| Soil erosion is one of the most serious ecological and environmental problems around the world,however,the time frame of current studies is focused on the recent decades,which limits the understanding of the evolutionary processes and driving mechanisms of soil erosion in the past,as well as constraints on the assessment of the state of soil erosion in the modern under the influence of intense human activities and the prediction of future changes in soil erosion.The study of soil erosion during the Holocene,the youngest geological period during which the environment changed in distinct phases and the brilliant human civilization developed greatly in different regions,is important for understanding the evolution of soil erosion and the role played by human activities in it.Due to the lack of a complete geological record of soil erosion and sedimentation in fluvial landforms,the study of soil erosion during geological history is still at the stage of qualitative research.The lake basin is a relatively independent geographical unit,and the lake is the "sink" of the basin.Surface debris is deposited in the lake through the process of denudation-transportation-deposition,and the lake sediment faithfully recorded the sediment deposition process and environmental evolution information,which becomes an ideal material for reconstructing and studying soil erosion in the geological-historical period.In this thesis,based on lake sedimentary records from different climatic regions of China and paleoenvironmental simulation products,soil erosion factors including precipitation,vegetation and human activities were reconstructed,soil erosion modulus was quantitatively calculated,and its dynamic processes and influencing factors were analyzed.Based on the above research,a geomorphological evolution model applicable to the lake basin was constructed using the geomorphological evolution numerical simulation platform(Landlab).The model was used to simulate the soil erosion and sediment deposition processes in the lake basin of the Yunnan-Guizhou Plateau and the middle and lower reaches of the Yangtze River Plain,which are dominated by water erosion,and to investigate the driving mechanisms of monsoon evolution and human activities on the geomorphological evolution of lake basin soil erosion and sediment deposition.The following conclusions and insights were obtained from the study:(1)Characteristics of soil erosion changes in different sub-regions of China and the response of influencing environmental factors.Reconstructing the Holocene soil erosion modulus of Chinese lake basins,the average values of northwest,the Tibetan Plateau,north,south and southwest were 2.1 t/(ha a),1.2 t/(ha a),12.9 t/(ha a),3.1 t/(ha a)and 18.9 t/(ha a),respectively.60%of all studied lakes showed a significant increasing trend of soil erosion modulus during the Holocene.In northwest and the Tibetan Plateau,the Holocene soil erosion modulus showed an overall increasing trend.Except for south where soil erosion modulus was high during 9.5-7.0 ka(ka=thousands of years before present,where the“present”is defined as the year AD 1950),soil erosion modulus in north,south and southwest was relatively low from 12.0-2.0 ka and increased rapidly after 2.0 ka.There were regional differences in the response of soil erosion changes to environmental factors under the influence of different climate monsoon systems.For example,Increased precipitation led to enhanced soil erosion in the Tibetan Plateau and north,with weak effects in south and southwest.Except for northwest,soil erosion modulus decreased with the increase of tree pollen in all sub-regions,indicating that the increase of tree pollen reduced soil erosion.(2)Soil erosion and sediment deposition processes of typical lake basins in the Yunnan-Guizhou Plateau and the middle and lower reaches of the Yangtze River Plain.Spatially,deposition occurred within the lakes,on the flat terrain of river valley flats,premountain alluvial flats and plains around the lakes,and erosion occurred in other mountainous regions,with the average soil erosion rate higher than the lake sedimentation rate.The average lake sedimentation thicknesses in the Lake Lugu,Lake Erhai and Lake Dianchi on the YunnanGuizhou Plateau were 4.19 m,7.80 m and 2.40 m,and the average lake sedimentation rates were 3.49 cm/100 a,6.50 cm/100 a and 2.00 cm/100 a,respectively.The average lake sedimentation thicknesses in the Lake Dongting,Lake Poyang and Lake Chaohu on the middle and lower reaches of the Yangtze River Plain were 3.34 m,3.44 m and 2.96 m,and the average lake sedimentation rates were 2.78 cm/100 a,2.86 cm/100 a and 2.47 cm/100 a,respectively during the Holocene.Soil erosion rates and lake sedimentation rates in the Holocene were characterized by similar phase changes,showing fluctuating changes before 4.0 ka and increasing after 4.0 ka.Vegetation contributed to soil erosion significantly more than precipitation on the Yunnan-Guizhou plateau and the middle and lower reaches of the Yangtze River Plain.Driven by the natural environment,with soil erosion rates increasing during periods of decreasing vegetation cover.The low vegetation cover of lakes in the YunnanGuizhou Plateau region at around 5.1 ka,4.7 ka,3.3 ka,1.8 ka,and 0.7 ka lead to enhanced soil erosion.Intensive soil erosion of lake basins in the middle and lower reaches of the Yangtze River Plain occurred during periods of low vegetation cover around 10.2 ka,6.5 ka,4.6 ka,3.5 ka and 2.2 ka.Vegetation cover gradually decreased in the late Holocene,leading to an increase in soil erosion rates.(3)Effects of human activities on soil erosion in Holocene lake basins.Human activities were the most important explained factor of Holocene soil erosion changes in all sub-regions of China.Since the late Holocene,human activities have led to rapid enhancement of soil erosion in north,south and southwest China through vegetation/land use changes.The history of human activities contributing significantly to soil erosion varied in different regions,and the earliest period of significant effect of human activities on soil erosion in the middle and lower reaches of the Yangtze River Plain was about 3.0 ka and in the Yunnan-Guizhou Plateau was 2.0 ka.There were differences in the extent of human activities on soil erosion in different regions during the historical period.In the Yunnan-Guizhou Plateau,the effect of human activities on soil erosion of lake basins at high altitudes was weak and increased with decreasing altitude,e.g.,the soil erosion rates of Lake Erhai and Lake Dianchi increased 0.45 and 2.03 times,respectively,at 2.0-0 ka compared with before.This study found that under natural conditions,there were significant differences in the response patterns of soil erosion to environmental factors in different regions,with vegetation changes playing a much greater role than precipitation in soil erosion in the Yunnan-Guizhou Plateau and the middle and lower Yangtze River Plain.Anthropogenic activities have been the main cause of the rapid enhancement of soil erosion in different subregions since the late Holocene,but there were regional differences in the history of influence and the degree of contribution.It is suggested that the reconstruction of high-resolution paleoenvironmental data is the basis for an in-depth study of past soil erosion in future studies.Moreover,when studying soil erosion based on lake sedimentary records,it is necessary to consider the soil erosion characteristics of lake basins and the autochthonous characteristics of lakes. |
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