Variations Of The East Asian Summer Monsoon Since The Last Deglaciation Recorded By Dali Lake In Northern China

As an important part of the global climatic system,the East Asian summer monsoon(EASM)and its variations are the focus of academic research.The spatial distribution of precipitation caused by the EASM fluctuations is closely related to the social economy,ecological environment,and water resources of China and its surrounding areas,and has a profound impact on the production and life of billions of populations.An accurate understanding of the history of the EASM,its ecological and environmental effects,and driving mechanisms,thus is helpful to reveal the evolution of human-environment interactions and adaptation mechanisms,and provide scientific reference for the interpretation of modern climate and the prediction of future climate change.Over the past few decades,the evolution of the EASM on different timescales has been extensively studied by means of observation data,historical documents,geological records and climate models.However,(1)Can lake-level change in the EASM margin represented by Dali Lake characterize the intensity of the EASM?(2)Whether the EASM strengthened or weakened during the Younger Dryas cooling period(YD,?12.9-11.7 ka)?Whether the maximum period of the Holocene EASM occurred in the early Holocene(EH)or the middle Holocene(MH)?(3)Can Chinese cave oxygen isotope(?18O)records represent the EASM precipitation?(4)Whether the Holocene climatic optimum(HCO)occurred synchronously throughout the EASM region,and if it did,whether it occurred in the EH,or in the MH?In addition,studies of variations of the EASM during modern and geological period reveal that when the EASM strengthens,precipitation increases in northern China.On the contrary,precipitation in northern China decreases.Meanwhile,northern China is the core region of the development of Chinese civilization.Therefore,northern China is the key region for the study of the EASM,and the definition of the EASM intensity based on the amount of precipitation in northern China not only has clear dynamic significance,but also has environmental archaeological significance.Taken together,Dali Lake(43.3°N,116.6°E,Altitude=1223 m),a closed lake located in northern China and sensitive to the change of the EASM,was selected as the research object.(1)On the one hand,based on the drilling of lake sediments in the center of Dali Lake and the investigation of paleolake shorelines in Dali Lake catchment,a sediment core(DL19B)that covers the past?15.5 ka and 47 optically stimulated luminescence(OSL)dating samples from 11 paleolake shorelines were obtained.Through the analysis of a variety of sedimentary records from the sediment core and the OSL dating on the paleolake shorelines,the history of regional hydrological changes in Dali Lake catchment since the last deglaciation was reconstructed.(2)On the other hand,based on the development and verification of the reliability of the“climate-vegetation-hydrology" coupled lake water balance model in Dali Lake under modern conditions,the variation of lake area in Dali Lake reconstructed by paleolake shorelines was coupled to the lake water balance model,and the precipitation change of Dali Lake catchment since the last deglaciation was simulated.Through the comparison of sedimentary records of Dali Lake with simulated precipitation,combined with published high-resolution climatic records in East Asia,the variations of the EASM since the last deglaciation and the timing of the HCO were explored.(3)Finally,based on continuous monitoring of modern processes of ?18O in lake water and precipitation in different locations of the EASM region for five years,and by comparing the carbonate oxygen isotope records(?18Ocarb)of Dali Lake sediments,the Chinese cave ?18O records,and the precipitation variations in Dali Lake catchment simulated by lake water balance model,combined with the accurate understanding of the evolution history of the EASM since the last deglaciation,this paper further discusses whether the Chinese cave ?18O records can accurately reflect the EASM precipitation changes.Based on the above work,the main findings are summarized as follows:(1)History of lake-level changes in Dali Lake since the last deglaciation:Based on the new OSL-based chronological data with the previously published ages of beach ridges,we constructed an integrated lake-level record for Dali Lake since the last deglaciation,which is more accurately dated and provides a clearer representation of lake-level changes than previous studies.The results indicate that the level of Dali Lake rose gradually during the last deglaciation while decreased obviously during YD period on the millennium timescale.During the EH,the level of Dali Lake increased further,and presented a highstand during?11.7-10.0 ka,and then fluctuated frequently during?10.0-8.0 ka.The level of Dali Lake reached a maximum during the MH,which was?60 m higher than today;the level then gradually declined after?6.0 ka.