Importance of pumiceous sediment in influencing chemistry of hydrothermal fluids in Okinawa Trough: insights from boron isotopic ratios2025 · 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。
Estimation of bubbled oxygen flux from a coral reef community2025 · 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.
Concentrations and isotope ratios of Mo and W in Okinawa Trough hydrothermal fluids: Novel probes for hydrothermal processes in a back-arc basin2025 · <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><sup>98/95</sup>Mo values ranged from –0.17 to 3.92‰, W concentrations ranged from 1.7–238 nmol/kg, and <i>δ</i><sup>186/184</sup>W values ranged from –0.03 to 1.08‰. We found that both <i>δ</i><sup>98/95</sup> Mo and <i>δ</i><sup>186/184</sup>W 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><sup> 98/95</sup>Mo 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><sup> 186/184</sup>W 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>
