Japan Geoscience Union Meeting 2021

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[J] Poster

M (Multidisciplinary and Interdisciplinary) » M-ZZ Others

[M-ZZ46] Marine Manganese Deposits: Origin, Exploration, and Development

Thu. Jun 3, 2021 5:15 PM - 6:30 PM Ch.19

convener:Akira Usui(Marine Core Research Center, Kochi University), Katsuhiko Suzuki(Submarine Resources Research Center, Japan Agency for Marine-Earth Science and Technology), Yoshio Takahashi(Department of Earth and Planetary Science, Graduate School of Science, The University of Tokyo), Takashi Ito(Faculty of Education, Ibaraki University)

5:15 PM - 6:30 PM

[MZZ46-P03] Submarine erosion of basement sedimentary rocks around ferro-manganese crust fields

*Takashi Ito1, Masaki Saito1, Eri Shimizu2, Takashi Miyazaki3, Bogdan S. Vaglarov3, Katsuhiko Suzuki3, Akira Usui4 (1.Faculty of Education, Ibaraki University, 2.JOGMEC, 3.JAMSTEC, 4.Center for Advanced Marine Core Research, Kochi University)

Keywords:ferromanganese crust, fish tooth fossil, Strontium isotope

The ferromanganese crust Sample HPD#958R12 collected from the depth of 1424 m at Takuyo-Daigo Seamount in the western Pacific Ocean was used as a research target to investigate the seamount erosion around the crust formation site. The number of fish tooth fossil samples separated from the 7-layers within the surficial 4 cm of this crust is 25 to 54, and the mass is about 0.1 mg to 0.2 mg. The fish tooth fossils for each of these strata were chemically treated and yield the 87Sr/86Sr ratio ranging from 0.70809 to 0.70880. The Sr isotopic composition of each fish tooth group in all layers is significantly lower than the coeval seawater Sr isotope ratio estimated from the 10Be age of each layer of the crust. No specific pattern is observed in the growth direction. Based on these results, it was clarified that the teeth of fish contained in the crust include not only the fish that lived during the crust growth but also the older tooth fossils with lower Sr isotope ratio. The field observation by JAMSTEC ROV and the examination of the collected samples reveal that the exposed sedimentary rocks near the summit of Takuyo-Daigo Seamount are hard, phosphatized Cretaceous reef deposits. The results of this study clearly suggest that the following mechanism are very common, i.e. these phosphatized basement rocks are physically eroded and the washed-out fish teeth are contained in the growing layer of the crust. It was shown that the phenomena of physical erosion of hard sedimentary rocks and chemical precipitation of manganese oxide coexist in the seamounts.