2:45 PM - 3:00 PM
[BCG05-17] Os isotopic composition of Early Permian seawater reconstructed from Kunimiyama umber in Kochi, central Shikoku, Japan
Keywords:umber, Os isotopic ratio, Early Permian, accretionary complex, hydrothermal ferromanganese sediment
Osmium (Os) isotopic composition of seawater is controlled by the relative balance of riverine, hydrothermal, and cosmic dust fluxes. Therefore, Os isotope ratios recorded in seafloor sediments can provide insights into the relative changes in these fluxes and associated paleoenvironmental changes.
Fe–Mn oxyhydroxides derived from seafloor hydrothermal activity at a mid-oceanic ridge effectively adsorb elements (e.g. P, V, As, REE) in seawater and then precipitate on the seafloor as hydrothermal ferromanganese sediment. Osmium, which dissolves in seawater with an extremely low concentration, is also adsorbed on Fe–Mn oxyhydroxide particles and settled into sediments. Such a hydrothermal ferromanganese sediment accreted onto a continental plate owing to the plate motion, resulting in a ferromanganese sedimentary rock called “umber.”
The Kunimiyama umber, distributed in the Northern Chichibu Belt, central Shikoku, Japan, is considered to have been derived from seafloor hydrothermal activity and accreted onto the continental margin in the same way as other umber deposits in the Japanese accretionary complexes (Kato et al., 2005; Nozaki et al., 2005). The depositional age of the Kunimiyama umber was constrained to the Early Permian based on radiolarian fossils in red chert associating with the umber deposit (Fujinaga and Kato et al., 2005). Therefore, the Kunimiyama umber is expected to retain the paleo-seawater Os isotopic ratio of the Early Permian.
In this study, we conduct Os isotope analysis of the Kunimiyama umber and reconstruct Os isotopic composition of Early Permian seawater. Analytical pretreatment and measurement were performed at the Ocean Resources Research Center for Next Generation, Chiba Institute of Technology. In the presentation, we will report preliminary results of the analysis.
Fe–Mn oxyhydroxides derived from seafloor hydrothermal activity at a mid-oceanic ridge effectively adsorb elements (e.g. P, V, As, REE) in seawater and then precipitate on the seafloor as hydrothermal ferromanganese sediment. Osmium, which dissolves in seawater with an extremely low concentration, is also adsorbed on Fe–Mn oxyhydroxide particles and settled into sediments. Such a hydrothermal ferromanganese sediment accreted onto a continental plate owing to the plate motion, resulting in a ferromanganese sedimentary rock called “umber.”
The Kunimiyama umber, distributed in the Northern Chichibu Belt, central Shikoku, Japan, is considered to have been derived from seafloor hydrothermal activity and accreted onto the continental margin in the same way as other umber deposits in the Japanese accretionary complexes (Kato et al., 2005; Nozaki et al., 2005). The depositional age of the Kunimiyama umber was constrained to the Early Permian based on radiolarian fossils in red chert associating with the umber deposit (Fujinaga and Kato et al., 2005). Therefore, the Kunimiyama umber is expected to retain the paleo-seawater Os isotopic ratio of the Early Permian.
In this study, we conduct Os isotope analysis of the Kunimiyama umber and reconstruct Os isotopic composition of Early Permian seawater. Analytical pretreatment and measurement were performed at the Ocean Resources Research Center for Next Generation, Chiba Institute of Technology. In the presentation, we will report preliminary results of the analysis.