COMPARISON OF OS ISOTOPE AND BE-10 AGES OF HYDROGENOUS FERROMANGANESE CRUSTS

Hydrogenous ferromanganese (Fe-Mn) crusts have been used as archives for monitoring long-term evolution of seawater chemistry during the Cenozoic (e.g., Frank, et al., 2002; Ling et al., 2005). Those paleoceanographic studies often apply Os isotope (¹⁸⁷Os/¹⁸⁸Os) and Be-10 ages to determine depositional ages and growth rates of the crusts (e.g., Nielsen et al., 2009). However, reliabilities of these methods are yet to be evaluated. In the present study, we report Os and Be isotope depth profiles of three Fe-Mn crusts from the Western Pacific Ocean (see also Goto et al. (2014) and Nishi et al. (in press)).

The ¹⁸⁷Os/¹⁸⁸Os depth profiles for all three samples show a small negative anomaly as low as ~0.7. Our results are consistent with those reported by previous studies that analyzed Os isotope of Fe-Mn crusts from the Central Pacific and Atlantic Oceans (Klemm et al., 2008; Goto et al., in press), although such an anomaly has not been reported from other pelagic sediments (e.g., Peucker-Ehrenbrink & Ravizza, 2016). The anomaly has been interpreted to reflect an increase supply of unradiogenic Os due to eruption and subsequent weathering of the Columbia River Flood basalt at ~16 Ma (Klemm et al. 2008). Hence, the observed anomaly would be as old as ~16 Ma. However, our new Be-10 ages of the anomaly are always younger than ~16 Ma (i.e., 8, 8, 10 Ma). The same thing seems true for the samples analyzed by Klemm et al. (2008): i.e., ~11 Ma based on extrapolation of Be-10 ages. Note that all the Be-10 ages are (re)calculated using a new decay constant reported by Korschinek et al., 2010). The observed discrepancy among the Be-10 ages for the similar ¹⁸⁷Os/¹⁸⁸Os anomalies could be an artifact due to low temporal resolution record of currently available seawater Os isotope curve for age calibration. Alternatively, because addition of Be from seawater to Fe-Mn crusts after deposition can result in higher cosmogenic ¹⁰Be abundance, the young Be-10 ages may reflect a non-closed system behavior of Be in Fe-Mn crusts. These findings suggest that either or both of Be-10 and Os isotope chronologies do not necessarily provide accurate depositional ages of Fe-Mn crusts. Detailed reconstructions of seawater Os isotope using pelagic sediments is required to further discuss these possibilities.