Concentrations and accumulation rates of platinum-group elements and rhenium in Pacific and Atlantic sediments, DSDP and ODP data

The nature of Re-platinum-group element (PGE; Pt, Pd, Ir, Os, Ru) transport in the marine environment was investigated by means of marine sediments at and across the Cretaceous-Tertiary boundary (KTB) at two hemipelagic sites in Europe and two pelagic sites in the North and South Pacific. A traverse across the KTB in the South Pacific pelagic clay core found elevated levels of Re, Pt, Ir, Os, and Ru, each of which is approximately symmetrically distributed over a distance of ~1.8 m across the KTB. The Re-PGE abundance patterns are fractionated from chondritic relative abundances: Ru, Pt, Pd, and Re contents are slightly subchondritic relative to Ir, and Os is depleted by ~95% relative to chondritic Ir proportions. A similar depletion in Os (~90%) was found in a sample of the pelagic KTB in the North Pacific, but it is enriched in Ru, Pt, Pd, and Re relative to Ir. The two hemipelagic KTB clays have near-chondritic abundance patterns. The ~1.8-m-wide Re-PGE peak in the pelagic South Pacific section cannot be reconciled with the fallout of a single impactor, indicating that postdepositional redistribution has occurred. The elemental profiles appear to fit diffusion profiles, although bioturbation could have also played a role. If diffusion had occurred over ~65 Ma, the effective diffusivities are ~10**?13 cm**2/s, much smaller than that of soluble cations in pore waters (~10**?6 cm**2/s). The coupling of Re and the PGEs during redistribution indicates that postdepositional processes did not significantly fractionate their relative abundances. If redistribution was caused by diffusion, then the effective diffusivities are the same. Fractionation of Os from Ir during the KTB interval must therefore have occurred during aqueous transport in the marine environment. Distinctly subchondritic Os/Ir ratios throughout the Cenozoic in the South Pacific core further suggest that fractionation of Os from Ir in the marine environment is a general process throughout geologic time because most of the inputs of Os and Ir into the ocean have Os/Ir ratios >/=1. Mass balance calculations show that Os and Re burial fluxes in pelagic sediments account for only a small fraction of the riverine Os (<10%) and Re (<0.1%) inputs into the oceans. In contrast, burial of Ir in pelagic sediments is similar to the riverine Ir input, indicating that pelagic sediments are a much larger repository for Ir than for Os and Re. If all of the missing Os and Re is assumed to reside in anoxic sediments in oceanic margins, the calculated burial fluxes in anoxic sediments are similar to observed burial fluxes. However, putting all of the missing Os and Re into estuarine sediments would require high concentrations to balance the riverine input and would also fail to explain the depletion of Os at pelagic KTB sites, where at most ~25% of the K-T impactor's Os could have passed through estuaries. If Os is preferentially sequestered in anoxic marine environments, it follows that the Os/Ir ratio of pelagic sediments should be sensitive to changes in the rates of anoxic sediment deposition. There is thus a clear fractionation of Os and Re from Ir in precipitation out of sea water in pelagic sections. Accordingly, it is inferred here that Re and Os are removed from sea water in anoxic marine depositional regimes.本研究通过欧洲两个半远洋沉积位点以及北太平洋、南太平洋各一个远洋沉积位点的白垩纪-古近纪界线（Cretaceous-Tertiary boundary, KTB）及其上下的海洋沉积物，探究了铼-铂族元素（Re-Platinum-Group Element, Re-PGE；Pt、Pd、Ir、Os、Ru）在海洋环境中的迁移特性。对南太平洋远洋黏土岩心跨越KTB的剖面进行分析后发现，铼（Re）、铂（Pt）、铱（Ir）、锇（Os）与钌（Ru）的含量均出现升高，且在KTB上下约1.8米的范围内呈近似对称分布。Re-PGE的丰度模式与球粒陨石（chondrite）的相对丰度存在分馏：相较于铱，钌、铂、钯与铼的含量略低于球粒陨石值，而锇相较于球粒陨石铱比例的亏损程度约达95%。北太平洋远洋KTB沉积物样品中同样观测到约90%的锇亏损，但相较于铱，该样品中钌、铂、钯与铼的含量均出现富集。欧洲的两个半远洋KTB黏土则呈现接近球粒陨石的丰度模式。南太平洋远洋剖面中宽约1.8米的Re-PGE含量峰值无法通过单一撞击体的溅射沉降来解释，这表明沉积后再分配过程确实发生。尽管生物扰动可能也起到了一定作用，但元素分布剖面与扩散剖面的拟合度较高。若扩散过程持续约6500万年，其有效扩散系数约为10^-13 cm²/s，远低于孔隙水中可溶性阳离子的扩散系数（约10^-6 cm²/s）。再分配过程中铼与铂族元素的耦合性表明，沉积后过程并未显著分馏它们的相对丰度。若再分配由扩散作用主导，则各元素的有效扩散系数应保持一致。因此，KTB地层中锇与铱的分馏必然发生在海洋环境中的水相迁移阶段。南太平洋岩心新生代地层中普遍存在的亚球粒陨石Os/Ir比值进一步表明，海洋环境中锇与铱的分馏是地质时间尺度上的普遍过程——因为绝大多数输入海洋的锇与铱的Os/Ir比值均≥1。质量平衡计算结果显示，远洋沉积物中锇与铼的埋藏通量仅占河流输入海洋总锇（<10%）与总铼（<0.1%）的极小比例。与之形成对比的是，远洋沉积物中铱的埋藏通量与河流输入的铱通量相当，这表明远洋沉积物作为铱的储库，其规模远大于锇与铼的储库。若将所有缺失的锇与铼假设赋存于大洋边缘的缺氧沉积物中，则计算得到的缺氧沉积物埋藏通量与观测值相符。但若将所有缺失的锇与铼都归于河口沉积物，则需要极高的浓度才能平衡河流输入通量，同时也无法解释远洋KTB位点的锇亏损——因为最多仅有约25%的K-T撞击体锇能流经河口。若锇优先在缺氧海洋环境中被固存，则可以推断远洋沉积物的Os/Ir比值应随缺氧沉积速率的变化而发生敏感响应。由此可见，远洋剖面海水中沉淀的沉积物中，锇与铼相对于铱存在明确的分馏。据此，本研究推断在缺氧海洋沉积环境中，铼与锇会从海水中被移除。