Environmental Changes In Western Mongolia Since The Last Glacial Maximum Recorded By Lake Tal

The western Mongolia region is located in the mid-latitude Eurasian continent and has a significant temperate continental climate due to its distance from the ocean and the strong influence of the Siberian-Mongolian high-pressure system.This regional ecosystem is vulnerable and sensitive to regional climate and environmental changes.Although previous studies have investigated the Holocene climate and environmental changes in this region using various geological archives and proxies,there are still significant discrepancies in the evolution of humidity,for example,most records show a dry early Holocene,while some reconstructed a dry middle Holocene.Furthermore,there is a lack of long-term climate records spanning the LGM in the region.Therefore,this study selected Tal Lake,a high-altitude lake in western Mongolia with an altitude of 2538 m,as the study site.A 3.9 m sediment core（TA19A）was collected and analyzed for 17 AMS¹⁴C dates（14 aquatic plant residues and three organic samples）to evaluate the carbon reservoir effect.Based on the reliable chronological framework,paleoenvironmental proxies such as pollen and grain size were used to reconstruct the climate and environmental changes in western Mongolia during the past～22 ka（1 ka=1000 cal yr BP）.By comparing the results with other climate records in adjacent areas,the possible driving mechanisms were analyzed.The main conclusions are listed below.（1）The pollen analysis results indicated that during the Last Glacial Maximum（LGM,22-19 ka）and the late Glacial,herbaceous pollen dominated the pollen assemblages,with very low percentages of tree pollen and total pollen concentration,indicating sparse vegetation,and a low A/（C+E）ratio indicated drier.During the last deglaciation（19-11.7 ka）,the A/（C+E）ratio and pollen concentration increased,indicating an increase in vegetation coverage and relative humidity in the region.Since7.8 ka,the percentage of tree pollen has rapidly increased and reached its peak during7.8-5 ka,indicating that the regional climate was relatively favorable,with strong summer solar radiation and a better water and heat combination.The A/（C+E）ratio showed an increasing trend after～6 ka,indicating an increase in effective humidity in the region.（2）The grain size analysis results indicated that the sedimentary environment of Lake Tal was relatively stable during the LGM and the late Glacial,with a high lake level.From the beginning of the Holocene to～7.8 ka,sand layers appeared in core TA19A,and the core chronology showed a dramatic jump（the depth from 185-173 cm spanning～4160 years）,suggesting possible sedimentary hiatus.The proportion of coarse-grained components（>63μm）increased sharply,indicating that the core location was closer to the lake shore,and the lake level was low,which was related to the early Holocene aridity in the westerlies core area.Since～7.8 ka,the proportion of fine-grained components in the core gradually increased,indicating a fluctuating rise in the lake level.（3）Comparison with humidity records from the dominant westerly zone during the Last Glacial Maximum（LGM）reveals that the humidity changes in western Mongolia recorded in Lake Tal are similar to the patterns in the westerly-dominated zone.This suggests that the region where Lake Tal is located is mainly influenced by the mid-latitude westerlies,and that its water vapor content and sources are affected by the combined impact of winter and summer solar radiation,the north-south movement of the subtropical high-pressure system,and the Siberian-Mongolian high-pressure system.These factors have contributed to a gradual increase in humidity in the region since the Holocene.（4）By comparing the lake level changes recorded in lakes on the marginal areas of the Asia summer monsoon with those in the transitional zone where Lake Tal is located reveals significant differences in hydro-climatic changes and exhibits opposite trends.The high lake level of Lake Tal during the Last Glacial Maximum（LGM）is related to weak evaporation and long freezing time,and the high lake level during the last deglaciation period is to some extent replenished by melting water from ice and snow.During the early Holocene,strong summer solar radiation intensified the lake water evaporation,and the supply of glacier melting gradually decreased,leading to a sharp drop in the lake level.Unlike the reasons for the high lake levels before the Holocene,the high lake levels during the middle and late Holocene were mainly due to an increase in regional precipitation or effective humidity.The phase changes of Arctic Oscillation and North Atlantic Oscillation also had a certain impact on the precipitation or effective humidity in arid central Asia.