Discovery of evidence of the Chicxulub impact in East Asia

The Cretaceous-Paleogene boundary (K-Pg boundary) at 66 million years ago marks one of the most severe mass extinctions through Phanerozoic. A clear peak in abundances of platinum group elements (PGEs) such as iridium (Ir) and osmium (Os) 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. Even though the northwest Pacific is considered to have suffered almost no direct tsunami-like damage, a complete K-Pg boundary has not yet been discovered. Therefore, the discovery of the K-Pg boundary in this region is essential for understanding the environmental changes and ecosystems of the time in distal sites from the impact site, the Yucatan Peninsula in the Gulf of Mexico.

We conducted detailed field surveys and sampling in the Shiranuka Hills, Hokkaido, Japan. Our primary study area was a tributary of the Kawaruppu River, where analysis of magnetostratigraphy, biostratigraphy (foraminifera and calcareous nannofossils), zircon U-Pb dating have identified sedimentary layers from the Upper Cretaceous to the lowermost Paleogene (Takashima et al., 2024). In this study, we measured PGE concentrations and ¹⁸⁷Os/¹⁸⁸Os ratios to constrain the stratigraphic position of the K-Pg boundary. The ¹⁸⁷Os/¹⁸⁸Os ratio in the stratigraphic interval of the Upper Cretaceous was approximately 0.6, while that in the interval of the lower Paleogene was approximately 0.4. These values are consistent with those of seawater recorded in pelagic sedimentary rocks (Ravizza and Peucker-Ehrenbrink, 2003; Robinson et al., 2009). Notably, a distinct decrease in the ¹⁸⁷Os/¹⁸⁸Os ratio was observed in the stratigraphic level between the two intervals, reaching approximately 0.235. High Os concentration has also been confirmed at the same level. These results allow us to identify the K-Pg boundary at that level.

This section was deposited in a tectonically active continental margin, resulting in extremely high sedimentation rates. Our magnetostratigraphic and Os isotope stratigraphic data revealed that the sedimentation rate in this section was at least 40 cm/kyr during the Upper Cretaceous and 24 cm/kyr during the lowermost Paleogene. Consequently, it is expected to provide the highest-resolution record in the world of both terrestrial and marine paleoenvironmental changes across the K-Pg boundary.

[References]
Ravizza, G., & Peucker-Ehrenbrink, B., 2003, Chemostratigraphic evidence of Deccan volcanism from the marine osmium isotope record: Science, 302(5649), p. 1392-1395.
Robinson, N., Ravizza, G., Coccioni, R., Peucker-Ehrenbrink, B., and Norris, R., 2009, A high-resolution marine ¹⁸⁷Os/¹⁸⁸Os record for the late Maastrichtian: Distinguishing the chemical fingerprints of Deccan volcanism and the KP impact event: Earth and Planetary Science Letters, v. 281, p. 159-168.
Takashima, R., Ota, H., Kuroda, J., Schmitz, M., Hayashi, K., et al., 2024, Integrated stratigraphy and radiometric ages across the K-Pg boundary in Shiranuka Hill, Hokkaido, Japan: Abstract T16-O-12 presented at 131st Annual Meeting of the Geological Society of Japan, Yamagata University, 8-10 September.