Impact Of Abrupt Late Holocene Monsoon Climate Change On The Status Of Lake In North China Recorded By Multi-proxy Records From The Sediments Of Lake Mayinghai

Alpine lakes are generally not directly affected by human activities and play an indispensable role in the regional hydrological cycle.Previous studies have shown that under the background of the overall weakening of the East Asian summer monsoon in recent decades,the number of alpine lakes in eastern China has decreased rapidly and the lake ecology has deteriorated.So will the state of the lake deteriorate further under the background of future global warming?How do alpine lakes respond to climate change?These problems deserve people's attention.However,the answer to these questions requires a new understanding of the relationship between climate change and lake ecology on a long time scale(e.g.,the Holocene)with the help of paleolimnology research methods and methods.Generally speaking,the East Asian summer monsoon declined in the late Holocene,and the precipitation in North China was significantly reduced.However,the extent of the impact of monsoon decline on lake state in northern China is still unclear.In this study,we measured the carbon isotope(?¹³C),magnetic susceptibility(?_lf)and exogenous elements content in the borehole sediments of Lake Mayinghai,an alpine lake located in the margin of the Loess Plateau,to investigate the history of East Asian summer monsoon precipitation since the Middle and Late Holocene.The results of this study were compared with other paleoclimate records of the Loess Plateau,and it was found that they have similar variation characteristics,which verified that they can be used as a proxy index for the variation of East Asian summer monsoon.In addition,we used ancient DNA sequencing methods to establish the time series of algae evolution since the Middle and Late Holocene in Lake Mayinghai,and compared them with other physical and chemical indicators in the core.Combined with other published data,we conducted comprehensive analysis and discussed the driving factors of the evolution of relative abundance of algae in Lake Mayinghai.Finally,based on the paleo-ecological and paleo-hydrological records of other lakes in northern China,the impacts of the East Asian summer monsoon on the lake state in North China were discussed.The main research results of this paper are summarized as follows:(1)The?¹³C values of Lake Mayinghai sediments showed a negative and stable state in the Middle Holocene,while the?¹³C values showed a rapid positive trend in the Late Holocene.By contrast,found that Lake Mayinghai sediments in late Holocene?¹³C values significantly positive end the change trend of the Lake Gonghai and adjacent borehole woody pollen content and quantitative reconstruction of the law of changes of annual precipitation is,this indicates that the change of the?¹³C may reflect the vegetation change of Lake Mayinghai catchment,the main controlled changes in the east Asia summer monsoon.In addition,the terrigenous detritus content of Lake Mayinghai is mainly controlled by the density of vegetation.When the density of vegetation is high,the amount of exogenic detritus material into the lake decreases.And vice versa.The?_lfvalue of Lake Mayinghai sediments is mainly related to the content of detrital material into the lake and the preservation degree of magnetic minerals,and the trend of?_lfis the same as that of?¹³C,which further proves that the?¹³C record of Lake Mayinghai is a credible indicator of the East Asian monsoon precipitation.(2)The high-resolution sediment records from Lake Mayinghai indicate the rapid weakening of the East Asian summer monsoon around 3000 cal yr BP.Other ancient climate records in the north China,such as the development degree of loess plateau ancient soil,sand dune area of eastern China ancient soil development situation,the loess-paleosol profiles of?_lfrecords,etc.With the change of Lake Mayinghai?¹³C values consistent,show that due to the precipitation decrease when late Holocene climate is relatively dry,which proves that the East Asian summer monsoon declined suddenly during this period.The abrupt decline of the monsoon was mainly driven by ENSO and solar radiation:the increase of ENSO frequency and the decrease of solar radiation intensity in the Late Holocene resulted in the weakening of the East Asian summer monsoon.At the same time,the rapid decline of monsoon also had a profound impact on the Neolithic culture in China.(3)Through the calculation and analysis of the DNA results of the algae in Lake Mayinghai,it is found that the main algae in Lake Mayinghai during the middle and late Holocene are Synechococcus,Microcystis,Merismopedia,Dolichospermum and Lobospharea.The algae DNA data of Lake Mayinghai results by Tilia analysis software is divided into four main stages:8000?6500 cal yr BP?6500?4650 cal yr BP?4750?3000 cal yr BP?3000?0 cal yr BP.We compered the result with the Lake Mayinghai sediment organic matter and in the northern hemisphere and Lake Daihai simulated temperature records,driving algae relative abundance changes as the temperature during 8000?3000 cal yr BP,and with lakes condition deteriorated rapidly,rapid decline in nutritional status which become the main factors influencing the relative abundance of algae after 3000 cal yr BP.(4)Through a comprehensive analysis of the evolution records of the lakes in northern China,such as Lake Gonghai,Lake Ganhai and Lake Daihai,it is found that the water level of these lakes decreased rapidly in the Late Holocene,and the Lake Ganhai dried up due to the small area and shallow water depth of the lakes.These evidences consistently show that the deterioration of lake environment in northern China is synchronous with the abrupt decline of the East Asian summer monsoon,which indicates that the rapid weakening of the East Asian summer monsoon has a greater impact on the overall condition of the lakes in northern China.With future global warming,the expected arid climate in arid and semi-arid regions will accelerate the degradation of alpine lakes,which are also vulnerable to intensive human activities.Therefore,it is imperative to take more effective measures to protect these precious lake ecosystems.