Holocene Lake Environmental Changes In The East Asian Summer Monsoon Limit Revealed By Lake Geomorphy And Lacustrine Sediment Core Record From The Wulanhushao Lake Basin

In this study,we selected Wulanhushao Lake,a typical closed lake located in the eastern part of Inner Mongolian Plateau,in the Asia monsoon-westerly transitional region,as the research area.Through conducting the lake geomorphological surveys and eulimnetic core drilling,we have collected lacustrine-aeolian stratigraphic profile samples,with the lacustrine sediment core.Quartz OSL dating was used to establish the age framework for lake landform sedimentary profile.And Quartz OSL dating and AMS ¹⁴C dating were used and cross-validated to establish a reliable chronosequence for lacustrine sediment sequence.In addition,we carried out a variety of environmental substitution indexes,such as pollen,grain size,carbonate,loss of ignition,element,chroma,magnetic susceptibility,to reconstruct the evolution history of palaeosedimentary environment and watershed vegetation semi-quantitatively.Consequently,we integrated geomorphologic records and lacustrine sedimentary records to reconstruct the lake evolution and environmental change process during the Holocene on millennium scale of Wulanhuolihu Lake,and discussed the change of east Asian summer monsoon and its possible driving mechanism.Our results suggest:?1?The quartz luminescence signal analysis of 5 lacustrine-aeolian sedimentary sections around Wulanhushao Lake shows that quartz OSL dating can be used as establishing the reliable chronological framework for eolian sediments in this area,and all quartz OSL ages fell into the Holocene.The cross-validation result of the core's quartz OSL ages and full-sample organic AMS ¹⁴C ages,which removed carbon library effect,shows that quartz OSL ages has been significantly underestimated,possibly due to inaccurate water content estimation in core sediments.It is more appropriate to choose AMS ¹⁴C age to construct chronosequence for lacustrine sediment sequence.?2?The periodical history about lake level changes of Wulanhushao Lake we reconstructed according to the section sedimentology,Pediastrum,Fe/Mn and some other indicators.The results show that the development time of Wulanhushao Lake was earlier than¹0.1 ka,during the early-middle Holocene?¹0.1-7.4 ka?was in a relatively stable high lake level,which approached to¹414 m a.s.l.,above the modern lake level¹2 m.Lake level began to decline from the middle Holocene?⁷.4 ka?,the high elevations around the lake has entered aeolian sedimentary environment completely about 6.8 ka.Since⁶.4 ka the lake level has dropped to below¹411 m a.s.l.,that is the water depth is<9 m,and never rose to this elevation again.During the late Holocene?after¹.5 ka?,the water level had decreased¹404m a.s.l,becoming a shallow lake with a water depth of<2 m.?3?The paleoclimatic and paleoenvironmental evolution process we restored,based on a variety of environmental indexes from geomorphic sedimentary records and lacustrine sedimentary records,show that during the Last Deglaciation?¹6 cal ka BP?,manifested as dry and cold climate,with rare precipitation and very weak chemical weathering,appeared the aeolian sedimentary environment.The early-middle Holocene?⁹.8-6.7 cal ka BP?was the climatic optimum of the Holocene in this region,which has abundant precipitation,high effective humidity,significant chemical weathering and leaching,and the highest lake productivity and biomass.During the middle-late Holocene?⁶.7-1.5 cal ka BP?,the effective humidity has begun to decreased,the environment significantly tended to drought,with the decreased the chemical weathering and productivity.After¹.5 cal ka BP,the trend of environmental drought was retarded.?4?The vegetation succession history reconstructed based on the core palynology records show that since the Last Deglaciation,Artemisia and Chenopodiaceae have always occupied an absolute advantage in the vegetation composition in Wulanhushao basin,and the pollen content of trees and shrubs has always been very low.According to the pollen concentration and combination,in the Last Deglaciation?¹6 cal ka BP?was desert grassland vegetation,with very low vegetation coverage.During the early-middle Holocene?⁹.8-6.7 cal ka BP?was a typical temperate grassland,the pollen content of trees and shrubs increased,and the vegetation coverage and diversity ware the highest stages in the whole Holocene.Since⁶.7cal ka BP,vegetation coverage and diversity started decreased,until the late Holocene?¹.5cal ka BP?some desert plant types increased,presenting temperate steppes with low vegetation coverage.?5?Compared with the palaeoclimatic and palaeoenvironment changes and vegetation succession in Wulanhushao basin with some other environmental records from the other region,show that the warm and wet climate and high lake level during the early-middle Holocene and the environmental deterioration during the middle-late Holocene,which indicated the intensity change of east Asian summer monsoon reached the highest in the early-middle Holocene?⁹.8-6.7 ka?,and then continued to weaken in the middle-late Holocene?after⁶.7 ka?.This result indicates that the variation of east Asian summer monsoon precipitation laged behind the northern Hemisphere summer insolation in the monsoon fringe region,which may be influenced by the high latitude driving regulation.