[B-CG06] 地球史解読：冥王代から現代まで

[BCG06-P11] イタリア南部上部三畳系半遠洋性堆積物におけるロシュシュアールクレーター起源イジェクタ層の発見

*大森 翔太郎¹、佐藤 峰南¹、尾上 哲治¹、Rigo Manuel² (1.九州大学、2.パドバ大学)

キーワード：天体衝突、後期三畳紀、放散虫

The Late Triassic is characterized by stepwise biotic turnover, and the onset of declining biodiversity extinction is close to the Norian/Rhaetian boundary (NRB). Faunal turnovers of marine taxa, such as ammonoids, conodonts, and radiolarians, occurred around the NRB (e.g., Tanner et al., 2004; Lucas and Tanner, 2018), and the negative organic carbon-isotope excursions at the NRB are recorded across the Panthalassa Ocean to both sides of the Pangaea continent (Rigo et al., 2020). Large igneous provinces and extraterrestrial impacts in the Late Triassic have been proposed as possible cause of biotic turnover events across the NRB (e.g., Zaffani et al., 2017; Rigo et al., 2020). Although the 23-km Rochechouart impact structure in France is marked around the NRB (206.92 ± 0.32 Ma; Cohen et al., 2017), deposits containing its ejecta are still unknown. Here, we report that the Upper Norian hemipelagic sediments in the Sasso di Castalda section, southern Italy, contain the concentrated siderophile and chalcophile elements, microspherules, and magnetic particles with dendritic structure that are characteristic of impact ejecta. The signals of the extraterrestrial impact are recorded in the red shale, with a thickness of 1.5 cm. Analysis of radiolarian fossils reveals that the red shale correlates with the latest Norian. Enrichment in siderophile and chalcophile elements (Fe, Ni, Cu, and Zn) are identified based on X-ray fluorescence analysis. The anomalously high abundances of Ni are defined by concentrations of up to 300 ppm, an order of magnitude higher than background level. The red shale contains microspherules, ranging in size from 20 to 30 μm. Geochemical mapping for using energy dispersive X-ray spectroscopy reveals a high concentration of Fe and Co. The red shale also contains magnetic particles with dendritic structure enriched in K. The textures of the dendrites are similar to those of Ni-rich spinels from at the Cretaceous/Paleogene boundary. The discovery of microspherules associated with the siderophile elements anomaly in the red shale suggests an important sedimentary record of an extraterrestrial impact in the Late Triassic. We propose that the Rochechouart impact structure would appear to be related to the deposition of the red shale, based on an age for the crater of ~206.92 Ma. Further research is needed to confirm the source of the latest Norian red shale in Italy, and to constrain the origin of the microspherules.