[M-IS13] 地質学のいま

[MIS13-03] 美濃帯の下部三畳系−下部ジュラ系遠洋深海堆積岩を用いた古環境解析

*曽田 勝仁¹、尾上 哲治²、安川 和孝³ (1.高知大学大学院教育研究部自然科学系理工学部門、2.九州大学 大学院理学研究院 地球惑星科学部門、3.東京大学大学院工学系研究科エネルギー・資源フロンティアセンター)

The Triassic–Jurassic periods of the Mesozoic are characterized by two mass extinction events in the Phanerozoic. The coeval pelagic deep-sea deposits in the Jurassic accretionary complexes of Japan are optimal archives as the geologic timescales and the global environmental records in a pelagic setting of the Panthalassic Ocean. The paleoenvironmental analyses are important to explore the evolutionary origin and formation processes of the modern-type marine biota in a pelagic realm. However, recent integrated stratigraphic studies have indicated stratigraphic gaps in the Lower-Middle Triassic and the Triassic-Jurassic boundaries of Japan. Additionally, the paleoenvironmental changes during these periods have been mainly reconstructed from the Tethysian, Boreal, and Pangean sections of China, Europe, and North America. This study shows reestablishments of the Lower Triassic–Lower Jurassic stratigraphy in the deep-sea sedimentary sequence of Japan and paleoenvironmental reconstruction based on geochemical and statistical analyses. Geochemical and statistical studies were conducted on the Lower Triassic–Lower Jurassic stratigraphic sections in the Kamiaso Unit of the Mino Belt, central Japan. To compensate stratigraphic gaps in the previous Lower Triassic–Lower Jurassic sections, the continuous stratigraphic sections across the Lower-Middle Triassic and the Upper Triassic-Lower Jurassic boundaries were defined in the Kamiaso and Inuyama areas, respectively. Statistical analyses were performed on compositional dataset of the deposits in the pelagic deep-sea realms of the Panthalassic Ocean. Extracted geochemical independent components explained most of the total variance of the analyzed elements. From these ICs, we proposes that combinations of the volcanism-induced global warming and the enhanced continental weathering controlled the oceanic redox conditions and the seawater-elemental inventories. These environmental changes triggered the mass extinction of marine organisms, which led to the subsequent biotic prosperity and the coeval enrichment of rare earth elements in the pelagic deep-sea realms of the Panthalassic Ocean.