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 187 Os/ 188 Os values, and its age can be estimated by fitting the isotopic ratios to the seawater 187 Os/ 188 Os curve. On the other hand, the stability of Fe–Mn oxyhydroxides is sensitive to redox change, and it is not clear whether the original 187 Os/ 188 Os values are always preserved in sediments. However, due to the lack of independent age constraints, the reliability of 187 Os/ 188 Os ages of pelagic clay has never been tested. Here we report inconsistency between magnetostratigraphic and 187 Os/ 188 Os ages in pelagic clay around Minamitorishima Island. In a ~ 5-m-thick interval, previous studies correlated 187 Os/ 188 Os data to a brief (< 1 million years) isotopic excursion in the late Eocene. Paleomagnetic measurements revealed at least 12 polarity zones in the interval, indicating a > 2.9–6.9 million years duration. Quartz and feldspars content showed that while the paleomagnetic chronology gives reasonable eolian flux estimates, the 187 Os/ 188 Os chronology leads to unrealistically high values. These results suggest that the low 187 Os/ 188 Os 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 redistribution 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.

中文翻译：

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远洋粘土中锇同位素记录沉积后变形的磁地层证据及其对矿物通量估计的影响

化学地层学可用于测定深海沉积物的年代，因为深海沉积物有时缺乏辐射或生物地层学限制。含氧远洋粘土含有 Fe-Mn 羟基氧化物，可以保留海水 187 Os/ 188 Os 值，其年龄可以通过将同位素比与海水 187 Os/ 188 Os 曲线拟合来估计。另一方面，Fe-Mn羟基氧化物的稳定性对氧化还原变化敏感，沉积物中是否始终保留原始的187 Os/188 Os值尚不清楚。然而，由于缺乏独立的年龄约束，远洋粘土187 Os/188 Os年龄的可靠性从未被检验过。在这里，我们报告了南鸟岛周围远洋粘土中磁地层年龄和 187 Os/188 Os 年龄之间的不一致。在大约 5 米厚的间隔中，之前的研究将 187 Os/188 Os 数据与始新世晚期的一次简短（< 100 万年）同位素偏移相关联。古地磁测量显示该区间内至少有 12 个极性带，表明持续时间超过 2.9-690 万年。石英和长石含量表明，虽然古地磁年代学给出了合理的风成通量估计，但 187 Os/188 Os 年代学导致不切实际的高值。这些结果表明，低 187 Os/188 Os 信号已从原始薄层扩散到当前 ~ 5 米间隔，导致对沉积持续时间的低估。极性模式的保留表明生物扰动等机械混合不能成为扩散的主要过程，因此 Fe-Mn 羟基氧化物和相关 Os 的成岩重新分布可能是原因。