Environmental Changes Recorded By Multiple Indicators In Lake Aksayqin,Northwestern Tibetan Plateau

Lake Aksayqin,a typical big lake located in the high altitude and arid area of the northwestern Tibetan Plateau,is an ideal lake for studying changes in the Westerlies and Indian Summer Monsoon and the interplay between them.In this paper,taking Lake Aksayqin in the high-altitude arid area of the northwestern Tibet Plateau as the research object,the relationship between lake area and climatic factors was established by extracting the changes of lake area and climatic factors in the past 30 years.A 5.31m-long lake sediment core(AKLC15-1)was obtained at 16.4 m water depth of Lake Aksayqin,under accurate dating techniques,a comprehensive analysis of sedimentary composition and lithologic characteristics,combined with high-resolution elements,grain size,and geochemical substitution indexes,the lake level changes of Lake Aksayqin in the northwest of Tibet Plateau in recent 6000 years have been reconstructed.By comparing the differences between ancient and modern times,the climatic and environmental changes of Lake Aksayqin Basin since the middle and late Holocene have been reconstructed.The main results are as follows:(1)The area of Lake Aksayqin gradually increased in the past 30 years,and the lake area changed slowly before 2000 and increased rapidly after 2000.The area change of Lake Aksayqin is compared with the meteorological data of the surrounding meteorological observations.The increase in the area of Lake Aksayqin in the past 30 years is caused by the increase of glacier meltwater caused by the increase of temperature,the increase of precipitation,and the decrease of evaporation.Through multiple linear regression analysis,it is known that temperature,precipitation,and evaporation contribute 69% to the area change of Lake Aksayqin,in which daily average temperature > daily average precipitation > daily average evaporation,that is,air temperature is the main reason for the change of Lake Aksayqin.(2)The evaporation in the Lake Aksayqin area is strong.The lake water is mainly supplied by glacier meltwater.The content of organic matter in lake sediments is low,which mainly comes from lake endogenous aquatic organisms.The sediment of Lake Aksayqin is dominated by silt,followed by clay,with the least sand content,with an average content of 70.48%,27.64% and 1.88%,respectively.(3)Combined with the multi-proxy analysis of Lake Aksayqin,it is concluded that the process of lake level change and basin environment change is as follows: In the first stage(6000～4400 cal yr BP),the climate is relatively warm,the evaporation of the basin is weakened,a large amount of glacier meltwater continues to supply lakes,and the aquatic productivity of lakes is low,which is a deep-water environment with weak hydrodynamic transport conditions.In the second stage(4400～1800 cal yr BP),the climate of the lake basin is cold and dry,the recharge of glacier meltwater decreases,the water level of the lake is low,and the aquatic productivity of the lake increases,which is a shallow water environment with strong hydrodynamic transport conditions.The multi-proxy changes indicate that three rapid extreme cold events(4400～3400 cal yr BP,3000～2600 cal yr BP,2400～2100 cal yr BP)have occurred in this stage.In the third stage(1800～0 cal yr BP),the climate of the basin gradually warms,the amount of water entering the lake is more,the lake surface expands,the lake turns to the deepwater environment,and the hydrodynamic transport conditions of the lake gradually weaken.(4)The area of Lake Aksayqin decreased at first and then increased in the middle and late Holocene.Figure [29] Table [11] Reference [137]...