Late Middle Jurassic Paleoclimate And Paleoenvironmental Evolution And Organic Matter Accumulation Mechanism,Qaidam Basin

Jurassic is a typical greenhouse period,and characterized by a highly changeable climate and environment.During the late Middle Jurassic to Late Jurassic,northwest China underwent a major warming event.In this critical period of climatic transition,thick coal and oil shales were successively deposited in the Shimengou Formation of the Qaidam Basin,which records important information about terrestrial climate and environment in the late Middle Jurassic（Batonian）.The research of climate and environmental evolution,as well as the organic matter accumulation,not only helps to reconstruct the evolution process of terrestrial climate and environment in the Qaidam Basin during the late Middle Jurassic,but can provide a useful guideline for understanding the current and future climate change.In addition,the research on the correlation between them can also provide new insight into the formation of coal and oil shale in the Qaidam Basin.In this study,we focused on these terrestrial fine-grained sedimentary rocks and conducted mineralogy,organic petrology,palynology,elemental geochemistry,organic geochemistry,and isotope geochemical analysis for better revealing of paleoclimatic evolution of the late Middle Jurassic（Batonian）in the Qaidam Basin.The evolution of land and lake environments triggered by the late Middle Jurassic climate fluctuations was further revealed,and eventually,the models for organic matter accumulation under different climates and environments were established.The main findings are as follows:1.Based on the botanical affinities of pollen and spores from the Shimengou Formation,the palynoflora of Bathonian in the Qaidam Basin was reconstructed.The palynoflora have undergone a shift of paleobotany from mixed forest,lowland fern forest,upland conifer forest to conifer forest marked by abundant Cheirolepidiaceae.The variable abundance and diversity of palynoflora at different time intervals of the late Middle Jurassic Bathonian stage are interpreted as deduced by a fluctuating climate in the Qaidam Basin.During the early Bathonian,a warm and humid climate was followed by a gradual decrease in humidity.By the late Bathonian,abundant xerophytic Cheirolepidiaceae（Classopollis）attested to a significant climate transition from humid to arid conditions.High-resolution geochemical analyses also revealed that the chemical weathering intensity weakened while lake water salinity enhanced during the late Bathonian.Simultaneously,the lakes have evolved from an open freshwater lake to a closed saline lake,where the abundant evaporite minerals were deposited.The high consistency of multiple proxy estimates confirms that the late Bathonian is the onset of the late Middle Jurassic climate transition from humid to arid conditions in the Qaidam Basin.2.Results of organic petrology and organic geochemistry indicate that coal seams from the coal-bearing member of the Shimengou Formation generally contain abundant records of palaeo-wildfire:inertinite and combustion-derived polycyclic aromatic hydrocarbons（PAHs）.The high consistency of these two evidences confirms the existence of widespread wildfires under the extremely humid climate conditions during the early Bathonian in the Qaidam Basin.These wildfires were dominated by ground and surface fires with low burning temperature,ranging from 222℃to642℃.These frequent wildfires under the overall humid climate are attributed to the sufficient atmospheric oxygen（25.8%）and fuel.The incidental mercury emissions from wildfires enhanced mercury input in the peatlands,which is the direct trigger for mercury anomaly in the coal（up to 8640 ppb）from the Shimengou Formation in the Qaidam Basin.3.The transition from a humid to an arid climate in the late Batonian has strongly disturbed lake system of the Qaidam Basin.As the climate aridification,the sharp rise in lake water salinity resulted in a salinity stratification of water column.Freshwater algae in surface were gradually extinct,instead,the halotolerant algae such as coccolith and cyanobacteria began to bloom.Meanwhile,the bottom water evolved into an extremely anoxic condition.The expansion of the anoxic zone stimulated the denitrification in the lake,and the lake salinization led to ammonia volatilization,both of which jointly promoted the accumulation of ¹⁵N and depletion of nitrate reservoirs of lake.Persistent nitrate depletion has triggered nitrogen fixation in the lake,compensating for the inorganic nitrogen content of the lake and maintaining the nutrient balance of the lake.4.The organic matter accumulation model of the Shimengou Formation（Batonian）within different climate and environmental zones was established.During the deposition of coal-bearing member,a warm and humid climate prevailed in the Qaidam Basin,where coniferous and fern vegetation flourished,providing sufficient terrestrial organic matter for the peat accumulation.The restricted swamp limited the inflow of terrigenous debris,which is an ideal condition for peat accumulation,promoting the formation of thick coal seams.These coal seams are characterized by high organic matter abundance and liptinite components.Therefore,these coals seams are oil-prone,which are coaly source rock with excellent hydrocarbon potential.During the deposition of lower part of shale member,the humid climate and tectonic subsidence led to the lake expansion,resulting in a deep lake environment and more contribution of algae to organic matter.However,the high input of terrestrial organic matter in humid climate led to a mixed organic matter source.The high injection of freshwater and terrigenous debris resulted in a dyoxic and freshwater,which is not the perfect condition for the organic matter preservation.Therefore,the massive oil shale with moderate organic matter abundance and hydrocarbon potential were deposited.During the deposition of upper part of shale member,the climate has converted to arid and hot,which led to a lake salinization and stratification.The halotolerant algae bloom on the surface greatly improved the primary productivity,and the anoxic bottom water environment promoted the organic matter preservation.This perfect model of organic matter accumulation promoted the formation of the lamellar oil shale with high organic matter abundance and strong hydrocarbon potential.Therefore,the humid climate in the early Batonian promoted the coal accumulation,while the aridification event in the late Batonian triggered the formation of higher quality oil shale.