Osmium geochemical behavior and global isotopic changes associated with the cretaceous-paleogene impact

Abstract

The Re-Os isotopic system is a powerful tool for investigating impact events because of osmium enrichment in chondrites relative to upper continental crust and large isotopic differences between these two end-members. Physical mixing models of chondrites and upper continental crust predict that the 187Os/188Os ratio of sediments found at terrestrial Cretaceous-Paleogene (K-Pg) impact horizons should be ≈0.13. This is thought to represent the Os isotopic composition of the ejecta plume fallout that results from the impact event. Results reported here do not support a chondritic Os composition for the ejecta plume but rather suggest a slightly elevated 187Os/188Os ratio of ≈0.14. This is consistent with previously reported data. This has significant implications for Os-based estimates of impactor size. These results require a bolide greater than three times more massive than previous Os based estimates. These estimates are based on impact induced global changes in the 187Os/188Os ratio of seawater. Profiles of 187Os/188Os of marine sediments across the K-Pg boundary exhibit an excursion from ~0.4 before the impact, to ~0.15-0.17 immediately following the impact, and then recovery back to pre-impact values of ~0.4. This excursion has been documented for many sites and is therefore thought to be homogeneous throughout global seawater. Results here suggest that the Os inventory of the Indian sector of the Southern Ocean was more unradiogenic than the rest of the global ocean for at least the first 3 myr of the Paleogene as 187Os/188Os ratios recover only to ~0.32. However, reworking of sediment with inherently low 187Os/188Os composition cannot be precluded as an alternative explanation at this time.