[M-GI28] Drilling Earth Science

[MGI28-03] IODP Expedition 369: Osmium isotope stratigraphy and platinum group elements composition at the Cretaceous-Paleogene boundary on the Mentelle Basin off the southwest coast of Australia

*Hayu Ota¹, Junichiro Kuroda¹, Maria Luisa Tejada², Akira Ishikawa³, Katsuhiko Suzuki², Naohiko Ohkouchi² (1.Department of Ocean Floor Geoscience, Atmosphere and Ocean Research Institute, the University of Tokyo, 2.Japan Agency for Marine-Earth Science and Technology, 3.Department of Earth and Planetary Sciences, Tokyo institute of Technology)

Keywords:osmium isotope, K-Pg boundary, Mentelle Basin, IODP Exp.369

International Ocean Discovery Program (IODP) Exp. 369 was conducted in 2017 on the Great Australian Bight and Mentelle Basin off the south coast of Australia. This program aimed to investigate Cretaceous paleoclimate and palaeoceanographic change on high-latitude Southern Hemisphere, and igneous activity associated with Mesozoic continental breakup of the Eastern Gondwanan. In this study, we measured platinum group element concentrations and osmium isotope ratio of core sample from Site U1514 in the Mentelle Basin to identify the stratigraphic interval spanning the Cretaceous-Paleogene (K-Pg) boundary.
At Site U1514, a clear peak of Ru, Os and Ir concentrations was identified at the top of Section U1514C-23R-2 (393.52 m depth below seafloor). In addition, a clear drop of ¹⁸⁷Os/¹⁸⁸Os values to ~0.2 was also identified at the same stratigraphic level. This level corresponds to the biostratigraphic K-Pg boundary based on calcareous nannofossil assemblages. The minimum value of ¹⁸⁷Os/¹⁸⁷Os of ~0.2 at the K-Pg boundary is consistent with those of the K-Pg boundary in other pelagic sites. Platinum group element compositions of U1514C-23R-2 show a flat pattern when normalized by CI chondrite composition, indicating a high contribution of extraterrestrial material. These lines of evidence strongly suggest that the impact ejecta was preserved at the K-Pg boundary in the Mentelle Basin. A gradual decrease in ¹⁸⁷Os/¹⁸⁸Os values was observed in the stratigraphic levels below the K-Pg boundary, indicating an increase in the contribution of mantle-derived materials, which may reflect the formation of the Deccan Trap or igneous activity around the southeast Indian Ocean.