| The Holocene is the most recent historical period,and Holocene climate change had a huge impact on the human living environment and social development process.The Holocene climate was warm and humid,but there were also secondary fluctuations of cold,warm,dry and wet climate,which were unstable in space and time.The Ulan Buh Desert,located on the edge of the East Asian monsoon,is more sensitive to regional and even global climate change,and its stratigraphy preserves environmental changes and climatic events during the evolution of the desert,making it an ideal site for Holocene climate change studies.The Holocene evolution of the Ulan Buh Desert is currently controversial,involving the spatial and temporal interactions of circulation,precipitation,rivers and lakes,and changes in climatic elements.Therefore,in this paper,the wind-formed sand-river-lake phase sedimentation profiles of the Ulan Buh Desert were selected,and the Holocene Ulan Buh Desert was reconstructed by analysing sediment grain size,chemical elements and organic matter proxies,using probability density statistics on the basis of OSL dating and previous research data,and using probability density statistics on the fluvial/lacustrine sedimentation(53)and aeolian sand sedimentation(34)records reconstruct the climatic and environmental evolution of the Holocene Ulan Buh Desert.This paper also explores the mechanisms driving the evolution of the desert environment in the context of other paleoclimatic and palaeoenvironmental records from the East Asian monsoon region.The following main conclusions are drawn.(1)The Holocene Ulan Buh Desert sediment grain size consists of three end-members:EM1 represents fluvial/lacustrine sediment deposited in the lowlands,reflecting regional precipitation conditions,and the EM1 fine-grained material is derived from fluvial/lacustrine sediment capturing airborne clay and silt;(EM2)is a distal aeolian sand sediment under weak wind-dynamic conditions in a shallow lake-lakeshore environment,indicative of a more arid climate.EM3 is a near-edge wind-formed sand component under strong wind-dynamic conditions,representing the intensity of regional dust storm activity and indicative of arid and cold climatic conditions.(2)Before 10.2 ka in the Ulan Buh Desert,the East Asian monsoon was strong,the regional climate was generally warm and humid,monsoon precipitation was abundant,wind and sand activities were weak,rivers and lakes developed in the desert localities,the ecological environment was good,and vegetation grew and developed;10.2-8.0 ka the East Asian summer wind weakened significantly,the climate changed from warm and humid to cold and dry,and the desert area expanded;8.0-3.0 ka the East Asian monsoon strengthens,temperatures rise,precipitation increases,and effective humidity increases,but there are secondary climatic fluctuations of warm and dry climate.The East Asian monsoon was strongest from 8 to 6.5 ka,with both temperature and precipitation reaching Holocene maximums and the expansion of lake areas.6.5 to 3.0 ka saw the East Asian summer winds continue to weaken,with the climate moving in a cold-dry direction,and the lakes began to shrink and disintegrate,undergoing a process of retreat from southwest to northeast,and the desert began to activate and expand.3 ka onwards the desert entered an active period of wind and sand activity,with the lakes drying up over a large area.The climate became dry and cold,monsoon precipitation decreased sharply,and the modern Ulan Buh Desert began to develop.(3)Climate change in the East Asian monsoon region of China is controlled by fluctuations in the strength of the East Asian summer winds.The amount of solar radiation and ocean circulation in the Northern Hemisphere control the strength of the East Asian monsoon,and as the amount of solar radiation on the Earth’s surface increases,the strength of the East Asian summer wind increases.However,there is a lag of about 2-3 ka in the response of the East Asian monsoon strength to changes in Northern Hemisphere solar radiation,because the Northern Hemisphere ice cap was the main control factor of the East Asian monsoon before it melted,and the anomalous freshwater input from the North Atlantic Ocean in the early Holocene weakened the solar radiation-driven strengthening of the East Asian summer wind.(4)The Holocene environmental evolution of the Ulan buh Desert is the result of a combination of climate change,water regime change and human activity.The Yellow River gradually moved westward in the early to middle Holocene under the control of tectonic subsidence in the western part of the Loop Basin,and the ancient lake Tu Shenze developed in front of the Wolf Mountain.The 5-2 ka river channel migrated eastwards to the vicinity of Deng kou,where it continued to move eastwards,and the abandoned river channel gradually formed shallow lakes scattered across the desert massif.Under dry and cold climatic conditions,the lakes began to shrink and disintegrate,and since 2 ka the influence of human activities on the evolution of the regional environment has gradually increased,with predatory exploitation of the land,waste and abandoned tunnels destroying the vegetation and causing the surface to be exposed,accelerating the process of regional desertification. |
