Middle Triassic To Late Jurassic Climate Change In The Zigui And Dangyang Basins On The Northern Margin Of The Middle Yangtze

During the Early Mesozoic era,the Triassic and Jurassic periods had atmospheric carbon dioxide levels that were on average 4-8 times higher than present-day levels,with peak levels exceeding 2000 ppm.The global average temperature was 5-10℃higher than present-day temperatures,resulting in a typical greenhouse climate.This period records the transition from the late Paleozoic ice age to the early Mesozoic greenhouse period,providing a window for studying and predicting the future greenhouse processes and their impacts on the modern Earth.Increasing evidence from marine and terrestrial studies suggests that the Mesozoic greenhouse climate was not as monotonously stable as previously believed but rather displayed significant temporal and spatial variability due to interactions within the Earth system.However,the lack of continuous records of paleoclimate changes under the tectonic scale of the early Mesozoic and insufficient attention paid to the reconstruction of terrestrial paleoclimate has hindered a deep understanding of the long-term evolution history and driving mechanisms of paleoclimate during this period.This thesis selects the Dangyang Basin and Zigui Basin in the northern margin of the Yangtze Plate in southern China as the research area.The Middle Triassic to Upper Jurassic strata continuously exposed in three study sections of Xiaozhanghe,Xuanwudong and Xiakou areas in the two basins and the Early-Late Jurassic paleosols in the Xiakou section are selected as research objects.Based on data collection and field geological research,combined with research methods including detrital rock and tuff zircon chronology,mudstone geochemistry and clay mineralogy,paleosol macro-micro morphology,element and carbon-oxygen isotope geochemistry,etc.,paleo-chemical weathering intensity,paleo-mean annual temperature（MAT）,paleo-mean annual precipitation（MAP）and PCO₂ are used as proxy to qualitatively and quantitatively reconstruct the long-term evolution history of greenhouse climate in the Middle Triassic-Late Jurassic of the study area.The driving mechanism of climate evolution during this period is discussed.The main research results and understanding are as follows:（1）The study of the sedimentary facies of the Zigui and Dangyang basins during the Middle Triassic to Late Jurassic.The typical characteristics of the Middle to Upper Triassic Badong Formation include mixed sedimentation of tidal bedding clastic rocks and storm sedimentary structures carbonate rocks,indicating a tidal flat to shallow marine depositional environment.The top of the Upper Triassic Jiuligang Formation transitions to continental sedimentation,with wave-dominated cross bedding and various types of unidirectional flow bedding indicating a pro-delta to delta front depositional environment.The sedimentary facies of the Lower Jurassic Tongzhuyuan Formation are diverse,with braided river,delta,lake,and meandering river depositional environments occurring in sequence from bottom to top.Unidirectional flow bedding in sandy-muddy rocks dominates in the river depositional environments at the bottom and top of the formation,and braided river sedimentation is differentiated from meandering river sedimentation by the lack of binary structures.The Qianfoya,Shaximiao,Suining,and Penglaizhen formations of the Middle to Upper Jurassic form a thick sequence of continental red beds,in which paleosol sedimentation and various sedimentary structures indicative of wave influence,such as ripple mark and wave-dominated cross bedding,are extensively developed.The lack of various types of unidirectional flow bedding indicates a lacustrine depositional environment in the foreland basin.（2）Paleoclimate evolution of the Northern Margin of the Middle Yangtze region from the Middle Triassic to Late Jurassic.Based on stratigraphic and sedimentological research,173 samples of mudstones from the Middle Triassic to Upper Jurassic strata in the region were subjected to whole-rock geochemical analysis,phase identification and relative contents semi-quantitative analysis of clay mineral.Proxy with good paleoclimate significance,such as various chemical weathering indices,trace element climatic indices and clay mineral assemblages,were selected for paleoclimate reconstruction.Results showed that the potassium-corrected chemical index of alteration（CIAcorr）of the Badong Formation samples from the Middle-Late Triassic ranged from 59.3 to 83.5,indicating weak to moderate chemical weathering intensity in provenance.The average of the Climate index value（C-value）was 0.34,and the average Sr/Cu value was 15.2.Combined with the clay mineral assemblage characterized by illite and chlorite,it indicated a prevailing hot and arid climate during this period.In the Xuanwudong section of the Zigui Basin,the relatively high CIAcorr（71.1-88.0）and C-value（0.91）,low Sr/Cu（<5）,and clay mineral assemblage rich in kaolinite indicated a regional shift to a warm and humid climate accompanied by intense chemical weathering in the Late Triassic to Early Jurassic Jiuligang and Tongzhuyuan formations.In the Xiakou section of the Zigui Basin,the CIAcorr of the lowermost samples of