Platinum-group elements (PGE) and rhenium in marine sediments across the Cretaceous–Tertiary boundary: constraints on Re-PGE transport in the marine environment
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.
Section snippets
Pelagic sites
We examined the distribution of the PGEs (Pt, Pd, Ir, Ru, Os) and Re at selected stratigraphic sections from KTB sites (Fig. 1). We analyzed a pelagic sediment core from the South Pacific (DSDP 596) at closely spaced intervals, including the KTB, which was previously identified at 20.10 m below the sediment–seawater interface (mbsf) on the basis of the highest Ir concentration peak (Zhou et al., 1991). The Ir anomaly in this core was found to span ∼1.8 m. The upper 40 m of cores at Site 596
Methods
Although several PGE studies have been conducted on the above KTB sites, we sought to improve on these studies by analyzing Pt, Pd, Ir, Ru, Os, and Re on the same aliquot of each sample and by conducting a PGE traverse across the KTB in DSDP 596. Sample powders were loaded in chilled, thick-walled borosilicate glass vessels (Carius Tubes; Shirey and Walker, 1995) along with a mixed Re-PGE tracer solution (191Ir, 198Pt, 105Pd, 99Ru, 190Os, 185Re) and 3 mL of aqua regia (glass vessels were
DSDP 596
Dry weight concentrations for DSDP 596 are shown in Table 1 located at 20.09 mbsf (Fig. 2). Re, Pt, Ir, Os, and Ru are elevated at the KTB (Fig. 2), with the highest levels of these elements occurring in sample 3-4 49-50, located at 20.09 mbsf. Like Zhou et al. (1991), we assign the KTB to the position of the highest Ir concentrations. Pt, Ir, Os, and Ru levels decrease monotonically both up and down section from the peak, producing roughly symmetric concentration profiles over a width of ∼1.8
Discussion
We first discuss the origin of the broad (∼1.8 m) Re, Pt, Ir, Os, and Ru anomaly at DSDP 596 (Fig. 2). This depth interval corresponds to an apparent deposition time of ∼6 Ma (based on a constant sedimentation rate), which is much too long to be reconciled with the fallout of a single impact event and therefore requires postdepositional Re-PGE redistribution in the stratigraphic column. We will also discuss the origin of the high degree of Re-PGE fractionation in the two pelagic sites compared
Conclusions
The nature of Re-PGE transport in the marine environment was investigated by using marine sediments at and across the KTB at two hemipelagic and two pelagic sites. A traverse across the KTB in a 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 1.8-m interval containing the Re-PGE anomaly cannot be reconciled with the fallout of a single impactor with chondritic
Acknowledgements
Diane McDaniel, Thomas Meisel, associate editor R. Walker, and an anonymous reviewer are thanked for reviews. Discussions with James Chen, Jerome Gaillardet, Damien LeMarchand, Graham Pearson, and Bernhard Peucker-Ehrenbrink are appreciated. Samples from DSDP 465 and DSDP 596 were supplied by Ocean Drilling Program, which is curated with support from the National Science Foundation. Samples SK10 and SM503 were collected and supplied by J. Smit. This study was supported by DOE DF FG03-88ER13851.
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