Iridium and Osmium as Tracers of Extraterrestrial Matter in Marine Sediments

Abstract

Platinum group elements, specifically iridium and osmium, are the most sensitive non-volatile elemental tracers of extraterrestrial (meteoritic) matter in marine sediments. The budget of these elements in sediments can be considered a mixture of extraterrestrial, eolian (i.e., wind-blown dust), and hydrogenous (i.e., seawater-derived) sources. Differences in the osmium isotopic composition between these three sources allow quantification of the amount of extraterrestrial osmium in sediments. Osmium isotope data for about 30 pelagic sediments from the Atlantic and Pacific oceans, spanning the past 80 Myr, yield an average annual flux of extraterrestrial matter of 30,000 ± 15,000 metric tons. The only clear exception is the large impact at the Cretaceous-Tertiary boundary. Temporal resolution of these flux estimates is limited to at least several thousand years by the need to obtain statistically representative samples. The scatter in the flux estimates is large and secular variations in the flux of less than a factor of about three cannot be excluded. However, samples from individual cores indicate that the flux of extraterrestrial matter has varied by less than a factor of two, if the flux is averaged over the scale of temporal resolution in marine sediment samples.