Post-depositional mobility of platinum, iridium and rhenium in marine sediments

THE discovery of high concentrations of iridium in Cretaceous/Tertiary boundary sediments engendered the hypothesis¹ that a meteorite collided with the Earth 65 million years ago, coincident with the mass extinction that occurred at that time. Iridium spikes of various magnitudes have subsequently been reported at more than 10 other extinction horizons^2-11. It has been suggested, on the other hand, that geochemical processes might create or modify many of these spikes^{5, 11-4}, but a critical evaluation of these suggestions has been hindered by incomplete understanding of low-temperature iridium geochemistry. Other platinum-group elements (Ru, Rh, Pd, Re, Os, Pt, Au) are often found to be associated with Ir spikes, and inter-element ratios have been used to assess the cosmic or terrestrial nature of the enrichments^5,7,9,28,29; but the geochemical influences on these relative abundances are also poorly constrained. Here we describe analyses of recent abyssal marine sediments which allow us to characterize the behaviour of Pt, Re and Ir during early diagenesis. These elements are redistributed by changes in sedimentary redox conditions. Such changes can probably account for many of the small platinum-group-element spikes found in the geological record, and may render ambiguous attempts to interpret inter-element ratios.