Relationship Between Monsoon Precipitation And Ecological Evolution In The Marginal Region Of Asian Monsoon In The Last 2000 Years

The research on climate change in the past 2000 years is one of the core contents of exploring the laws of global climate change.Its research can not only excavate in depth the laws of climate change in the past 2000 years on a centennial to annual timescales,and evaluate the effects of anthropogenic forcing and natural variability on climate,and also predict the trend of climate change in the future.The Asian summer monsoon(ASM)is the most active components in the monsoon area and has become an indispensable part in the study of global climate change because of its sensitive response to global climate change.By changing the availability of water resources,it has a significant economic and social impact on the areas within its sphere of influence,which in turn affects the lives and well-being of communities.Therefore,the research of ASM plays an important role in revealing the changes of palaeo-environment and palaeoclimate and discussing the social life of human beings.As a unique geological research carrier,stalagmite has been widely used in the reconstruction of palaeoclimate change.However,in previous studies of stalagmites,most of the previous stalagmite studies focused on the relationship between oxygen isotope(?¹⁸O)and the change of hydrological and climatic states,while ignoring the important role of carbon isotope(?¹³C)and other geochemical indicators in climate change.The Wanxiang Cave is located in the transitional zone of the Qinghai-Tibet Plateau and Loess Plateau climatic region where the modern Asian monsoon interacts with the westerly jet,and is very sensitive to the change of the Asian summer monsoon,thus it is an ideal location for palaeoclimate research.This article selects the Longnan Wanxiang Cave WX42B stalagmite as the research object,based on 721 groups of carbon and oxygen isotopes and trace element data reconstruction for nearly 2000 years Asian monsoon marginal zone monsoon precipitation and the ecological environment evolution.The change characteristics of?¹⁸O and?¹³C in stalagmite were analyzed,and the relationship between summer monsoon precipitation and ecological evolution in the last 2000 years was discussed.Combined with the time series of trace element ratios,the climatic and environmental implications of trace element ratios in WX42B stalagmite were revealed.This paper explores the relationship between the changes of Asian summer monsoon,regional ecological environment evolution and solar activity records,northern hemisphere temperature,ENSO,NAO,PDO and ITCZ in the last 2000 years,and illustrates the coupling relationship among them.The main conclusions are as follows:(1)The?¹³C of WX42B stalagmite in the Wanxiang Cave responds to the local hydrological and climatic changes,indicating the change of vegetation density over the cave.On centennial to multidecadal scales,stalagmite?¹⁸O and?¹³C change correlation,and the?¹³C of stalagmite responds to the change of monsoon precipitation indicated by?¹⁸O.The WX42B stalagmite records show four periods with different climatic and vegetation conditions:The Dark Age Cold Period(DACP,192-892 AD)was relatively cold and wet with scarce vegetation;The Medieval Climate Anomaly(MCA,892-1300 AD)was warm and humid with good vegetation cover;The Little Ice Age(LIA,1300-1870 AD),the climate in the study area was cold and dry,and the vegetation coverage was poor.The Current Warm Period(CWP,AD 1870-2003)had a relatively warm and humid climate with the well vegetation coverage.It shows that the?¹³C of the stalagmite in Wanxiang Cave can be used as an indicator of vegetation change for the reconstruction of regional ecological environment evolution and hydrological changes.(2)The trace element ratios(Mg/Ca,Sr/Ca,Ba/Ca)of the WX42B stalagmite in the Wanxiang Cave indicate the local hydrological and climatic changes and responds to the changes of monsoon rainfall and overleaf vegetation.The higher the trace element ratios(Mg/Ca,Sr/Ca,Ba/Ca)of stalagmite,the drier the environment,the scarcer the vegetation,and the ecological environment is degraded.On the contrary,the wetter the environment,the thicker the vegetation,the better the ecological environment.The trace element ratios stalagmite of WX42B is coupled by many factors,including water-rock interaction,percolation water retention time and stalagmite growth rate,but PCP is may not be the main factor.Although there are multiple factors influencing it,in the final analysis,it is influenced by the climate and responds to changes in the Asian summer monsoon.(3)The Wanxiang Cave?¹⁸O and?¹³C records of WX42B stalagmite show that six significant monsoon weakening events occurred in the past 2000 years,and the monsoon rainfall index synchronization,indicating the monsoon precipitation is significantly reduced,the climate is dry,is not conducive to the growth of plants,ecological degradation,these events center in A.D.370,892,1050,1400,1624,1930,respectively.Within the dating errors range,it was synchronized with six major precipitation weakening events recorded by the precipitation index in northern China.(4)The solar activity,the Northern Hemisphere temperature,ENSO,NAO,PDO and ITCZ were compared with the stalagmite?¹⁸O and?¹³C records,finding well corresponding relationship between these records.We found that the variation of Asian summer monsoon on centennial to decadal timescales in the last 2000 years was controlled by the variation of the ocean-atmospheric circulation ENSO,ITCZ,PDO and NAO.However,it is ultimately controlled by the change of solar activity intensity,which is the result of the coupling of multiple factors and indirectly affects the change of ecological environment.