Norian conodont biostratigraphy and carbon isotopic perturbations in the Lanping-Simao terrane, Eastern Tethy

Norian time interval is a crucial period in Earth’s evolutionary history, characterized by several organic carbon-isotope perturbations in the carbon cycle that are closely related to global environmental change and biotic turnover (Zaffani et al., 2017). Among them, the Norian “chaotic carbon episode” has been recently reported globally in North America, Italy, and Japan, from shallow to pelagic environments (Whiteside and Ward, 2011; Zaffani et al., 2017; Jin et al., 2022; Sato et al., 2023). However, the mechanisms and timing of these events are poorly understood, especially in the eastern Tethys. In this study, we have investigated the carbon isotope profiles and conodont assemblage changes from the middle to upper Norian bedded carbonate rock succession in the Sanhedong section, Southwest China, to provide new insights into the Norian carbon cycle perturbation and its impact on marine biodiversity. We thus collected several samples for conodont biostratigraphic investigations and organic and carbonate carbon isotope (δ13Corg, δ13Ccarb) and oxygen isotope data (δ18Ocarb) along the study section. We identified various conodont genera, including some typical Norian Arcyrogondolella, Norigondollela, Mockina, and Primatella, and established detailed conodont biostratigraphy that indicates a middle to upper Norian age for the Sanhedong section. Furthermore, we detected a significant negative carbon isotope excursion (NCIE) across the middle/upper Norian, which can be globally correlated with the NCIE events reported from North America, the central Panthalassa Ocean, and the Tethys Ocean. We have also observed changes in conodont diversity and abundance associated with the NCIE.Our study provides an accurate age and a high-resolution carbon isotope record for the Sanhedong section, revealing the global nature of the Norian carbon cycle perturbation and its impact on the biosphere in the eastern Tethys. Our preliminary results suggest that also the carbon isotope perturbation in Eastern Tethys may be associated to the middle-late Norian carbon-cycle perturbations, which were triggered by the emplacement of the Angayucham Large Igneous Province (Alaska). Further studies are needed to constrain the timing and magnitude of Norian volcanic activity.Keywords: Norian, conodont biostratigraphy, carbon isotope perturbation, Eastern Tethys  Reference:Jin, X., Du, Y., Bertinelli, A., Shi, Z., Preto, N., Zou, H., Ogg, J.G., Han, L., Wu, Q. and Rigo, M., 2022. Carbon-isotope excursions in the Norian stage (Upper Triassic) of the Baoshan terrane, western Yunnan, China. Journal of Asian Earth Sciences, 230, p.105215.Sato, H., Nozaki, T., Onoue, T., Ishikawa, A., Soda, K., Yasukawa, K., Kimura, J.I., Chang, Q., Kato, Y. and Rigo, M., 2023. Rhenium-osmium isotope evidence for the onset of volcanism in the central Panthalassa Ocean during the Norian “chaotic carbon episode”. Global and Planetary Change, 229, p.104239.Whiteside, J.H. and Ward, P.D., 2011. Ammonoid diversity and disparity track episodes of chaotic carbon cycling during the early Mesozoic. Geology, 39(2), pp.99-102.Zaffani, M., Agnini, C., Concheri, G., Godfrey, L., Katz, M., Maron, M. and Rigo, M., 2017. The Norian “chaotic carbon interval”: new clues from the δ13Corg record of the Lagonegro Basin (southern Italy). Geosphere, 13(4), pp.1133-1148.