High Resolution Environmental Changes In Jianghuai Region Over The Last 1,800 Years Recorded In Lake Sediments

Climate change in the past millennium(or past two millennium),as the focus of the Past Global Changes(PAGES)in the International Geosphere-Biosphere Programme(IGBP),has always been the focus of the academic research worldwide.The period of the past one or two millennium is also charcterisized by the co-existence of historical documents and geological records and the rapidly increasing human activities.Therefore,it is critically important to study the climate change in these periods and its relationship with human activities because it can help us better understand the mechanism of climate change and the relationship between the human and environment.The southern China that has the large population density and large number of economic industries is an ideal study area for investigating the monsoon precipitation reconstruction and human-activity responses in the past millennium.In the past few decades,there are increasing numbers of paleoclimatological and paleoenvironmental reconstructions together with a deepening understanding of the related research.However,different scholars have different viewpoints on the dry-wet evolution mechanism of the two typical climatic periods in southern China during the past millennium.The pattern of the precipitation in the past millennium in southern China is still unclear,mostly due to the scarcity of high-quality and reliable proxies,the age errors of sedimentary sequences,and the ambiguous interpretation of the paleoclimatic proxies.Therefore,it is urgent to find accurate and high-resolution geological records and alternative reliable indicators of precipitation variations in southern China;it contributes to making a breakthrough in the precipitation reconstructions in the past millennium in southern China.In this study,the sediments of Nvshan Lake in the Jianghuai region of southern China are investigated.A variety of dating materials are selected for comparative analysis to build an accurate chronology,and a variety of precipitation indicators are tested for the mutual verification.We verify the relaiblity of the precipitation proxies used in this study by comparising them with modern meteorological observation data.Then,the high-resolution record of monsoon precipitations over the past 1800 years in the Jianghuai region is reconstructed.We also compared other precipitation records in eastern China with the Jianghuai region precipitation records and then discussed the spatial pattern of precipitation changes in eastern China in the past thousand years.Based on the data analysis of the modern atmospheric circulation process in the East Asian monsoon region,the driving mechanism of the precipitation change in the past millennium in the Jianghuai region is discussed.Furthermore,based on the close relationship between the precipitation in the Jianghuai region and the sea surface temperature(SST)in the tropical Pacific,the spatial pattern of the precipitation change in the past millennium in the eastern China is discussed,The El Nino-Southern Oscillation(ENSO)records of the past 1800 years are reconstructed.Finally,by analyzing the historical data of climate change and economic development,extreme drought and flood events in sedimentary records are identified and the relationship between precipitation change and social and economic developments in different spatial and temporal distributions is discussed.The main achievements are as follows:(1)The evolution history of monsoon precipitations in the Jianghuai region over the past 1800 yearsThrough the comparison of the annual records of the residual,charcoal,and total organic carbon(TOC)in the core NSH2016,the "Carbon pool effect" in the lake sediments is defined,and an accurate chronology is obtained.On this basis,the sedimentary chroma is selected as the alternative indicator of precipitation changes.The records of the grain size and elements as the alternative paleoclimate proxies are compared with the modern meteorological observation data.which verifies the reliability of the chroma index as to the proxy of of the precipitation reconstruction.The high-resolution record of monsoon precipitations in the past 1800 years in Jianghuai region was reconstructed by the multi-proxies.It was clear that the Medieval Warm Period(MWP)and the Little Ice Age(LIA)in Jianghuai region were in the "warm-dry" and"cold-wet" precipitation modes,respectively.(2)Regional comparison and the temporal and spatial distribution pattern of precipitation records in Jianghuai regionThe reconstructed precipitation records over the past millennium in North China are consistent.Therefore,this study compares the precipitation records of the past 1800 years in Jianghuai area with that of North China.It is found that the Jianghuai region and North China have the opposite precipitation patterns in the MWP and the LIA.The Jianghuai region is the"warm-dry,cold-wet" precipitation model,whereas North China is the "warm-wet,cold-dry" model.In terms of the spatial pattern,the MWP is the "South-drought,North-flood"mode,yet the LIA is the "South-flood,north-drought" mode.(3)The mechanism driving precipitation change in the past 1800 years in Jianghuai regionThe ENSO and precipitation in Jianghuai area change synchronously,as indicated by with the data analysis of the modern atmospheric circulation in East Asia monsoon region and tropical Pacific.As a result,the change of summer monsoon intensity is shown to be closely related with the change of the latitudinal position of the West Pacific subtropical high(WPSH)and the precipitation in eastern China.The the SST change in the eastern equatorial Pacific can largely influence the intensity and latitude position of the WPSH.It is,thus,decuded that the change of dry and wet in the past millennium in Jianghuai region is mainly driven by ENSO mode.(4)ENSO records reconstructed by the precipitation changes in Jianghuai region over the past 1800 yearsBased on the close relationship between the precipitation in Jianghuai region and the SST of tropical Pacific Ocean and the reliability of rainfall changes in Jianghuai region as the indicator for the ENSO reconstruction,ENSO records of the past 1800 years are reconstructed.The records show that the MWP in the past millennium is like La Nina state and the LIA like El Nino state.The results are further supported by the SST-reconstructed ENSO records in the tropical Pacific.It is found that the change of solar activity may be the important factor influencing the ENSO changes during the MWP and LIA periods in the past millennium.(5)The precipitation changes and the response of human activity in Jianghuai regionWe reviewed the historical records of extreme droughts and floods events in the past 500 years in Jianghuai region and identified the correspondence of changes of sedimentary color and granularity index to these historical redords.We suggest that the extreme drought and flood events had a great impact on human activities in the past 500 years.The national economic center moved from north to south in the historical period of the past 1800 years.Accordingly,the precipitation changes of North China,Jianghuai region,and Jiangnan region during the same periods are compared with the the historical economic index.It is found that the precipitation change may play an important role in the history of China's social and economic developments.