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% H₂O₂ solution was used. The range of ¹⁸⁷Os/¹⁸⁶Os obtained by this procedure was between 2 and 8.2. A milder leach (0.15% H₂O₂) 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.