Geochemical Characteristics And Paleoenvironmental Significance Of Mudstones From The Lower Jurassic In The Zigui Basin,Western Hubei Province,China

Since the industrial revolution,with the continuous growth of the population,the CO₂ emission from human life activities into the atmosphere has led to a sharp increase in the content of CO₂ in the atmosphere.The concentration of CO₂ in the atmosphere affects climate change.Climate changes in different periods will definitely leave sedimentary records in the strata at that time.The Jurassic period is a representative greenhouse climate period in geological history.The study of its palaeoclimate and palaeoenvironment can effectively help us to have a clearer understanding and understanding of the climate change laws under the greenhouse state.The Xiangxi Formation in the Zigui Basin in western Hubei is a typical lithostratigraphic unit of the Lower Jurassic in South China,this paper reveals the paleo-sedimentary environment and paleoclimate information of the Early Jurassic Xiangxi Formation in Western Hubei Basin.We use XRF and ICP-MS to analyze the major and trace elements of mudstones from the terrestrial Lower Jurassic Xiangxi Formation in the Toudaohe section of the Zigui Basin,western Hubei Province,in order to recover the sedimentary environment and palaeoclimate conditions.The mean Sr content is 113.8 ppm for the 1-12th layers of the upper Xiangxi Formation but rises to 2838.3 ppm for the 13th layer,indicating a sudden increase in water salinity.And the changes in Ba content and Sr/Ba ratio show the same results.Indicators such as Ni/Mo,U/Th,V/Cr,Cu/Zn and Ce_anom suggest that the sedimentary water medium changed from oxygen-rich condition during the early depositional stage of the upper Xiangxi Formation to reduction condition during the late stage.Indicators such as Sr,Mg/Ca,CIA reflect the occurrence of dry heating events when the upper Xiangxi Formation was deposited.These results reveal a dry heat paleoclimate during the deposition of the upper Xiangxi Formation in the Zigui Basin.We use GC-MS to analyze the biomarker compounds in the mudstone of the Lower Jurassic Xiangxi Formation in the Toudaohe section of Zigui area in western Hubei.Trisnorhopane ratio Ts/(Ts+Tm),saturated hydrocarbon Pr/n C₁₇-Ph/n C₁₈ graph,sterane C_29-S/(S+R)-C_{29-??/(??+??)}and methyl phenanthrene indicators such as the MPI index of phenanthrene isomers judged that the samples in the Toudaohe profile were low-maturity samples,which could be used to indicate the source of biogenic-parent-material and the depositional environment.According to relevant indicators such as n-alkane TIC and Pr/n C17-Ph/n C18-Pr/Ph triangle diagram,it shows that the parent-material of the Toudaohe section is mainly low-grade aquatic bacteria and algae and terrestrial higher plants,and terrestrial higher plants is the main biogenic-parent-material.Indicators such as SA,LA,MA,?21^-/n-alkane,?22⁺/n-alkane,and the relative percentages of C₂₇,C₂₈,and C₂₉ steranes indicate that the contribution ratio of terrestrial higher plants is weakened in layer B13,correspondingly the higher contribution of submerged plants.The main reason for this phenomenon is likely to become hot and dry climate,terrestrial higher plants death indirectly led to increased continental weathering,a lot of nutrients flowing into the water,resulting in an increase in the proportion of submerged plants.At the same time,polycyclic aromatic hydrocarbons,benzofluoranthene,benzo[e]pyrene,indeno[1,2,3-cd]pyrene were detected in layers B1,B4,B12 and B13 of Toudaohe section in Zigui Basin.Benzo[ghi]pyrene and halobenzene,which indicate the occurrence of ancient fires,prove that ancient fire events may have occurred in the late Early Jurassic in Zigui Basin,western Hubei.One of the reasons for ancient fires may be due to the climate has become hot and dry.In the late Early Jurassic,a global Oceanic Anoxic Events occurred.This period led to significant changes in the terrestrial ecosystem,and a global climate warming event occurred.The Toarcian paleo-atmospheric p CO₂ background value reconstructed using ginkgo plant leaf fossils was about 1000ppm,and the p CO₂ at the top of the Xiangxi Formation was as high as 1400ppm;indicating a hot and arid climate.The content of large fossil cheirolepidiaceous plants and their pollen components in the top of Xiangxi Formation increased significantly.At the same time,various geochemical indicators in this article indicate that a climate warming event and paleofire event occurred at the top of Xiangxi Formation in the late Early Jurassic,and most likely it is the response of T-OAE in terrestrial ecosystems in southern China.