Climate Change Recorded By Sediments From Lake Lugu Since 16.0 Cal.KaBP

Since the Last Deglaciation the climate has changed dramatically,and there are abundant paleo-environmental and paleo-climatic records,which are the main periods for regional correlation,Asian monsoon evolution and catastrophic events to extract the driving mechanism of climate change.The period experienced a period of sharp fluctuations that was similar to modern warming.Therefore,understanding and grasping the change rule of Asian monsoon since the Last Deglaciation has important practical significance for the prediction of climate change with the main feature of warming.Yunnan is a typical Indian monsoon region,whose special monsoon climate features provide good conditions for studying the different response mechanism of Indian monsoon region to global climate change since the Last Deglaciation.The lake sediment contains a large amount of climate and environmental information,which is an ideal carrier to explore climate change.Lugu lake,located at the edge of the southwest monsoon region,is sensitive to the strong and weak changes of summer monsoon.It is also a large lake in Yunnan province with little human disturbance,with water depth and sediment environment stable for a long time.Therefore,it has become one of the hot spots of paleoclimate research in recent years.In this paper,the LGH-2 core of Lugu Lake was studied by using carbonate content,magnetic susceptibility and Sr/Ba,Rb/Sr and Sr/Ca.Based on the results of sporopollen analysis conducted by the research team,the climate evolution history of the Lugu Lake Basin from 16.0 cal.ka BP was reconstructed,and the driving mechanism of paleoclimatic change in the Lugu Lake Basin was analyzed.The following conclusions were obtained:1.The paleoclimate and environment evolution sequence since 16.0 cal.ka BPin Lugu Lake Basin has been reconstructed.1)During the late glacial period(16.0-14.7 cal.ka BP),the Rb/Sr ratio increased rapidly,the carbonate content decreased rapidly,the Sr/Ba ratio decreased,the broad-leaved tree species began to increase and the low-frequency magnetic susceptibility decreased,which indicated that the southwest monsoon increased rapidly,precipitation increased rapidly,lake water level rose,vegetation coverage increased gradually,and surface soil erosion weakened.Generally speaking,the climate changes from cold and dry to warm and humid.2)During the BA(B?lling-Aller?d)warm period(14.7-12.9 cal.ka BP),the Rb/Sr ratio was at a high level and the carbonate content decreased rapidly to about 0,indicating that the southwest monsoon was strong,the water level was high,the lake water outflow,the pine forest expansion,the vegetation coverage was high,and the climate was warm and humid.3)In YD(Younger Dryas)cold period(12.9-11.6 cal.ka BP),Rb/Sr ratio is low,Sr/Ba ratio is high,Sr/Ca ratio is low,carbonate content is high,susceptibility is relatively high,vegetation is scarce,erosion is strengthened,dry and cold-resistant deciduous broad-leaved forests such as birch,elm and birch increase,indicating that the southwest monsoon is weakening,lake water level is decreasing,salinity is increasing,and climate is characterized by cold and dry.4)During the Early-Middle Holocene(11.6-3.5 cal.ka BP),Rb/Sr value was high,Sr/Ba value was low,carbonate content was low,magnetic susceptibility changed little,pine forest expanded and deciduous broad-leaved forest shrank,indicating there were strong southwest monsoon,abundant precipitation,low salinity of Lake water,outflow of Lake water,weak soil erosion,and generally warm and humid climate.5)During the mutation period(3.5-3.0 cal.ka BP),carbonate increased rapidly,Rb/Sr,Sr/Ca ratio decreased rapidly,Sr/Ba increased,hard-leaved evergreen oak forest expanded significantly,while pine forest decreased significantly,indicating that precipitation decreased,water level decreased,salinity increased,and experienced a drying process of about 500 years.6)In the Late Holocene(3.0 cal.ka BP to date),Rb/Sr and Sr/Ca values were low,Sr/Ba values were high,carbonate content was high,and magnetic susceptibility values suddenly increased and maintained high levels;it showed that the southwest monsoon was weak,precipitation was reduced,vegetationwas reduced and human activities caused soil erosion was strengthened;vegetation was obviously thinner than that in the Early-Middle Holocene,the overall temperature was significantly lower,precipitation was less,and climate fluctuations were