Geochemical studies of the Cretaceous-Tertiary boundary in ODP Holes 113-689B and 113-690C

In a study of ODP Hole 689B no iridium (Ir) anomaly was found in Sections 1 through 6 of Core 25X or in Core 26X from the top down to section 2, 3-12 cm. The background Ir abundance averaged 11 parts per trillion (ppt) and a clay-enriched region had nearly the same average, 26 ± 12 ppt. If the Cretaceous-Tertiary (K-T) contact is in the region studied, then sedimentation was not continuous, and the K-T boundary was probably either not deposited or it was eroded away. In a study of Cores 15X and 16X of ODP Hole 690C, an iridium peak with a maximum abundance of 1566 ± 222 ppt was found in Section 4 of Core 15X at 39-40 cm with a half-width of 6.6 cm. Background abundances were ~15 ppt and distinctly higher Ir abundances were observed from 119 cm below to 72 cm above the main peak. The Ir distribution below the main peak is attributed to bioturbation by organisms with burrows extending at least 0.4 m. The Ir distribution above the main peak may be due to the same cause but other explanations may be significant. There are variable enrichments of clay in the mainly CaCO3 sediment of Core 15X, and the stratigraphically lowest part of the most abundant clay deposits is found (within 2 cm) in the same position as the main Ir peak. The clay deposit, which is estimated to be about 50% of the sediment, extends upward ~19 cm and then slowly decreases to a background level of 10% over 1 m. The degree of homogeneity of the clay-rich interval suggests it was not due to episodic volcanism but may have been due to a decrease of the CaCO3 deposition rate which was possibly triggered by the impact of a large asteroid or comet on the Earth.

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.