Vegetation And Climate Changes During The Holocene Reflected By Lacustrine Recoreds In Chagan Nur,Otindag Sandy Land

The study of Holocene climatic change in the monsoon marginal zone of north China is of great significance for understanding evolutionary laws and driving mechanism of East Asian summer monsoon.Because the Otindag sandy land located in Monsoon marginal zone which is sensitive to climate changes,so the study of Holocene vegetation and climate change will help us understand the temporal and spatial patterns of Holocene climate change and acclimated climate dynamics in the monsoon marginal zone.In this paper,on the basis of the AMS¹⁴C dates,we use the pollen,grain-size and pediastrum records of the lake core of the Chagan Nur?CG core?in Otindag sandy land and reconstructed the vegetation and climate changes in Chagan Nur during last twenty thousand years.Furthermore,temporal and spatial patterns and driving mechanism of Holocene climate change in the monsoon marginal area were discussed through comparison analysis.The main conclusions are as follows:?1?Under the control of 15 AMS¹⁴C dates and reliable Bayesian chronology age model and on the basis of pollen,grain size and Pediastrum proxies data,the sedimentary environment,vegetation and climate change during past 20,870 cal.yr BP were reconstructed.Specifically,during 20,870¹5,830 cal.yr BP,cold desert developed under extreme cold and arid climate,and aeolian sedimentary environment dominated.Between 15,830 cal.yr BP and 14,160 cal.yr BP,the vegetation changed into steppe desert and the climate ameliorated a little in spite of that the aeolian sedimentary environment continued.During 14,160¹1,400 cal.yr BP,alluvial facies dominated and the vegetation deteriorated into desert under extreme dry climate.During the 11,400⁸880 cal.yr BP,the lake developed and gradually transgressed under gradually wetting climate,and resultantly the vegetation changed into desert steppe.Between 8880 cal.yr BP and 6270 cal.yr BP,woodland steppe developed and mega-lake sustained under extreme wet climate.The 6270²950 cal.yr BP witnessed a lake regression and regional vegetation deteriorated into steppe desert,the climate was dry.During 2950¹280 cal.yr BP,lake environment sustained and the precipitation slightly increased,the vegetation changed into desert steppe.Since the 1280 cal.yr BP,the water level of the lake fluctuates drastically with high level in times broadlycorresponding to Medieval Warm Period and low level in time corresponding to the Little Ice Age,in general the climate was a little wet and the vegetation is steppe with exception of desert steppe during times corresponding to LIA.?2?On the basis of reliable chronology,the pollen assemblages sequences of the CG core clearly recorded climate fluctuations during past 2000 years.It is notable that the pollen-based precipitation and Artemisia/Chenopodiaceae ratio?A/C?both recorded a depression in times corresponding to LIA and peaks in times corresponding to MWP.The comparison may suggest that the climate fluctuations disclosed in Chagan Nur may be of significance in extro-regional and global scale.?3?Put the CG climate sequences under the context of the forcing and boundary sequences including NH summer insolation,the oscillation of Arctic Oscillation/North Atlantic Oscillation?AO/NAO?and the tropical Pacific El Nino and the Southern Oscillation?ENSO?,climatic dynamics were discussed.In detail,suppressed by the high latitude ice cover and associated changes in sea level,land cover in early Holocene,the warming of the Asian interior was delayed,and therefore the Asian Low was inhibited.This lead to the weakthermal and pressuredifferences between sea and land and thus hinder the enhancement of the East Asian summer monsoon.During the early-middle Holocene?8500⁵500 cal.yr BP?,in response to the still higher northern hemispheric summer insolation,the thermal conditions of the Asian interior increased and thus the Asian Low?AL?deepened.At the meantime,the AO/NAO tend to be positive and the ENSO tend to be negative?i.e.,more La Nina?,and this lead to the weakening of Siberia High and the increasing in SST of the tropical west Pacific Ocean.This enforced the Hadley circulation and thus enhanced the West Pacific Subtropical High?WPSH?.As result,the pressure contrast between the AL and the WPSH enlarged and thus boosted a strong Asian Summer Monsoon.In middleto late late Holocene,in response to the decrease of NH summer insolation,the thermal condition of the Asian interior decreased and thus the AL shallowed,the AO/NAO tend to be negative and the ENSO tend to be more positive,the Hadley circulation weakened and thus the WPSH decreased.Those decreased the thermal and pressure differences between sea and land and thus weakened the Asian summer monsoon.