Paleoenvironment fluctuations and conodont size variations in the Baoshan block of eastern Tethys: implications for the late Norian warming event (Late Triassic)

Abstract

The Norian Stage of the Late Triassic represents a significant interval in geological history marked by environmental and biological evolutions. One notable event during this time is the late Norian warming event (W3), occurring in the late Norian. Globally recorded, the warming event is associated with carbon cycle perturbations, temperature increases, changes in biodiversity, and enhanced weathering intensity. Previous studies have focused on carbon isotopes, sedimentology, and biostratigraphy in the Baoshan block. However, information on environmental changes in this area during the Norian Stage remains limited. In this study, we utilized redox conditions indexes (V/Cr, U/Th, Mo_EF/U_EF, and Ce/Ce^⁎) and paleo-productivity proxies (P_org, Ba_xs) to reconstruct the ocean paleoenvironment in the HYB section of the Baoshan block. Two stages of reducing conditions and paleo-productivity fluctuations were identified during the late Norian warming event interval. In addition, the size of 1606 well-preserved adult conodont pectiniform elements from the HYB section was measured. The mean length of conodonts increased before the Alaunian-Sevatian boundary and then decreased significantly after the boundary in the Sevatian. Conodont size variations are typically related to changes in living environments, but our study found no direct relation between changes in redox conditions, paleo-productivity, and variations in conodont size in the Baoshan block. It may be due to the predominant conodont fauna, Mockina, being surface water dwellers. These surface-dwelling conodonts were not influenced by changes in seafloor redox conditions in relatively deep water. The direct evidence is instead that the size variation of conodonts is more likely affected by temperature changes in Tethyan surface seawater during the late Norian warming event.