The osmium isotopic composition change of Cenozoic sea water as inferred from a deep-sea core corrected for meteoritic contributions

A detailed study was made of the oceanic 187Os/ 186Os variation through the Cenozoic by using a hydrogen peroxide leaching procedure on a pelagic clay core from the North Pacific (Long Lines-44 GPC3). A 6% H 2O 2 solution was used. The range of 187Os/ 186Os obtained by this procedure was between 2 and 8.2. A milder leach (0.15% H 2O 2) in three out of four samples yielded a slightly higher 187Os/ 186Os value than the stronger leach implying that the stronger leach released some meteoritic Os with 187Os/ 186Os of 1. Using published 3He concentrations in GPC3 and cosmic dust 3He and Os fluxes to deep-sea sediments, the extraterrestrial Os concentration was estimated for each core segment and subtracted to yield “corrected” values of 187Os/ 186Os. The results based on the milder leaching protocol and the 3He correction protocol yield similar values. The “corrected” ocean water 187Os/ 186Os pattern for the past 25 million years is similar to that obtained by other workers and is compatible with other results for the entire Cenozoic. The variation with time in GPC3, uniquely, provides a statigraphic signature for the Paleogene.