Study On Phytolith-vegetation Community Evolution Analysis And Reconstruction Of Modern Climate In Nanjishan Wetland

Sediment phytolith assemblage analysis has become an important means to interpret the study of the Quaternary paleoenvironmental reconstruction in the region,and the research on the modern process of phytolith assemblage is the fundamental cornerstone for scientifically and objectively understanding its environmental significance and for the study of paleoenvironmental restoration.This paper takes Nanjishan wetland as the research object,and selects 19 soil samples from 7 lakes(including 17 surface soils and 2 sedimentary column samples)that can represent regional climate change.The physical and chemical indexes(total organic carbon,Ph,reduction potential)and paleoenvironmental proxy(Phytolith)in soil samples from Nanjishan wetland were analyzed by paleo-limnology,based on the time series framework of Sedimentary depth-time series established by ²¹⁰ PB dating of radioactive elements and the recent climatic data of the basin,canonical correspondence analysis(DCA),principal component analysis(PCA),cluster analysis(CA),factor analysis(FA)and stepwise regression analysis(SRA)were used to study the corresponding relationships between the phytolith assemblages and typical vegetation communities in the south of Nanjishan wetland,the main environmental factors affecting the spatial distribution of phytolith were obtained,and the spatial distribution patterns of climate change and vegetation succession were further revealed.The main research contents and results are as follows:(1)Through the analysis of lithology and physical and chemical properties,it can be seen that the soil in the study area is yellow-brown clay,silty clay and gray-black clay,and the TOC content and redox potential decrease with the increase of depth,and the pH increases with the increase of depth.Silty clay obviously affects the preservation of short-cell phytoliths.(2)After extracting and identifying 75 samples of phytoliths from the surface soil and sediments of Nanjishan Lake Wetland,it was found that a total of 19 types of phytolith,including square type,rectangle type,Fan type,cross type,dumbbell type,polylobate type,short saddle type,long saddle type,elevate saddle type,wavy-trapezoid type,trapezoid type,point type,smooth-elongate type,prickly-elongate type,sinuate-elongate type,‘I'type,triangular prism type and globular granuate type.The total number can reach 19,997,including 4248 topsoil phytoliths and 15,749 phytoliths in sediments.(3)Multivariate Statistical Analysis(DCA and PCA)showed that temperature and humidity accounted for 39.29%and 26.48%of the total explanatory factors,respectively.According to the phytolith assemblages of Zizania Latifolia and Phragmites Australis communities in central and southern wetlands,there were significant differences in phytolith types under different vegetation communities.(4)Under the environment change of nearly 100 years,the typical vegetation community of Zhanbei Lake changed from Carex cinerascens community to Triarrhena lutarioriparia-Zizania latifolia community,and the typical vegetation community of Shangbeijia Lake changed from the wet vegetation community such as Carex cinerascens community to Triarrhena lutarioriparia–phragmites australis community.(5)The phytolith-climate conversion function was established by factor analysis and stepwise regression analysis:the F test was used for calibration and it was found that climate factors such as average annual air temperature,annual precipitation,average annual relative humidity and average annual wind speed presented a good correlation with the phytolitic content change(P<0.05).(6)After quantitative assignment of phytolith in the sediment,the combination zones of phytolith in the two sedimentary profiles were divided into three stages by using cluster analysis and comparing with the real climatic factors in different ages,generally speaking,the climate change trend of Nanjishan wetland in the past century is characterized by warm-dry-warm-wet-cold-wet structure.