Evolutions Of The Lake Level And Paleoclimate And Paleoenvironment In Dali Lake During 17.8-6.8 Ka BP

According to OSL ages of thirteen samples from sedimentary section DLB which is situated at the northwest shore of Dali lake,we established the chronosequence of the sedimentary section which is 17.8-6.8 ka BP.In addition,based on analyses of environmental proxies,including grain size,loss on ignition,color,elemental content,pollen and ostracoda,the evolution of the lake level and paleoclimate and paleoenvironment changes in Dali Lake during17.8-6.8 ka BP was reconstructed.The results show:At ~17.8 ka BP,the sedimentary section turned from alluvial deposition to typical lacustrine sediment,indicating the Dali Lake entered a period of relatively high lake level.During 17.8-13.5 ka BP and 13.1-12.2 ka BP,the compositions of sediments consisted mainly of sand and contents of soluble elements were relatively high,indicating relatively high lake levels.In these two stages,pollen content in the sediment was mainly herbs,the contents of organic matter and carbonate were relatively high,suggesting that the regional climates were becoming warm.The fact indicates that the rise of the lake level since the 17.8 ka BP was likely due to snow and ice ablation in the catchment caused by increased temperature after entering the last deglaciation from the last glacial maximum.However,during the last deglaciation relatively lower lake levels occurred at13.5-13.1 ka BP and 12.2-11.8 ka BP,indicated by the large median grain size of sediment and low chemical weathering rate.During these two stages,the contents of pollen,organic matter and carbonate were remarkable lower,implying that the regional climate was relatively cold.Compared with the global climate change records,it can be found that the two periods corresponded to the two Dryas Events.Hence the dropped lake levels during these two stages were likely due to large amounts of regional precipitation stored in ice and snow caused by relatively low temperatures during the two events.In 11.8 ka BP the lake level rose again,indicated by relatively larger median grain size and higher content of soluble elements in the sediment.During 11.8-7.6 kaBP,the contents of organic matter and carbonate were relatively high,the pollen content also increased significantly,suggesting that the lake level rise during this period was caused by warmed regional climate and increased effective precipitation.In the early Mid-Holoence(7.6-6.8 ka BP),the median grain size reached the minimum and the clay content reached the peak,indicating the highest lake level during the studied period.Compared with other typical lakes in adjacent regions and global climate records,it can be found that the hydrological evolution of the Dali Lake was consistent with other lakes during 17.8-6.8 ka BP.More importantly,during the two Dryas Events,the lake level showed rapidly and significantly declining trends,implying temperature changes significantly affecting the lake level.This further indicated that the fluctuations of lake level during the last deglaciation were mainly related to ice and snow melt which was controlled by temperature changes.However,after entering the Holocene,owing to the melted ice and snow in the catchment,the fluctuations of the lake level were mainly affected by the monsoon precipitation changes.