The Environmental Evolution Of The Jingchuan Red Clay On The Loess Plateau Since The Late Miocene

The eolian depositional sequence of the Loess Plateau in my country contains rich palaeoclimate environmental information,which is a good carrier for recording environmental evolution.Predecessors have obtained rich research results on the sedimentary sequence of the Loess Plateau.They believe that loess-paleosoil and red clay belong to the same aeolian origin.Relatively speaking,the research on the evolution of the paleo-climatic environment of the Loess Plateau focuses more on the overlying loess-paleosoil deposition.In the sequence,there are relatively few studies on the underlying red clay.In this paper,through experiments,the characteristics of trace elements,carbonate carbon and oxygen isotopes,and mineral content of the Jingchuan red clay profile have been obtained.Based on previous research results,a variety of environmental alternative indicators have been selected for comprehensive analysis,and the late Jingchuan red clay on the Loess Plateau is analyzed.The environmental evolution recorded by geochemistry since the Miocene has been studied,and the following conclusions have been obtained:(1)According to the change trend of the magnetic susceptibility curve,the magnetic susceptibility characteristics of the red clay sections in Jingchuan and Lingtai have been established in the paleomagnetic age.For comparison,the chronological sequence of Jingchuan red clay is indirectly obtained,which is 7.1?2.58 Ma.(2)The study of the rare earth element geochemical characteristics of Jingchuan red clay found that the Neogene red clay and the Quaternary loess-paleosoil sedimentary sequence have similar element distribution patterns,which are very close to the average composition of the upper crust,indicating that their materials are homogeneous.From the desert area with a broad wind direction on the Loess Plateau,it reflects the similarity in the genesis of red clay and loess palaeosoil,and supports the view that red clay is aeolian.(3)Comprehensive analysis of trace elements,isotopes,and mineral content indicators,the climate and environmental changes recorded by Jingchuan red clay since the Late Miocene are divided into the following stages: 7.1?5.5 Ma,and the climate is shown as the background of the overall warmth of the Neogene.In the relatively dry and cool stage above,the winter and summer monsoons are relatively weak;5.5?4.8 Ma,which is the warmest and humid period of the entire red clay deposition period,the summer monsoon is continuously strengthened,the pedimentation is enhanced,and the winter monsoon is weakened to the lowest;During 4.8?3.4 Ma,the winter and summer monsoons are relatively stable,the climate is milder,and the evaporation effect is stronger than the previous period,and it is relatively arid.Generally speaking,it is in a warmer and semi-arid period;during 3.4?2.8 Ma,the winter and summer monsoons increase simultaneously with seasonal changes.Intensified;during the period of 2.8?2.58 Ma,the Northern Hemisphere began to enter the Late Cenozoic Great Ice Age,the winter and summer monsoon entered the prevailing East Asian monsoon period,and the climate also transitioned from the overall warmth of the Neogene to the warm,humid,dry and cold of the Quaternary.At the same time,it is believed that the uplift of the Qinghai-Tibet Plateau and the evolution of the Arctic ice sheet may be the main factors controlling the changes in the Neogene climate and environment.(4)At 3.4 Ma,the winter and summer monsoons both strengthened and the seasonal changes were significant.This may be related to the overall uplift of the Qinghai-Tibet Plateau at 3.4 Ma.On the one hand,the tectonic uplift of the Qinghai-Tibet Plateau cuts off the water source from the Indian Ocean to central and western China.Accelerating the aridification of inland Asia,on the other hand,strengthening the monsoon circulation,and the seasonal distribution of rainfall is more significant.(5)The existence of primordial dolomite in the Jingchuan red clay section reveals that the Jingchuan red clay has a warm and seasonal arid climate,indicating that the red clay deposition period is mainly composed of a combination of strong rainfall and strong evaporation.Warm and dry climate with short and wet conditions.