Program > By author > Shiohara Takuma

The Triassic climate is generally considered a period of arid or semi-arid conditions, and there are two humid climate events in the late Anisian (Pelsonian) and late Ladinian (Longobardian) of the Middle Triassic. Although the causes of these humid climate events are uncertain, these events are likely to have led to the diversification of major pelagic groups (e.g., radiolarians and conodonts) during the Middle Triassic. To investigate the response of radiolarians to the Middle Triassic humid climate events, we examined radiolarian biostratigraphy and obtained geochemical profiles from a Middle Triassic bedded chert sequence (Section O) in the Inuyama area, central Japan. Section O consists mainly of rhythmic brick-red bedded cherts with a thickness of 21 m that accumulated in a pelagic, open ocean setting within a low-latitude zone of the Panthalassa Ocean. A total of 65 chert samples were collected from Section O for biostratigraphic study. Our radiolarian biostratigraphy shows that the six Sugiyama's radiolarian subzones were recognized in the study section: the TR 2C (Triassocampe deweveri) to TR 5A (Capnuchosphaera) zones, which can be compared to the late Anisian to early Carnian. Based on our radiolarian biostratigraphic data, the radiolarian faunal change from Fassanian (early Ladinian) to Longobardian (late Ladinian) taxa occurred across the thick siliceous claystone bed (4 cm thick) in Section O at 10.5 m above the section base. A geochemical analysis indicates that no significant marine redox changes were observed in the study section. In contrast, changes in biogenic apatite productivity that originated from marine vertebrates (e.g., conodonts and fish) increased across the Fassanian/Longobardian boundary. Furthermore, the continental weathering proxies such as CIA (Chemical Index of Alteration) and the WIP (Weathering Index of Parker) suggest that chemical weathering of hinterland rocks intensified in the Longobardian. Our results suggest that the humid events in the Logobardian may have triggered an increase in pelagic vertebrate productivity and the radiolarian faunal turnover in the pelagic realm of the Panthalassa Ocean.