(2)The record-model integration confirms that the sedimentary records from Dali Lake represents the intensity of the EASM:In this study,based on the reliability of the newly developed lake water balance model under modern conditions,the variation of lake-level(area)reconstructed from the paleolake shorelines of Dali Lake was coupled to the lake water balance model to simulate the history of precipitation change since the last deglaciation in Dali Lake catchment.The results indicate that the variation of lake-level reconstructed by paleolake shorelines of Dali Lake is in good agreement with other climatic records from lake core and the simulated precipitation,which proves that the variation of lake-level of Dali Lake can be used to accurately reflect the evolution history of the EASM.(3)Evolution history of the EASM since the last deglaciation:On the basis of confirming that the sedimentary records of Dali Lake can represent the variation of the EASM,we further compare the lake-level record of Dali Lake with the published precipitation records from northern China to reveal the variation history of the EASM since the last deglaciation.Since the last deglaciation,the EASM gradually increased,and the intensity of the EASM weakened during YD period on the millennium timescale;At the beginning of the Holocene,the EASM gradually strengthened;During the MH,both the sedimentary records from Dali Lake sediments and the precipitation changes in Dali Lake catchment revealed by water balance model show that the EASM reached the maximum.The characteristic of maximum EASM during the MH is also supported by many quantitative precipitation reconstruction records in northern China;During the late Holocene,the EASM gradually weakened and the regional precipitation gradually decreased.(4)Chinese cave ?18O records cannot be used as an indicator of precipitation change in northern China:By observing modern processes of oxygen isotopes in lake water(?18OL),we revealed that during the non-monsoon season,the variation of ?18OL exhibits a variety of patterns and the controlling factors across the EASM region are complex.During the monsoon season,although the effect of evaporation-induced isotopic enrichment still affects ?18OL,the variations in ?18OL are more influenced by the supply of abundant precipitation with lower precipitation oxygen isotope(?18OP)to the lakes,which caused a substantial decrease in ?18OL in this season.Further,through the analysis of the modern process change of ?18OP,it is found that the change of ?18OP may be mainly related to the variations of moisture source.Based on the analysis of modern observations,and taking into account the fact that the ?18Ocarp in Dali Lake sediments and Chinese cave ?18O records essentially inherited the signal of ?18OP,and Chinese cave ?18O records have a good consistency with ?18Ocarb in Dali Lake sediments on the millennium timescale,but are significantly different from the EASM precipitation evolution history.We proposed that Chinese cave ?18O records may mainly represent the variations of moisture source,which response sensitively to largescale atmospheric circulation changes,but cannot serve as the indicator of the Holocene precipitation change in northern China.(5)The stable MH climate can be used to define a synchronous HCO in East Asia:Based on the instability of the EASM during the EH revealed by the sedimentary records of Dali Lake,this paper further evaluates the stability differences of the EASM during the EH and MH by collecting 23 high-resolution EASM records.The results indicate a more stable climate which occurred synchronously during the MH in the EASM region.Thus,we proposed that the stable MH climate can be used to define a synchronous HCO in East Asia.(6)Influence of the EASM on prehistoric culture:From the perspective of climatic stability,the stable climate of the MH provided an optimum environment for the development of agriculture and animal husbandry,which contributed to rapid population growth and the flourishing of the Yangshao culture which was the most prosperous Neolithic culture in the EASM region of China during the Holocene.Thus,we proposed that the stable MH climate can be used to define a synchronous HCO which was more supportive of societal development and population growth than that during the EH.From the perspective of abrupt climate change,the pattern of“drought in northern China,flooding in central China" occurred at?4.2 ka,triggering the migration of the prehistoric human population of the area to the central Plain of China.This population migration may have destabilized the existing social order and promoted the emergence of more complex societies,leading to the development of early civilization in north-central China.(7)Possible driving mechanisms of the EASM:On the orbital timescale,the evolution of the EASM was mainly controlled by the Northern Hemisphere summer insolation(NHSI),but due to the AMOC changes caused by the existence of global ice volume during the last deglaciation and the EH,the EASM was suppressed,which led to the lag response of the EASM to NHSI during the Holocene;As for the possible mechanisms of the difference in climate stability between the EH and MH,we proposed that with the retreat of the Northern Hemisphere ice sheets,the variability of the westerly jet(WJ),which was previously perturbed by large ice sheets,was significantly reduced from the EH to the MH.This reduced WJ variability during the MH enhanced the stability of the climate of the EASM region,via the interactions of the EASM and the WJ.