Magnetostratigraphic Evidence for Post-Depositional Distortion of Osmium Isotopic Records in Pelagic Clay and Its Implications for Mineral Flux Estimates

Abstract

Chemical stratigraphy is useful for dating deep sea sediments which sometimes lack radiometric or biostratigraphic constraints. Oxic pelagic clay contains Fe-Mn oxyhydroxides that can retain seawater 187Os/188Os values, and its age can be estimated by fitting the isotopic ratios to the seawater 187Os/188Os curve. On the other hand, the stability of Fe-Mn oxyhydroxides is sensitive to redox change, and it is not clear whether the original 187Os/188Os values are always preserved in sediments. However, due to the lack of independent age constraints, the reliability of 187Os/188Os ages of pelagic clay have never been tested. Here we report inconsistency between magnetostratigraphic and 187Os/188Os ages in pelagic clay around Minamitorishima Island. In a ~ 5 m thick interval, previous studies correlated 187Os/188Os data to a brief (< 2 million years) isotopic excursion in the late Eocene. Paleomagnetic measurements revealed at least 12 polarity zones in the interval, indicating a > 3.3–7.4 million years duration. Quartz and feldspars content showed that while the paleomagnetic chronology gives reasonable eolian flux estimates, the 187Os/188Os chronology leads unrealistically high values. These results suggest that the low 187Os/188Os signal has diffused from an original thin layer to the current ~ 5 m interval, causing an underestimate of the deposition duration. The preservation of the polarity patterns indicates that a mechanical mixing such as bioturbation cannot be the main process for the diffusion, so diagenetic re-distribution of Fe-Mn oxyhydroxides and associated Os may be responsible. The paleomagnetic chronology presented here also demands reconsiderations of the timing, accumulation rate, and origins of the high content of rare-earth elements and yttrium in pelagic clay around Minamitorishima Island.