High-resolution Climatic And Environmental Change Since The LGM Recorded By Lake Hala,Northeastern Tibetan Plateau

The northeastern Tibetan Plateau is influenced by Asian Summer Monsoon?ASM?,mid-latitude westerlies,alpine topography effect and Siberian anticyclone,whose moisture sources are closely linked to atmospheric circulations.Although there have been some studies about the relationship between ASM and westerlies in northeastern Tibetan Plateau,long-time and high-resolution climatic records are still rare,especially the alpine lake record.Furthermore,the Last Glacial Maximum?LGM?and Last Deglaciation are the crucial periods for understanding how climatic and environmental changes from the LGM period to modern interglacial period and also how the ASM and westerlies respond to the global climatic change.Lake Hala,as the alpine close lake in the backland of Qilian Mountain,was selected here based on modern catchment study,obtaining short cores at different water depths and long cores in deep water area;the dating methods of ²¹⁰Pb and AMS-¹⁴C were adopted seperately and the chronology frame was built after evaluating the ¹⁴C age reservoir effect;combined with proxy significances of modern sediment,the supply water sources,lake level and evolution of ASM and westerlies since 21.5 ka were recovered with the continuous and high-resolution XRF-element,grain size,carbonate content,TOC,?¹⁸O_carb,?¹³C_crab,C/N ratio and?¹³C_org multi-proxies;and compared different lake sediment records at different altitudes in northeastern Tibetan Plateau for further understanding the regional climatic and environmental changes.The?¹³C_org and<19?m grain size content of lake sediment reflected the lake level changes together.The carbonate content and?¹⁸O_carb of lake sediment indicated summer precipitation and summer precipitation/glacier?snow?melt water amount ratio respectively,and one of summer precipitation and temperature increased implied stronger ASM.The carbonate content and?¹⁸O_carb had the highly same trend with Atlantic Meridional Overturning Circulation?AMOC?,indicating that the influence of AMOC on climatic changes in Lake Hala catchment was stronger than that of Intertropical Convergence Zone?ITCZ?and El Ni?o-Southern Oscillation?ENSO?,so the climate in Lake Hala catchment was influenced mainly by westerlies.The climatic and environmental changes in Lake Hala catchment since the LGM were as followings:?1?in the LGM period?21.5¹9.4 ka?,the cold and dry climate was dominated by westerlies,and the lake was supplied mainly by glacier?snow?melt water,resulting in low lake level with obvious fluctuation;?2?in the last deglaciation period?19.4¹1.6 ka?,the climate and environment fluctuated obviously and recorded the main climate events in north Hemsphere.ASM reached the Lake Hala catchment during 19.4¹7.9 ka with warm and humid climate,and the lake was supplied mainly by summer precipitation,which made lake level increase and evenly close to modern lake level;westerlies in H1 event?16.4¹5.3 ka?and YD event?12.8¹1.6 ka?periods was strengthed with more winter snow,which was stronger in H1 event period,resulting cold,humid and cold,dry climate,making the lake supplied mainly by glacier?snow?melt water and the lake level decrease;in B/A warm period?15.3¹2.8ka?,the glacier?snow?melt water increased dramatically,but summer precipitation did not show increasing trend,so weaker ASM and weaker westerlies existed at the same time with warm and dry climate.Abundant glacier?snow?melt water generated high lake level and formed the deep still environment;?3?at the beginning of Holocene?11.6 ka?,ASM reached the Lake Hala catchment and increased gradually,but the lake was still mainly fed by glacier?snow?melt water,making lake level increase rapidly and close to modern lake level in early Holocene?11.6⁹.3 ka?.ASM became the strongest in early middle Holocene?9.3⁶.0 ka?corresponding to the Holocene optimum,and the lake was supplied mainly by summer precipitation with warm and humid climate;ASM decreased slowly in the middle late Holocene?6.0².0ka?,the summer precipitation decreased and the lake was still mainly supplied by summer precipitation,with relative warm and humid climate;in the late Holocene?after 2.0 ka?,westerlies increased with more winter snow and made the lake supplied mainly by glacier?snow?melt water,with relative cold and wet climate.The lake level maintained high levels in the whole Holocene,so we inferred that the input water and evaporation amount affected by temperature were generally in the balance state.The regional comparision indicated that the alpine lake and norther lake in northeastern Tibetan Plateau were much more affected by westerlies than that in lowland,therefore the differences of precipitation recorded by different lake sediment were attributed to the influence of locat atmosphere circulation.