Grant-in-Aid for Scientific Research(B)

  Project Year: 2020.04  -  2023.03 

  Direct: 13,500,000 (YEN)  Overheads: 17,550,000 (YEN)  Total: 4,050,000 (YEN)

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Other External funds 【 display / non-display 】

• Project Name: Research Project

  Project Year: 2019.04  -  2027.03 

  Offer Organization: Research Institute for Humanity and Nature

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Social Activity 【 display / non-display 】

Social Activity 【 display / non-display 】

• 2026.03

  　

  　

  

• 2026.02

  　

  　

  

• 2025.12

  　

  　

  

• 2025.03

  　

  　

  

• 2024.12

  　

  　

  

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Media Coverage 【 display / non-display 】

Media Coverage 【 display / non-display 】

• 竹富島の地層サンプルを琉大准教授へ寄贈/温泉掘削調査で採取  TV or radio program

  琉球朝日放送・報道制作局  CATCHY  2025.4

  Author： Other 

  　View Summary

  竹富島温泉の掘削試料を，研究に役立ててほしい，というご意思を受け，私が受け取った件に関する記者会見の様子。

• 周辺に海底温泉があるとされる竹富島　地元企業が1000メートル分の地質サンプルを研究者に寄贈  Internet

  RBC  2025.4

  Author： Other 

  　View Summary

  竹富島温泉の掘削試料を，研究に役立ててほしい，というご意思を受け，私が受け取った件に関する記者会見の様子。

• 地下千㍍分の地層試料が琉大に　掘削工事の土質標本寄贈　竹富島黒潮観光  Newspaper, magazine

  八重山毎日新聞  八重山毎日新聞  2025.4

  Author： Other 

  　View Summary

  竹富島温泉の掘削試料を，研究に役立ててほしい，というご意思を受け，私が受け取った件に関する記者会見の様子。

• 「流体」の謎…海底の泥を分析  TV or radio program

  ㈱テレビ朝日  サンデーステーション  2024.3

  Author： Other 

  　View Summary

  能登半島地震では，地球深部から供給されている”流体”が地震を引き起こしていることが指摘されてきているものの，その正体はよくわかっていない。当研究室では，能登半島地震の震源域近くの海底堆積物中から間隙水を抽出して持ち帰り，陸上において化学分析を行うことで，流体の化学組成を明らかにすることを目指している。

• 種子島沖海底泥火山における表層堆積物中の希ガスを用いた流体の起源深度の推定  Internet

  Chem-Station  Chem-Station  2023.5

  Author： Other 

  　View Summary

  種子島沖海底泥火山の表層堆積物中から抽出したガス中の希ガス濃度から，溶解平衡温度を見積もり，その深度を推定したところ，海底下数kmであることが明らかとなった。このことから，本泥火山の表層堆積物中の希ガスが，採取される直前まで海底下数kmにあったことが示唆され，泥火山の噴出現象によって海底直下まで移動させられたことが示唆された。

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Academic Activities 【 display / non-display 】

Academic Activities 【 display / non-display 】

• [International contribution]

  日本地球化学会  （ 琉球大学・千原キャンパス ）

  2018.9

  　

  　

  種別： Academic society, research group, etc. 

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  口頭発表及びポスター発表，夜間集会，学会賞記念講演，総会，懇親会など。関連イベントとして，9月9日（日）に市民講演会，9月10日（月）にショートコース，9月14日（金）にエクスカーションを開催した。

• フジテレビ  （ 奇跡体験！アンビリーバボー ）

  2025.4

  　

  　

  種別： Scientific advice/Review 

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  インドネシアで，村ごと沈んだ泥の噴出現象について，アニメーションで解説したいというディレクターの相談に乗り，イメージを伝え，ナレーション原稿の添削を行った。