[Objective]The paleoclimate and environment have changed significantly across the Norian-Rhaetian transition(NRT,Late Triassic),as demonstrated by carbon-isotope fluctuations and biological extinction events.However,the causes of climate perturbations and biotic crises during the NRT remain controversial.It is believed that the eruption of contemporaneous volcanisms(e.g.,the Angayucham Large Igneous Province)was the main cause of paleoenvironmental changes during the NRT.The large-scale volcanic activity released a large amount of greenhouse gases,which resulted in global temperature rise,carbon-isotope perturbation,and biological crises during the NRT.At present,the majority of NRT studies have focused on the shallow marine strata in the Tethys region,but knowledge on the changes in terrestrial paleoclimate,paleoenvironment,and their driving mechanism during the NRT is extremely limited.Studies have shown that terrestrial strata can faithfully record paleoclimatic and paleo-environmental changes during geological events,such as the end-Permian mass extinction.However,previ-ous studies mainly focused on paleobotany,sedimentology,organic carbon isotopes,and wildfires during the NRT in the Sichuan Basin,but lacked element geochemical evidence,limiting the accurate understanding of the climatic changes in this time interval and the comparison between different research methods.[Methods]To tackle this scientific question,we examined the Norian-Rhaetian section(Xujiahe section)located 4.5 km NE of Guangyuan city,northwest Sichuan Basin.Thirty-four samples were collected at a resolution of 10 cm to 2 m in the Xujia-he section for major and trace element compositions.The surface dust and weathered portions of samples were removed with a rasper and then washed with deionized water.After 8 hours of oven drying at 50 ℃,samples were ground into powder using agate mortars.Sample preparations were completed in the School of Materials and Chemistry&Chemical Engineering,Chengdu University of Technology.Analysis of major and trace elements in samples was completed at the Institute of Earth Environment,Chinese Academy of Sciences,Xi'an.A glass bead was created by fusing 0.6 g of the powdered sample with 6 g of dry lithium tetraborate(Li2B4O7)for 5 minutes at 1000 ℃.The glass bead was further scanned by an X-ray fluorescence spectrometer(WD-XRF;PANalytical,Ea Almelo,The Nether-lands).The analytical accuracy was better than 2%.[Results and Discussions]The analyzed samples have high Chemical Index of Alteration(CIA)values and Rb/Sr at 105.5,110.2-122.5,119,123.4-123.45,and 137.7 m in the Xujiahe section,and low CIA values and Rb/Sr at 107,109.5,115.5-116.5,121.5,and 127-135m.The upper-most part of Upper Norian successions(105.5-129.5m)have CIA values ranging from 59 to 82,with a mean of 73;Rb/Sr values ranging from 0.2 to 2.6,with a mean of 1.2;and R values ranging from 3.8 to 16.9,with a mean of 8.2.For the Norian-Rhaetian boundary interval(NRB,129.5-135 m),CIA values range from 59 to 63,with a mean of 60;Rb/Sr values range from 0.5 to 0.6,with a mean of 0.5;and R values range from 13.1 to 13.9,with a mean of 13.5.[Conclusions]Results show that the climate fluctuated frequently during the NRT of Xujiahe section in the Sichuan Basin.The Late Norian was dominated by warm and humid climate,which was interrupted by a short-term cooling event close to the NRB.The prevailing mega-monsoon in Pangaea during the Late Triassic may be the main trigger for the frequent climate change and NRB cooling events in the Xujiahe section,but the influence of volcanic activity and wildfire events on the paleoclimate system cannot be eliminated in this time period.To determine the precise timing of volcanic eruptions and wildfires during the NRT and how they contributed to climate change,more research is needed.
Late TriassicXujiahe Formationpaleoclimateweathering indexSichuan Basin
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