| Since the formation of the Earth,its long history of evolution has also been a history of the succession of the Earth’s climate system.At present,mankind is facing the great challenge of global warming,and the development of human civilization urgently requires us to have a deeper understanding of the development trend of global warming and its environmental and resource effects.Through in-depth study of the Earth’s surface system in the past and clarifying the pattern of succession of the Earth’s climate system in geological periods,"using the ancient to discuss the present" will help us better understand the current climate and environment and predict the future climate change.In paleoclimate research,the Quaternary(2.58 million years ago)is the closest period to human beings,which has been studied widely because of its easy dating and better availability of materials,and abundant of paleoclimate data has been accumulated.However,the Quaternary accounts for only 0.056% of the entire Earth’s history(about4.6 billion years).To clarify the succession law of the earth’s climate system,it is necessary to study the paleoclimate on a longer time scale.The climate change in the pre-Quaternary(deep time)is only controlled by natural factors,which is of great significance to explore the impact of current human activities on climate and to predict the future climate more accurately.Therefore,deep time paleoclimate research has become an important part of global climate change research.Recovery of paleoclimate by extracting paleoclimate information from the corresponding geological environment is an important way to reconstruct the evolutionary history of the Earth’s climate system and to understand the intrinsic connection between climate and the Earth’s environmental catastrophic events during the geological period,since climate always interacts with and maintains dynamic equilibrium with the geological environment.Research on "deep-time" paleoclimate is mostly focused on the major geological events during the geological period.On the one hand,a relatively high degree of geological research on major geological events provides a wealth of basic geological data for the study of "deep-time" paleoclimate;on the other hand,major geological events are often accompanied by catastrophic changes in the Earth’s paleoclimatic environment and even cause mass extinctions,which is an important factor for exploring the evolutionary laws of the Earth’s climate system,especially for exploring the evolutionary laws of the Earth’s climate system.Volcanism is an important form of material and energy exchange between the earth’s inner and outer circles,which has a significant impact on the global climate system.The Large Igneous Province,characterized by large-scale volcanism,developed in almost all parts of the world,which has become the focus of current paleoclimate research.Large igneous provinces(LIPs)cover an area of 105 km2 or more and are characterized by the eruption of large amounts of magma in a short period of time(1-2 million years),accompanied by the release of gases such as CO2,H2 S,SO2,etc.,which often have a significant impact on the global climate.LIPs have been the focus of deep-time paleoclimate studies because of their significant influence on ecosystems.Emeishan large igneous province(ELIP),located in southwest China,was almost contemporaneous with the end Guadalupian extinction event,and many scholars believe that the extinction was triggered by paleoenvironmental changes caused by massive volcanism in ELIP.However,much of the current research on the paleoclimate effects in ELIP is based on marine stratigraphic data,while research results on terrestrial paleoclimate are lacking,especially quantitative reconstruction of paleoclimate,which makes it difficult to understand the climatic effects of ELIP at the global level.The product of paleo weathering is a direct record of terrestrial paleoclimate.Paleosols formed by paleo weathering are considered ideal for the quantitative recovery of terrestrial paleoclimate and have been applied to the recovery of terrestrial paleoclimate in many regions of the world during different geologic periods.The massive volcanic eruption of ELIP formed a huge amount of basalt(called Emeishan Basalt),during which there were 9-11 inter-eruption periods,and paleosol profiles of different scales were developed on the Emeishan Basalt,among which,the paleosol profile developed on the last Emeishan Basalt(the top of Emeishan Basalt)is large in scale and closely related to the genesis of the paleo-weathering crust sedimentary type REE deposits,in the same region,which provides valuable materials for exploring the influence of ELIP on the regional and global climate and its resource effect.The quantitative reconstruction of paleoclimate from paleosol is generally based on the response relationship between climatic proxies of modern soil B-layer and climate factors(MAT,MAP)to construct a "soil-climate" conversion function.While existing "soil-climate" conversion functions are constructed assuming that other soil-forming factors are consistent or close to each other,actually selected soils for "soil-climate" conversion functions are often developed on different types of parent materials or rocks.As the response of soils to climatic factors may vary greatly among different matrices or rocks,this will affect the reliability of existing "soil-climate" paleoclimate recovery results.The construction of a soil-climate conversion function based on the same parent material can help improve the accuracy of quantitative paleoclimate recovery.Therefore,based on the work of previous authors,this paper collects newly published geochemical data of soil profiles developed on basalts and identifies those in situ developed soil profiles without artificial disturbance and aeolian,and revised the "soil climate" conversion function based on the comprehensive investigation of the existing potential climate substitution index.In this paper,the physical and chemical properties of the paleosols,such as micromorphology,mineralogy,and elemental geochemistry,are comprehensively studied on the basis of a detailed field investigation of the extensively developed paleosols on top of the Emeishan basalts.On this basis,the terrestrial paleoclimate at the end of volcanism in the ELIP was restored using the revised "soil-climate" conversion function in this thesis.The transport and concentration of rare earth elements(REEs)in epigenetic environments are closely related to climate,and the climatic conditions of their formation can be qualitatively reversed by the geochemical characteristics of REEs in geological bodies.By studying the migration and enrichment characteristics of REEs in the Emeishan basalts and the clay rocks of Xuanwei Formation,this thesis investigates the formation process of paleo-weathering crust sedimentary type REEs deposits,related to the weathering of Emeishan basalts,and qualitatively verifies the quantitative reconstruction results of paleoclimate in this period.The main results of this thesis were as follows:(1)Further revised the “soil-climate” relation equation developed on the basalt material.The "soil-climate" relation equation constructed with the Saf index was used for quantitative paleoclimate reconstruction of paleosols with a high degree of development: MAT = 32.81e-0.4166X(R~2 = 0.55,S.E.= ± 4.6℃);MAP =-875.5ln(X)+ 1792(R~2 = 0.69,S.E.= ± 328mm),where x = Si O2 /(Al2O3 + Fe2O3).The “soil-climate”relationship equation constructed with the WI-2 index was used for weakly developed paleosols: MAT = 33.15e-0.4714X(R~2 = 0.60,S.E = ±4.7℃);MAP =-826.2ln(X)+ 1722(R~2 = 0.70,S.E.= ± 333mm).(2)The terrestrial paleoclimate data(MAT: 18.9℃,MAP: 1546mm)were obtained at the end of volcanic activity(253 Ma)in the ELIP using the revised "soil-climate" relation equation,which makes up for the lack of quantitative terrestrial paleoclimate data and the low temporal resolution of paleoclimate studies from paleontology and sedimentology.It provides reliable data for climate comparison and global climate change modeling at larger temporal and spatial scales,and also provides a case study for exploring the response of the paleoenvironments to volcanic activity in the LIPs.(3)Weathering crust deposits are an important type of rare earth deposits,and the current understanding of this type of rare earth deposits is mainly limited to granite weathering crust rare earth deposits.Through the study of the mineralogical and REEs geochemical characteristics of paleosols and clay rocks at the bottom of Xuanwei Formation,this thesis proposes that the paleosols and clay rocks at the bottom of Xuanwei Formation are potential ore-bearing layers.On this basis,the mineralization model of basalt paleo-weathering crust sedimentary type REEs deposits was constructed,which enriches the mineralization theory of this type of deposits and is of great significance for finding potential REEs resources. |
