University of the Ryukyus · Faculty of Science · 教授

Research keywordsGeology・海洋科学・地球化学・Mineralogy・Igneous Petrology

Research fieldsPetrology/Mineralogy/Science of ore deposit・Solid earth and planetary physics・Civil engineering・Civil engineering materials/Construction/Construction management・Earth and planetary science・地球科学・Engineering・Mathematics and Physics

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• Can sclerosponge skeletons record ocean acidification?: Boron and carbon isotope ratios (δ11B and δ13C) in Acanthochaetetes wellsi from Okinoerabu Island, southwestern Japan2025 · Tanaka, K., Shirai, K., Uyeno, D., Shinjo, R.
• Penultimate glacial sea surface temperature and hydrologic variability in the tropical South Pacific from 150 ka Tahiti corals2025 · Abstract. Constraining climate models under extreme boundary conditions of the past on societally-relevant timescales is complicated by a common lack of high-resolution reconstructions of sea surface environmental variability for glacial periods. Here, we present subseasonally-resolved Sr/Ca and oxygen isotope (δ18O) records from well-preserved fossil corals of the penultimate glacial and last glacial periods drilled by Integrated Ocean Drilling Program Expedition 310 “Tahiti Sea Level” in the central tropical South Pacific. The proxy records document the mean and seasonality of sea surface temperature (SST) and seawater δ18O (δ18Osw) at 153 and 148 ka, during Marine Isotope Stage (MIS) 6b, and around 30 ka during MIS 3a. Results show mean SST 2.8–4.0 °C lower than present for MIS 6b, and about 3.8 °C lower for MIS 3a. The MIS 6b SST differences are greater during the austral winter (3.7–4.4 °C lower) than during the austral summer (2.0–3.7 °C lower), indicating a greater SST seasonality relative to today during the penultimate glacial. A reconstructed higher mean δ18Osw for both MIS 6b and MIS 3a (+0.41 ‰ to +0.51 ‰ relative to today) suggest more saline surface waters in the central tropical South Pacific over the entire year. Our coral-based reconstructions of SST and hydrology may indicate a reduced mixed layer depth around Tahiti during the penultimate glacial and last glacial. A potential explanation is a westward-expanded South Pacific subtropical dry area relative to today, probably accompanied by lower activity and/or displacement of the South Pacific Convergence Zone.
• Recent monogenic volcanism in North Central Vietnam: Implications for the regional mantle geodynamics2025 · Pliocene to Pleistocene monogenic basalt volcanism in North Central Vietnam is typical of the East and Southeast Asian' diffuse igneous province'. Exposed at Khe Sanh (4.5 Ma), Gio Linh (1.5–1.3 Ma), and Con Co Island (0.35 Ma) the Miocene activity marks the southeastern tip of the Red River Shear Zone (RRSZ) and appears to relate to the rifting of the Hue Sub-basin, triggered by the Song Ca-Rao Nay Fault system (SCRNFS). Petrological analysis of primitive samples suggests that their formation occurred via decompression melting of spinel peridotite mantle at temperatures of 1350–1400°C and pressures of 15–29 kbar. The basalts display oceanic island basalt (OIB)-type geochemical signatures, characterized by enrichment in radiogenic isotopes of Sr, Nd, Pb, and Hf. They are relatively high in TiO2 and K2O, and incompatible element concentrations. These suggest a fertile, spinel-peridotite source, influenced by in situ metasomatism and/or the presence of recycled subducted oceanic sediment melt. Given the lack of evidence for mantle plume activity beneath the region, we propose that North Central Vietnamese monogenic volcanism reflects decompression melting of a hotter-than-average (Tp ≈ 1400°C) mantle, triggered by lithospheric stretching (greater than 1.5) within the Red River and Hue Sub-basins. In the context of previous regional studies of east/southeast Asian Cenozoic volcanism, Dupal-like mantle affinity is ascribed to eastward asthenospheric flow from the Neo-Tethyan mantle following its closure due to the India-Eurasia collision in the Tertiary.
• Tracing young subduction beneath northwest Iran: Insights from cosmogenic 10Be recycling and Sr-Nd-Hf isotopes in late Miocene volcanoes2025 · Abstract Late Miocene–Quaternary volcanoes in northwestern Iran, characterized by andesitic to dacitic compositions including adakitic rocks with high Sr/Y and La/Yb ratios, exhibit zircon U-Pb ages ranging from 12 to 2 Ma. The isotopic compositions of whole-rock samples (εNd = +0.1 to +4.4; εHf = +2.8 to +11.8; 87Sr/86Sr < 0.705) indicate that the primary magma originated from partial melting of subcontinental lithospheric mantle. The whole-rock chemistry supports an arc magmatic signature, which is consistent with an active continental margin setting. To investigate sediment recycling, cosmogenic beryllium-10 (10Be) isotopes, with a short half-life (1.39 m.y.), were measured in 40 fresh whole-rock samples. Elevated 10Be concentrations (>106 atoms/g) in certain volcanoes confirm a young metasomatized mantle beneath the northwest Iran block. Variations in 10Be concentrations that increase from the front-arc to the rear-arc magmatic rocks suggest greater incorporation of pelagic sediments (rich in 10Be) away from the trench zone. These findings indicate that late Miocene–Quaternary magmatic activity coincided with or followed the formation of an active margin linked to the subduction of the Khoy-Oshnavieh oceanic branch of the northern Neo-Tethys Ocean beneath northwest Iran. Evidence suggests that this branch remained active until the late Miocene. This study highlights the significant role of subducted terrestrial and pelagic sediments in Pliocene–Quaternary magmatism, as supported by 10Be data. This study underscores the utility of cosmogenic 10Be as a reliable tracer for sediment incorporation in young magmatic systems along active margins, providing insights into the geodynamic processes shaping northwestern Iran.

• 沖縄県特有の土壌分布は特発性間質性肺炎の経過に影響をおよぼすのか？2025 · 基盤研究(C)
• サンゴの生息に許容可能な陸域由来のリン酸塩負荷量の決定：蓄積型リンに着目して2023 · 基盤研究(C)
• 前兆現象が発生している活火山の噴火予測：過去の噴出物からの物質科学的アプローチ2022 · 基盤研究(C)
• 南西諸島南西端域における「島弧胴切り」型海底活断層群と巨大津波発生に関する研究

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