the early Middle Jurassic Qianfoya Formation significantly decreased from 85.4 to 57.6,maintaining an average low value of 68.9 in subsequent strata.The Sr/Cu（5.99）,C-value（0.72）,and clay mineral assemblage containing illite/smectite mixed-layer clay indicated that this chemical weathering weakening was related to the aridification event in the Middle Jurassic,and the region was dominated by an increasingly arid continental climate during the Middle-Late Jurassic.Through a comprehensive comparison and discussion with factors such as atmospheric carbon dioxide variations,the development of the Megamonsoon system,the latitudinal drift and the evolution of paleogeographic patterns of South China Plate during the Early Mesozoic,it was determined that these four factors co-drove the long term paleoclimatic evolution of the study area during different periods of the Middle Triassic to Late Jurassic.（3）The qualitative study of the paleoclimate evolution of the Middle to Late Jurassic in Zigui Basin based on the macroscopic classification and microscopic morphology of paleosols.Paleosols are located at the core intersection of various sublayers of the Earth Systems,and their various pedogenesis at macro and micro scales and geochemical characteristics can record rich terrestrial climatic environmental information.A total of 138 paleosol sequences were identified in the Xiakou section of the Middle-Upper Jurassic in Zigui Basin.Using the Mack classification scheme,four soil orders,including Calcisol,Protosol,Gleysol and Vertisol,were identified,and based on the development of secondary soil characteristics and the actual soil horizons development state in the field,seven ancient soil suborders and 19 standard soil types were further built.After conducting micromorphology studies of paleosol thin sections,12 microstructures,4birefringent fabric,and 11 pedofeatures that can be subdivided into three categories were found under the microscope in paleosol thin sections.Qualitative paleosol-climatic research based on macroscopic and microscopic soil forming characteristics of these soil showed that the early Middle Jurassic Qianfoya Formation paleosols were mainly gleied Protosol suborders,with the pedofeatures development of Fe/Mn depletion coatings,Fe/Mn impregnate nodules and other micro-pedogenic features indicating a humid climate.The Calcisol with vertic features and microscopic characteristics that appeared in the middle part of the Qianfoya Formation indicated a dry paleoclimate with seasonal features during the pedogenesis period.The six Calcisol sequences that appeared at the top of the Shaximiao Formation in the late Middle Jurassic indicated a persistent dry climate.In the subsequent Upper Jurassic Suining and Penglaizhen formations,Calcisols accounted for more than 80%of the total developed soil thickness,and under microscopic observation,the cracked red matrix with abundant semi-arid to arid-type pedofeatures,indicating that the continuous and intensified aridification of the Middle-Late Jurassic climate which was consistent with the results of previous studies based on chemical weathering,clay mineralogy,and paleobiology.（4）Quantitative reconstruction of the paleoclimatic evolution in the Zighui Basin during the Middle-Late Jurassic.Based on the study of macro and microscopic morphology of paleosols,39 samples of paleosols illuvium horizons（B horizon）were selected for whole-rock geochemical research,and the core paleoclimatic parameters of MAT and MAP during the Middle-Late Jurassic were quantitatively reconstructed.The results show that the average MAP at the bottom of the Qianfoya Formation in the early Middle Jurassic decreased rapidly from 1034mm/year to 970mm/year,while the MAT increased from 18.0°C to 20.2°C,quantitatively constraining the process of warming and aridification of the paleoclimate in the area during the early Middle Jurassic.The average MAP of the Suining and Penglaizhen formations in the Late Jurassic continued to decrease to 898mm/year and 904mm/year,while the MAT record remained relatively stable,indicating the sustained aridification of the paleoclimate.Carbon and oxygen isotope tests were conducted on 56 carbonate nodules in the calcic horizon（Bk）,and the PCO₂ was calculated.The results show that the Middle-Late Jurassic had typical greenhouse conditions with PCO₂ 4-10 times higher than modern levels.The average PCO₂ at the bottom of the Qianfoya Formation of Aalenian-Bajocian ages was1400ppm,followed by a decrease to an average of 886ppm in the middle part of the Qianfoya Formation.Then,in the late Middle Jurassic Shaximiao Formation,the average PCO₂ increased significantly to 1679ppm,and further increased to higher levels of 2270ppm and 2105ppm in the Late Jurassic Suining and Penglaizhen formations,respectively.The PCO₂ results obtained in this study are comparable with other independent reconstructions during the same period.The range and trend of the PCO2data are also consistent with the results of the GEOCARB III geochemical model.The reliable PCO2 data reconstructed from calcareous nodules of ancient soils in this study could provide valuable boundary constraints for the adjustment of geochemical carbon cycle models and climate simulations during the Middle to Late Jurassic.