Climate Change Since The 12 Ka Recorded In The Sediments Of Lake Fuxian

The Holocene is the latest interglacial period and a period of highly developed human civilization,the climate change and its driving mechanism since the Holocene are the hotspot and keystone of the researches on the Quaternary climatology.It plays an important role in exploring the evolution law of paleoclimate,paleoenvironment and the rise and fall of ancient civilizations,also it is of great significance to predict the future climate development.Lakes can store all kinds of information from the atmosphere,basin surface and lake water,which are ideal objects for studying changes of regional paleoenvironment because of their stable depositional environment and high resolution.The Lake Fuxian is located in the central Yunnan Province in the southwest monsoon region of China,with long forming ages and continuous lake sediments,which is less influenced by human activities and has unique geographical location and regional environment,so lake sediments are ideal objects for researches on paleoclimate change.In this paper,the Lake Fuxian in central Yunnan Province is taken as the research object,296 cm long sediment cores were drilled from the depths of about80 m in the south of the Lake Fuxian,with sampling at interval of 0.5 cm from 0 to50cm and 1cm from 50 to 296 cm,so as to analyze the environmental proxies such as carbonate,particle size,calcite,quartz,Ca,Si and Sr/Rb ratios in detail,and discusses with the research results on stable isotope of carbonates.Nine?AMS?¹⁴C dating data obtained were corrected by Int Cal13 and converted to calendar age by corresponding calculation,thus rebuilding the history of environmental change since12 cal.Ka BP in Fuxian lake basin and the change process of the Lake Fuxian.?1?At the period of 12-8.85 cal.ka BP,the content of Carbonate substances and Sr/Rb ratio in the sediments of Lake Fuxian are increasing and has gradually risen from its minimum since the Holocene.The content of element Si shows the change of rise-steady-decreases again;The change of median particle diameter is not obvious.It shows that the Lake Fuxian Basin has been in a warm and humid climate period of early Holocene warming since the last deglaciation to the early Holocene,with the decrease of water level,precipitation reduction in the watershed,weathering erosion weakening and southwest monsoon intensity weakening.At the period of 8.8-4.4 cal.ka BP,the content of calcite,CaCO₃,relative content of Ca and Sr/Rb ratio all increased compared with the previous stage,the content of Si fluctuated greatly and the content of quartz increased slightly;It shows that the Lake Fuxian Basin has been in a climatic period of continuous warm and dry,with temperature continuing to rise,precipitation continuing to decrease,lake water level declining,the intensity of southwest monsoon increasing first and then decreasing.During this period,there was a short climatic event of climate cooling and monsoon weakening in the Fuxian Lake Basin at 5.1 cal.ka BP.At the period of 4.37-2.0 cal.ka BP,the sediments indexes of the Lake Fuxian have changed greatly and the environment of the basin has changed obviously.Especially during the period of2.2-2 cal.Ka BP,the climate changed dramatically from the warmest and driest since 12 Ka to near modern humidity.During this period,human civilization had risen in the Fuxian Lake Basin,but the impact of human activities on the environmental changes in the watershed was not significant.The change of southwest monsoon may be the main controlling factor affecting the environmental change in the watershed,which reveals the particularity and suddenness of regional precipitation in the southwest monsoon control region.Since 2 cal.ka BP,the relative content of CaCO₃,element Ca,calcite and quartz,Sr/Rb ratio and median particle diameter at the period are the lowest values of the whole borehole.It shows that the depositional environment of the Lake Fuxian is stable after rapid rise of lake water,which is the period that water level is highest and maintained for a long time since the Holocene,and also the period with the wettest climate.The obviously reduced vegetation coverage and human activities have very significant impact on the watershed environment.?2?The carbonate substances in sediments of the Lake Fuxian is mainly calcite?Relative content of other carbonate minerals is extremely low or even negligible?,Ca in the sediments mainly comes from carbonate,so its content is determined by the content of carbonate,that is,calcite minerals.The variation of carbonate content in sediments can be expressed by the results of XRF scanning of Ca in sediments,which makes it possible to reconstruct the evolution history of lakes with high resolution by using XRF scanning data of Ca in lake sediment cores in carbonate areas.?3?Analysis of climate driving mechanism since holocene in fuxian lake basin:climate since holocene in xianhu basin is mainly affected by solar radiation,Indian monsoon,plateau topography and atmospheric conditions,showing the particularity of regional precipitation and abrupt climate characteristics in India's seasonal risk control region.The climate evolution of fuxian lake basin recorded in FXH-B2borehole over the past 12,000 years is basically consistent with the southwest monsoon and other records.Based on this assumption,the reconstructed lake water level changes during last 12 ka demonstrates a gradual decrease 12 cal.ka BP to2.2cal.ka BP.During this processes,the change of all indexes indicated that the water reveal a lowest lake wate lever period between 4.37-2.2 cal.ka BP,during this period the water level decreased by modern lake,indicating a driest event since 12cal.ka BP.At abrupt water level rising occurred between 2.2-2 cal.ka BP and reached the level almost continuous to the present.