frequent.Climate change during the last glacial period is consistent with global climate change.During the Holocene,there were 12 cold-dry weather events,11.38-11.1,10.35-10.22,9.4-9.0,8.39-7.66,5.95-5.75,5.3-5.06,4.56-4.13,3.87-3.71,3.0-2.82,2.55-2.30,1.3-1.1 and 0.43-0.25 cal.ka BP.2.Reconstruction of lake water salinity since 16.0 cal.ka BPThe change of lake water salinity has been divided into six stages since 16 cal.ka BP:During the late glacial period(16.0-14.7 cal.ka BP),the Sr/Ba ratio rapidly decreased from 0.6 to below 0.4,indicating that the lake water salinity rapidly fades;the BA warm period(14.7-12.9 cal.ka BP),the Sr/Ba ratio is Small fluctuation between 0.35 and 0.25,water salinity is fresh water;YD cold period(12.9-11.6 cal.ka BP),Sr/Ba ratio is in a low valley area,slightly higher than 0.4,lake water is fresh water;early-middle Holocene(11.6-3.5 cal.ka BP),Sr/Ba ratio is from 0.25 to 0.35,the lake salinity reaches the minimum,the lake water level is high,then the lake belongs to the open lake,3.5 cal.ka BP may have been high before the water level phase;the Sr/Ba ratio reflects an increase in salinity 8 times,which occur at 11.1,10.0,9.2,8.2,6.7,5.6,4.2,and 3.8 cal.ka BP,respectively,in response to climate cold-dry events.Correspondingly,it indicates that the climate is also unstable during this period;in the late Middle Holocene(3.5-3.1 cal.ka BP),the Sr/Ba ratio rapidly changed from 0.3 fresh water to 0.6 in 400 years;Late Holocene(Since 3.1 cal ka BP),the Sr/Ba ratio is always in the high value range of 0.4-0.7,the salinity of the lake water changes frequently and rapidly,and the lake water is relatively salty(0.37,1.14,2.5,3.0 cal.ka BP)and the lake water were relative to weakened for 2times(0.7 and 0.11 cal.ka BP).3.Climate Change Model in Lugu Lake Basin since 16 cal.ka BPDuring the Last Deglaciation,the climate change patterns in East and South Asian monsoon regions were similar,with BA warm and wet period,and YD coldand dry period.After entering the Holocene,the climate pattern changed.During the early Holocene,the precipitation in the South Asian monsoon area was more north than south,while that in the East Asian monsoon area was more south than north;during the Middle Holocene,the precipitation in the South Asian monsoon area was more north than north,and that in the East Asian monsoon area was more north than south.The "seesaw" effect also exists in the eastern and western directions of the two circulation systems: in the southern part of the Early Holocene,the eastern part is less than the western part,while in the northern part,the opposite is true in the Middle Holocene.The climate change in the Lugu Lake Basin has the feature of significant regional uniqueness.The combination of water and heat varies according to different time scales and cannot be generalized.The long-term scale of the Last Deglaciation and the Holocene,the combination of warm-wet and cold-dry characteristics;the long-term scale of the early-middle and late Holocene,may be warm-wet and cool-dry;the Medieval Warm Period and the Litter Ice Age are the combination of warm-dry and wet-cold.4.Analysis of Climate Driving Mechanism in Lugu Lake Basin since 16.0cal.ka BPThe climate evolution of Lugu Lake basin since 16.0 cal.ka BP has a high degree of global consistency.It is driven by summer solar radiation,Indian monsoon and ice changes in high latitudes of the Northern Hemisphere.The cold-dry YD period and nine drought events in Holocene in Lugu Lake basin during the last glacial period correspond to ice raft events in North Atlantic,which indicates that Lugu Lake basin has a strong remote correlation with high latitude areas.In the early Holocene,the precipitation in Lugu Lake Basin reached the maximum,which was consistent with the change pattern of Indian monsoon;in the Middle Holocene,the temperature in the Lugu Lake Basin reached the highest and the precipitation decreased;in the Late Holocene,the temperature decreased and the precipitation decreased sharply,but in the later period,the temperature increased and the precipitation increased.5.The indices of Rb/Sr,Sr/Ba,Sr/Cu using trace element scanning results cansensitively respond to the changes of precipitation,have high resolution(14.15yr)and clear climatic significance,and can capture abrupt climatic events.Therefore,the indices of Rb/Sr,Sr/Ba and Sr/Cu from trace element scanning results are effective ways to retrieve and reconstruct long-scale and high-resolution climatic and environmental changes.