研究業績

Strata-bound ferromanganese (Fe–Mn) deposits (umbers) are thought to have formed by precipitation of Fe–Mn oxyhydroxides derived from seafloor hydrothermal activity at a mid-oceanic ridge. Fe–Mn oxyhydroxides effectively adsorb various elements (e.g., P, V, As, rare earth elements, and Os) dissolved in seawater. Of these elements, osmium (Os) is particularly important for reconstructing the Earth’s surface environment during formation of the deposits. Here we report the Os and rhenium (Re) contents and marine Os isotopic ratio (187Os/188Os)i of the early Permian reconstructed from the Kunimiyama umber deposit in the Northern Chichibu Belt, central Shikoku, southwest Japan. The Kunimiyama umbers exhibit comparable to slightly lower Os (42–183 ppt) and much lower Re (1.3–49 ppt) contents than other umbers in Japanese accretionary complexes and modern seafloor metalliferous sediments, the latter of which are regarded as umber precursors. The (187Os/188Os)i ratio of the Kunimiyama umbers is 0.523–0.676, which is comparable to or slightly higher than the values of the middle to Late Cretaceous and early Paleogene. This work is the first report of seawater Os isotope ratios in the Panthalassa Ocean of the early Permian, representing an important achievement in understanding the marine Os isotopic record of the Paleozoic Era.