High-resolution lithostratigraphy and organic carbon isotope stratigraphy of the Lower Triassic pelagic sequence in central Japan

After the severest mass extinction event in the Phanerozoic, biotic recovery from the extinction at the Permian-Triassic boundary required approximately 5 my, which covers the entire Early Triassic. It is important to obtain information on the superocean Panthalassa, which occupied most of the world ocean, to explore paleoenvironmental changes during the Early Triassic at the global scale. In order to establish the continuous lithostratigraphy of pelagic sediments in Panthalassa during the Early Triassic, high-resolution reconstruction of the Lower Triassic pelagic sequence in Japan was conducted for the first time based on detailed field mapping and lithostratigraphic correlation in the Inuyama area, central Japan. The reconstructed Early Triassic sequence is approximately 9.5 m thick, consists of five rock types, and is divided into eight lithological units. For the reconstructed continuous sequence, measurement of carbon isotopic composition of sedimentary organic matter (delta C-13(org)) was carried out. Stratigraphic variation of the delta C-13(org) value shows large-amplitude fluctuations between -34.4 and -21.0 parts per thousand throughout the sequence. In order to establish a higher resolution age model for the reconstructed Lower Triassic pelagic sequence, we correlated delta C-13(org) records in the Inuyama area with high-resolution isotopic profiles of carbonate carbon (delta C-13(carb)) from shallow-marine carbonate sequences in southern China based on the similarity in general variation patterns with age constraints by radiolarian and conodont biostratigraphy. The result provides a high-resolution time scale for the pelagic sequence of Panthalassa during the Smithian and Spathian. The age model suggests a drastic increase in sedimentation rate during the late Smithian, which should have been caused by the increase in terrigenous input to this site.