East Asian Paleoclimate Change In The Weihe Basin Since The Middle Eocene Revealed By Sedimentology And Clay Mineral Analysis

As the concentration of global atmospheric greenhouse gas rises,the future climate may be a reappearance of the warm period since the early Cenozoic.Asian climate is deeply affected by the monsoon circulation.However,the origin and evolution of the East Asian monsoon climate and its variability under the background of the doubling concentration of atmospheric CO₂are important scientific issues that have not yet been resolved.The Weihe Basin is located at the margin affected by the modern Asian monsoon precipitation.It contains a complete sedimentary sequence from the Eocene Honghe Formation to the Pleistocene Sanmen Formation,which can better reveal the Cenozoic East Asian climate evolution history.In this study,we starts from three aspects:（1）We combine the sedimentary facies and sedimentary environment since the Cenozoic Honghe Formation in the Weihe Basin;（2）We observe the micro-morphological characteristics of the soil according to the thin slices of the typical layers of each strata group in the Cenozoic Weihe Basin,qualitatively describe the distribution,morphological characteristics and content of bone particles,pores,and soil formations,and establish a quantitative method to estimate the relative proportion of the bone particles and soil formations in the slices and to obtain a training sample database of images of different components in soil slices;（3）We perform XRD analysis on oriented mounts of non-calcareous clay-sized particals and randomly orientated samples,SEM and EDS analysis,obtain the the types,contents and crystal parameters of the clay minerals in the Weihe Basin,and try to reveal the weathering intensity and paleoclimatic evolution of the Weihe Basin since the late Middle Eocene,and provide new evidence for the driving mechanism of the Cenozoic monsoon changes.The main conclusion are as follows:1.There is a shallow lake as a whole in the Eocene Honghe Formation.The sediment at the bottom 19 m is the intermountain alluvial deposit in the early stage of the basin’s development;There is in a delta sedimentary system in the Oligocene Bailuyuan Formation as a whole.The lower part of 150 m is lakeside deposit.Shallow lacustrine facies and lakeside facies alternate continuously in the middle part of 180 m,representing the sedimentary cycle of the lake surface,and the upper part of 30 m is lake shore deposits;There are braided channel deposits mixed with a small amount of floodplain and shallow lake deposits in the lower part of 40 m in the Miocene Lengshuigou Formation,Koujiacun Formation,and the upper part of 39 m is shallow lake deposit;There are braided channel,floodplain mixed with paleosol and lacustrine deposits in the Miocene Bahe Formation;In the Lantian Formation from late Miocene to Pliocene,there is the sedimentary environment from shallow lake to lakeside in the lower part of 70 m.The river bed,floodplain,and shallow lake alternately appear in the middle 118 m,and the upper 80 m is a floodplain deposit;It is a shallow lake to lake shore deposit in the Pleistocene Sanmen Formation.2.The micromorphological feature of the thin slices shows that the sediments in the Weihe Basin are in a strongly weathered environment in a drainage basin.A large amount of iron is decomposed and freed from the primary minerals and mixed with the secondary clay,making the clay deep red.However,after being transported by the river and deposited in the Weihe Basin,the weathered product in the drainage basin has not been exposed to the air in a large area to re-experience the soil formation process.The thin-section micrographs of the argillaceous horizons in each period show that they are aggregates of coarse-grained minerals such as quartz and feldspar and fine-grained minerals such as clay minerals and carbonates,indicating that the provenance may not have changed significantly.Typical features of pedogenesis are found in the Bailuyuan Formation,indicating that there was a period of time when the sediments were exposed to the surface and experienced a typical leaching process,at least indicating that the Weihe Basin was relatively humid during this period and was not under long-term arid climate conditions.3.The clay minerals of the Cenozoic fluvial-lacustrine sediments in the Weihe Basin are mainly smectite and illite,followed by chlorite and kaolinite.Illite crystallinity,illite chemical index,the ratio of smectite to illite and chlorite indicate the gradual weakening of chemical weathering since the Honghe Formation in the middle Eocene under a semi-arid and semi-humid environment,unraveling that the Weihe river watershed was in a relatively humid environment in the middle Eocene.The appearance of palygorskite may be controlled by weaker diagenesis,and has no clear climatic significance.4.The strong Eocene chemical weathering in the Weihe Basin and the large area of freshwater lake deposits indicate that it has not been in the arid zone for a long time,and may be similar to the Neogene monsoon-dominated climate pattern.From the middle Eocene to the Oligocene,the sedimentary facies of the Weihe Basin changed significantly,from shallow lake sediments to lake shore sediments.During the shallow lake period,the decrease of the crystallinity of illite and the chemical index of illite in the Weihe Basin also further indicated the process of climate drying and weakening of chemical weathering on the smaller time scale which may be the response of climate change to the Eocene-Oligocene transition（EOT）cooling event.The composition,content and crystal parameters of clay minerals in the sediments of the Weihe Basin indicate that the intensity of chemical weathering has gradually weakened since the Miocene,which may be related to the simultaneous stepwise changes of the East Asian monsoon and the climate in arid regions.