The Paleoclimate Evolution Of The Early-Middle Triassic In The Jiyuan Basin Of The South Of North China Basin

While climate disasters tend to often occur,how will the future climate be changing remains a hot issue in scientific research today.The study of deep-time climate change is undoubtedly the key.The Triassic featured the typical greenhouse climate,so the study of the paleoclimatic evolution during the period can help identify the characteristic climate changes under the greenhouse conditions.To learn about the paleoclimate evolution in the Early–Middle Triassic,this paper takes the Heshanggou Formation and Ermaying Formation in the Jiyuan area in the south of North China Basin as the research objects and makes use of various analysis methods to investigate how the local paleoclimate change was characterized.These analysis methods embrace sedimentary petrology,isotope chronology,and geochemical methods.Then,we carry out the discussion about the influential factors of paleoclimate evolution and its impacts on the ecosystem.The research results are shown as follows:（1）The sedimentary facies analysis discloses the sedimentary facies sequence from the Heshanggou Formation to the middle of Ermaying formations:meandering stream,shore-shallow lake,fluvial-delta,meandering stream and fluvial-delta.The laser ablation inductively coupled plasma mass spectrometry（LA-ICP-MS）was used to analyze the U-Pb dating of tuff zircons newly found in the upper Heshanggou Formation,showing its weighted average age of 244.6±2.8 Ma,and the local rock deposits from the Anisian.（2）The major elements and microelement analyses suggest that the Chemical Index of Alteration（CIA）ranged from 63.34 to 87.39 in the between Heshanggou Formation and middle part Ermaying Formation,with the index of sodium depletion fraction(τ_Na)being-0.92 to-0.4.It indicates that the CIA apparently dropped in the lower Ermaying Formation,and then rose up in the middle of the Ermaying Formation.The annual precipitation reflected by the depth of the B horizon was 506.41-1210.62mm/yr,with two peaks in the upper Heshanggou Formation and in the middle part of the Ermaying Formation.The changing precipitation corresponded well to the evolution of sedimentary facies.The average annual temperature calculated by theτ_Nawas between 8.71-21.33℃,which fluctuated from a rapid decline to a rise in the Ermaying Formation.The recovered paleo-atmospheric carbon dioxide concentration（pCO₂）by the pedogenic calcite nodule was 652-3714 ppm V.It reveals an apparent drop and then a slow increase in the lower Heshanggou Formation.The recovered paleo-atmospheric pCO₂data have made up for the insufficient pCO₂data in the Middle Triassic.Precipitation,temperature and atmospheric pCO2 are coupled with chemical weathering degree.According to the restored paleoclimatic parameters,from the Heshanggou Formation to the middle part Ermaying Formation were divided into four stages:hot and arid climate（first stage）;hot and semi-arid climate（second stage）;warm and arid climate（third stage）;and warm and semi-arid climate（fourth stage）.（3）The paleo-temperature transition in the Jiyuan area is subject to the global atmospheric CO₂content.Not only that,the regional precipitation level may have more to do with the paleo-geographic evolution in the south of the North China Basin,which affected the interaction of sea-land monsoon.The comparisons between the characteristics of trace fossils with the restored paleoclimatic changes show that the increasing precipitation gave rise to ichnogenera;humidity may be a key factor for terrestrial organisms to live on;the changes in paleo-temperature may exert an impact on the body shapes of trace makers.Besides,rapid paleoclimatic variations were likely to prejudice the organisms with a narrow niche to survive.There are 34 figures,3tables and 174 references.