T18［トピック］地球史

8:45 〜 9:00

[T18-O-1] 中新世以降のインド洋のオスミウム同位体比変動

*黒田 潤一郎¹、松本 廣直²、桑原 佑典^3,1、Auer Gerald⁴、鈴木 勝彦⁵、久保 雄介⁵、池原 実⁶ (1. 東京大学、2. 筑波大学、3. 関西学院大学、4. University of Graz、5. 海洋研究開発機構、6. 高知大学)

キーワード：

中新世、オスミウム同位体、インド洋

In 2023, the Japan Drilling Earth Science Consortium (J-DESC) and the Kochi Core Center (KCC) launched the Repository Core Re-Discovery (ReCoRD) Program. This initiative aims to maximize the scientific value of the core assets stored at the KCC by having relatively larger scientific teams conduct re-description, X-ray CT scanning, and re-analysis of the cores. Continuous marine sediments ranging from the early Miocene to the Quaternary were recovered at Site 707 on the Mascarene Plateau and Site 752 on the Broken Ridge. In 2023, the ReC23-01 Science Party conducted a comprehensive redescription and resampling of these sediment cores, resulting in an updated and detailed biostratigraphic age model (Lyu et al., 2025). These sediments serve as excellent archives of past marine osmium isotope ratios (¹⁸⁷Os/¹⁸⁸Os) in the Indian Ocean over the past 20 million years. Our new dataset clearly captures a gradual increase in the ¹⁸⁷Os/¹⁸⁸Os ratio from 0.72 at ~15 Ma (middle Miocene) to 1.02 at 0.3 Ma (late Pleistocene). This gradual increase has been widely recognized in the previous studies, suggesting that Indian Ocean sediments faithfully preserve seawater Os isotope signatures. A transient decrease in the Os isotope ratio (to 0.69) around 17 Ma was also observed, corresponding to the well-known mid-Miocene Os isotope minimum identified in the Pacific (Goto et al., 2023). This excursion is likely linked to the eruption and emplacement of the Columbia River Flood Basalt, one of the youngest large igneous provinces on Earth. Compared with Pacific records, the Os isotope values at Site 752A are slightly lower but exhibit a parallel long-term trend, suggesting basin-scale differences in seawater Os composition. This offset may reflect hydrographic factors, such as variations in ocean circulation and water-mass exchange between the Indian and Pacific Oceans. As the spatial and temporal coverage of marine Os isotope records expands, these data are expected to provide a robust global chemostratigraphic framework comparable to that based on strontium (Sr) isotopes. Because the residence time of Os in the ocean is estimated at 10-50 kyr—much shorter than that of Sr—regional Os isotope variations may preserve valuable information regarding past ocean connectivity and basin restriction. In summary, while the Os isotopic record of Indian Ocean sediments broadly reflects secular variations in global seawater composition, detailed interbasinal comparisons are likely to provide key insights into ocean circulation in the geologic pasts.

References
Goto, M.L.G. Tejada, E. Tajika and K. Suzuki, Communications Earth & Environment, Volume 4, Article no. 21, 2023.
Lyu, S. Barragán-Montilla, G. Auer, O.M. Bialik, A.V. Del Gaudio, A.-C. Sarr and D. De Vleeschouwer, Front. Earth Sci. Volume 13, Article no. 1528487, 2025.

ReC23-01 Science Party includes G. Auer, J. Kuroda, A.J. Drury, B. Christensen, D. De Vleeschouwer, O.M. Bialik, M. Ikehara, X. Puentes-Jorge, Tamara Hechemer, A.-S. Lee, A. Seki, J. Yoshioka, Y. Kubo and N. Okutsu