Soil Erosion In Recent 10,000 Years On The Loess Plateau And Its Influence On Sedimentation In The Lower Yellow River

The Loess Plateau,a typical fragile ecological zone in China,and soil erosion is sensitive to climate change and human activities.It is currently in the region with the most severe soil erosion in the world.In the study of soil erosion,we must understand the geological background,evolutionary process,driving mechanism and environmental effects of soil erosion.Since the Holocene（nearly 10,000 years）,the erosive environment of the Loess Plateau,such as climate,natural vegetation,and human activities,has undergone significant changes.Studying the evolution of erosive environment,soil erosion process and environmental effects during this period is an important foundation for understanding the relationship between soil erosion and ecological environment interaction in the Loess Plateau during the historical period.The purpose of this study is to investigate the evolution of the erosive environment in the middle and lower reaches of the Yellow River,discuss the soil erosion in the Loess Plateau since the Holocene,especially in the past 2000,and reveal the interactions between the middle and lower reaches of the Yellow River.We collected historical records,geomorphological evidence,meteorological and hydrological observatory data,and remote-sensing satellite imagery to reconstruct the time series and climate,hydrology,vegetation,sediment,and deltaic sediments since the Holocene.Based on the sequences of environment change,we used soil erosion models to simulate the soil erosion scenarios in the past 2000 years,discussed the basic process of river change and delta formation in the lower Yellow and the interaction between soil erosion in the Loess Plateau,channel change delta formation.We proposed the basic model of the evolution of the geomorphology of the middle and lower reaches of the Yellow River.Through the above research,we have obtained the following important conclusions:（1）Evolution of erosive environment and soil erosion processes.The climate of the Holocene Loess Plateau was controlled by the East Asian monsoon.The transition time in the early,middle,and late Holocene was 8.5 ka and 3.1 ka,showing an alternate cooling and warming climate.The characteristics of the Holocene in the early-mid and late Holocene climate were cool and dry-warm and humid-cool and dry.The evolution of natural vegetation in the Loess Plateau was affected by climate change,and the overall evolutionary features are expansion–flourishing-atrophy.Since the late Holocene,human activities have had a strong influence on the natural environment.In the past 10,000 years,monsoon activity had gradually increased.The most significant feature of the Loess Plateau was the loess slow accumulation and strong erosion.In the early Holocene,the soil erosion was stronger than before,but the erosion intensity was at a low level,and the average erosion modules was less than 10t?ha^-1?yr^-1,belonging to micro erosion.In the middle Holocene,the soil erosion intensity was stronger than the early,especially in the climate optimal period（8.5 ka-5.0 ka）.In the late Holocene,the soil erosion intensity on the Loess Plateau increased sharply.In the past 2000 years,especially after the Tang Dynasty,the Loess Plateau erosion reached the strongest period during the Holocene,and the soil erosion modules of the Eastern Han,Sui and Tang Dynasties,Ming and Qing Dynasties and modern times were 28t?ha^-1?yr^-1,39 t?ha^-1?yr^-1,43 t?ha^-1?yr^-1and 30t?ha^-1?yr^-1respectively.Before the late Holocene,the population of the Loess Plateau was few,and the impact on the natural environment was limited.Soil erosion was a natural process and was mainly affected by changes of climate.Strong soil erosion in the late Holocene was the result of climate fluctuations and human activities,and human activities gradually became the dominant factor.（2）Erosion and sedimentary interactions in the middle and lower reaches of the Yellow River.During the Holocene,soil erosion,sediment transport and delta formation in the middle reaches of the Yellow River have similar dynamic characteristics:In the early Holocene,Loess Plateau soil erosion intensity was weak,sediment transport was low,and the Yellow River Delta was in erosion-accumulation dynamic development;In the middle Holocene,the intensity of soil erosion,amount of sediment transportation and the rate of delta formation were increased,but it was relatively low.In the late Holocene,the soil erosion,sediment transport and the delta formation rate increased significantly.Soil erosion and sediment transportation in the middle reaches of the Yellow River are affected by climate change and human activities.During the early Holocene,the East Asian monsoon strengthened,rainfall increased,and soil erosion in the Loess Plateau increased,resulting in an increase in sediment transportation.However,during this period,the global sea level rose and the delta was mainly eroded;during the middle Holocene,the climate was warm and humid,and the vegetation recovered.The amount of erosive rainfall is large,and the soil erosion is stronger than in the previous period.In the late Holocene,especially in the past 2000 years,the strong soil erosion induced by the activity leads to an accelerated rate of downstream deposition and delta formation.Since the Holocene,sedimentation in the lower reaches of the Yellow River has been sensitive to changes in soil erosion in the middle reaches of the Yellow River and are mainly controlled by soil erosion.Before the late Holocene,erosion,sediment transportation,and sedimentation were mainly affected by climate change.In the late Holocene,human activities gradually became the dominant factor in the evolution of river geomorphology.