ABSTRACT

Os isotope compositions in ferromanganese crusts (Fe–Mn crusts) have been used for the dating of model ages from present to the Late Cretaceous. This dating method assumes that the Fe–Mn crusts preserve a paleo-seawater Os isotope composition at the timing of Fe–Mn crust deposition. However, available Os isotope data are limited to dredged samples without precise indications of water depths, and the Os isotope variation in relation to water depth remains uncertain. Here, we report on the Os isotope ratio data in the surface layer of Fe–Mn crusts from 956–2987 meters below sea level at the Takuyo Daigo Seamount in the northwestern Pacific Ocean collected by a remotely operated vehicle (ROV). Since the ¹⁸⁷Re/¹⁸⁸Os ratios of the surface layer samples exhibited low values ranging from 0.020 to 0.0078, the age correction of the ¹⁸⁷Os/¹⁸⁸Os ratios by subtracting radiogenic ¹⁸⁷Os from total ¹⁸⁷Os was not necessary for the Takuyo Daigo Fe–Mn crusts. Regardless of water depth, the surface layer samples possessed a narrow range of ¹⁸⁷Os/¹⁸⁸Os ratio (1.003–1.017). As their Os isotope ratios were very similar to or slightly lower than the present-day seawater value (~1.06), the Fe–Mn crusts are inferred to preserve the modern seawater Os isotope composition at the investigated water depths. Therefore, Os isotope stratigraphy using Fe–Mn crusts is a powerful dating tool in paleoceanography.