[S-GL22] Frontier research on chronostratigraphic unit boundaries

[SGL22-05] Osmium isotope stratigraphy of the Cretaceous-Paleogene boundary in the Nemuro Group

*Hayu Ota¹, Junichiro Kuroda¹, Akira Ishikawa², Katsuhiko Suzuki³, Reishi Takashima⁴ (1.Department of Ocean Floor Geoscience, Atmosphere and Ocean Research Institute, the University of Tokyo, 2.Department of Earth and Planetary Sciences, Tokyo institute of Technology, 3.Submarine Resources Research Center, Japan Agency for Marine-Earth Science and Technology, 4.Tohoku University Museum, Tohoku University)

Keywords:osmium isotope, K-Pg boundary, the Nemuro Group

The Cretaceous-Paleogene boundary (K-Pg boundary) at 66 million years ago marks one of the most severe mass extinctions through Phanerozoic. Many fossil records show that up to 75% of species including non-avian dinosaurs became extinct at the K-Pg boundary. A clear peak in abundances of platinum group elements such as iridium (Ir) has been observed in clay beds at the K-Pg boundary around the world and is one of the most important pieces of evidence of a massive meteorite impact that could have triggered the mass extinction. A sharp decline in osmium isotope ratio (¹⁸⁷Os/¹⁸⁸Os) can also be used to detect an extraterrestrial impact.

The K-Pg boundary can be defined by a clear peak in abundances of iridium and osmium, and a sharp and short-term decline of Os isotope ratio. In Japan, upper Cretaceous and lower Paleogene sedimentary successions have been reported in the Katsuhira Formation of the Nemuro Group in Hokkaido (Saito et al. 1986), but none of sections have been studied in detail for platinum group elements. In this study, we investigated the K-Pg stratigraphy by investigating the Kawaruppu Formation that is adjacent to the Katsuhira Formation. We sampled at the Kawaruppu tributary especially at high resolution near the K-Pg boundary, and measured platinum group element concentration and Os isotope ratio at JAMSTEC. We will present the results and discuss the stratigraphy and Os isotope ratio excursion across the K-Pg boundary.

In this study, we analyzed platinum group element compositions and osmium isotope ratios at intervals around the K-Pg boundary using samples from the Kawaruppu Formation of the same group. Samples were taken at 20 locations along a 200 m outcrop along a stream tributary of the Kawaruppu River, and high-resolution sampling was conducted at stratigraphic levels inferred to be close to the K-Pg boundary. The ¹⁸⁷Os/¹⁸⁸Os ratios in the uppermost Cretaceous interval were approximately 0.6, which are consistent with the Maastrichtian values for pelagic sedimentary rocks reported in the previous studies. The the isotopic ratios of the lowest Paleogene interval were ~0.4, which are also consistent with the Paleogene values for pelagic sedimentary rocks. The K-Pg boundary is expected to be located at the boundary between these two intervals. In our presentation, we will provide new data from high-resolution sampling and discuss the variation of osmium isotope ratios across the K-Pg boundary.