Study On Lower Triassic Stratigraphy Of High Resolution Geochemistry For Chaohu Section, Anhui Province:Implication For Paleo-environment

The Latest Permian Mass Extinction, the severest biotic crisis of Phanerozoic, killed over92%of the global species,68%of genera and even90%～100%in South China. Following this event was delayed biological recovery and prolonged reconstruction of paleoenvironment during the Early Triassic period. Researches on the oceanic environment show that the ocean was in a particular abnormally situation, which may cause the mass extinction in Late Permian and delayed biodiversity recovery in Early Triassic, and fluctuated for several times. The recovery during this period was much longer than any other time in the Phanerozoic according to the accurate chronology which took nearly5Ma. As the transition of the two ecosystems in the geological history, Lower Triassic is the key to understand the evolution process of life and its environment straddling the Paleozoic and Mesozoic. Meanwhile, it is important for the exploring the origin and early evolution of the present biodiversity and ecosystem.Since the Chaohu Section was proposed as the candidate of the Global Stratotype Section and Point of the Induan-Olenekian boundary, the Lower Triassic of Chaohu has been extensively studied. Based on the studies on the lithostratigraphy, biostratigraphy, magnetostratigraphy, isotope stratigraphy and cyclostratigraphy, we have detailed initial data for this work.The carbon and oxygen isotopes of carbonate have already become normal measurements combined the fossil records and sedimentary characters, as they associated well with the mass extinction event and extreme environment in Early Triassic. However, other geochemical proxies are barely used for this period until now. In order to search for new method, we measured the trace elements of conodont in situ taking advantaged of laser ablation inductively coupled plasma mass spectroscopy (LA-ICP-MS), and compared these with the trace elements of surrounding rocks that was tested by ICP-MS, and then reconstructed the paleo-environment and chemistry of the seawater at that time. It’s the first time for analysis of the bio-shell in situ in our country, which is a new exploring for the traditional paleontology and stratigraphy.Comparative analyses of surrounding rocks, the different species and the various histology, show the different compositions among them, which may be attributed to the particular ultrastructure of the species and demonstrate the more sensitive potential than the carbonate with respect to the paleoenvironment. There are two possible causes:First, fresh carbonate rocks without calcite veins are chosen to analyze, while conodonts are distinguished by CAI (Color Alteration Index) that can judge the thermal metamorphism during the diagenesis. The original signals of sedimentary rocks are overprinted by the diagenesis soon after its formation, including two processes:cementation and dissolution-recrystallization, which would change the initial compositions. Second, rocks are measured by high pressure acid dissolution-isotopic dilution method, which mean that the values are the comprehensive results after its formation. During that time, every diagenesis or hydrothermal process will overprint the original chemistry composition. However, the trace elements of conodont can avoid the contaminated tissues according to the in situ microanalysis.Based on the trace element compositions of conodont, we conclude that:①The fluctuation of the total REE(∑REE) around the Induan-Olenekian boundary demonstrated that enhanced weathering process, intensified soil erosion, frequently volcanic activities and so on, which resulted in the delayed recovery and prolonged environment reconstruction all together.②The Eu/Eu*fluctuation may reflect the accession of the deep vulcanian eruption matters into the ocean, which were absorbed and well preserved by conodont. But such large swings are not observed around the Smithian-Spathian boundary, because the same volcanism haven’t been reported yet.③The Th/La curve exhibited no obvious variation in the total section, although a maximum value can be observed just below the SSB followed by a rapid decrease, which may reflect a eustatic sea level change around the boundary.④The cerium anomaly demonstrated the repeated variation of the oxidation-reducing conditions. The oxia environment and low concentration of trace element below the boundary was beneficial to recovery, yet the suboxia or anoxia condition shown in the early Olenekian could be harmful.As a new potential tool for the research on oceanic paleoenviroment, the trace element microanalysis in situ of conodont species combined with the biostratigraphy and isotope stratigraphy, demonstrated the inner relationship between the Paleozoic mass extinction, the following delayed recover and the chemistry changes of the contemporaneous seawater.