Platinum-group elements (PGE) and rhenium in marine sediments across the Cretaceous–Tertiary boundary: constraints on Re-PGE transport in the marine environment

Abstract

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 cm²/s, much smaller than that of soluble cations in pore waters (∼10⁻⁶ cm²/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.

Introduction

The platinum-group elements (PGE) are Pt, Pd, Ir, Os, Rh, and Ru. Along with Re, these elements are gaining importance in studies of weathering, diagenesis, and paleoclimate. For example, the Re-Os radiogenic decay system has been used to track secular changes in the relative proportions of continental, hydrothermal, and extraterrestrial input of Os into the oceans (e.g., Peucker-Ehrenbrink 1996, Peucker-Ehrenbrink and Ravizza 2000. In addition, the variable speciations of Re and the PGEs make them potentially useful indicators of ocean oxidation states Anbar et al 1996, Crusius et al 1996, Morford and Emerson 1999. However, interpreting Os isotopic ratios and PGE elemental abundance data is complicated by an incomplete understanding of the behaviors of Re-PGE-Au during weathering, diagenesis, and transport in the marine environment. Furthermore, the full sources of Os (including cosmic dust infall and mantle additions from spreading centers) and the sinks for Os are not readily quantifiable (e.g., Sharma and Wasserburg 1997, Sharma et al 1999, Sharma et al 2000.

Sediments from the Cretaceous–Tertiary boundary (KTB) illustrate this point. The KTB coincides with a large Ir anomaly, which is now widely agreed to represent the fallout of a large meteorite impact. This leads to a simple prediction that the Ir anomaly should be manifested as a spike in the stratigraphic record corresponding to the time of impact and that the PGE abundance pattern of the spike should match that of the impactor, which, on the basis of the uniform relative abundances of PGEs in meteorites (Larimer and Wasson, 1988) and the Cr isotopic composition of the KTB sediments (Shukolyukov and Lugmair, 1998), was probably chondritic. However, a number of studies have shown that the Ir anomaly is often spread out over substantial distances in the sedimentary section that includes the KTB Kastner et al 1984, Kyte et al 1985, Kyte et al 1993, Kyte et al 1996, Robin et al 1991, Zhou et al 1991. In some cases, this distribution corresponds to several million years of deposition (Zhou et al., 1991), which is too long to be reconciled with a single impact event. There are very few KTB studies that consider most of the PGEs. The available data show that PGE abundance patterns at various KTB sites can depart significantly from chondritic values (e.g., Kyte et al., 1985). This suggests that the PGE fallout was fractionated from its initially chondritic proportions and redistributed before, during, or after deposition, or some combination of these. The goal of the present study is to use the distribution of Re-PGEs around the KTB to study the processes that fractionate and transport Re and the PGEs in the marine environment. The results of the KTB study will be combined with an investigation of Re-PGE content in pelagic sediments away from the KTB to draw more general inferences about the mechanisms of Re and PGE transport in the marine environment.