Geochemical Characteristics And Environmental Implications Of Lake Sediments In Wuhan

As humans continue to modify the Earth’s environment on a large scale,many scholars believe that we have now entered the Anthropocene,an era in which humans are the new geological force transforming the planet.However,there are conflicting opinions about the start of the Anthropocene and the boundary of its stratigraphy,so more evidence of its occurrence and the regions where it can be recorded are needed.Lake sediments are a good vehicle for recording environmental,geological and anthropogenic changes faithfully.Compared to alpine lakes,urban lake sediments can store more information related to human disturbance and can not only reconstruct the evolutionary history of the lake,but also reflect the extent of the role of human activities in it and provide a reference for the determination of the Anthropocene.In this thesis,urban lakes located in Wuhan,Hubei Province-Houguan Lake,Tangxun Lake and Chen-were selected as the study areas,and the sediment cores with depths of 65 cm,95 cm and 68 cm were collected,respectively.The dating was carried out using the²¹⁰Pb-¹³⁷Cs method to establish a date-depth model,and then combined with grain size,biogenic elements（C,N）,major elements（Al,Fe,K,Mg,Na,Mg,Mn and Ti）,trace elements（Lanthanide rare earth elements、Cu,Pb,Zn,Cr,Ni,As,Cd,Hg,Co,Rb,Sr and Zr）,stable isotopes(δ¹³C andδ¹⁵N,²⁰⁶Pb/²⁰⁷Pb,²⁰⁸Pb/²⁰⁶Pb,Δ¹⁹⁹Hg,δ²⁰²Hg)and other geochemical indicators to reconstruct the environmental change process of urban lakes on a century scale.The interrelationships between lake sediments and human activities were also explored by combining information on regional population size,industrial structure,GDP,mineral distribution,and historical events.The main results are as follows:1.the deposition history of the 65 cm sediment core in Houguan Lake is 1882-2020,with an average deposition rate of 0.47 cm/a respectively;the deposition history of the 95cm sediment core in Tangxun Lake is 1826-2020,with an average deposition rate of 0.49cm/a;the core deposition history of the 68 cm in Chen Lake is 1909-2020,with an average deposition rate of 0.61 cm/a.The high values of sandy（0.16-6.88%）in the sediment core of Houguan Lake,sandy（0.26-4.83%）in the sediment core of Tangxun Lake and sandy（1.17-6.86%）in the Chen Lake all occur around 1998a and 1931a,reflecting the enhanced hydrodynamics of the lakes in Wuhan,corresponding to the larger flooding events in the recent history of the city.2.The biogenic elements in the sediment core of Houguan Lake and Chen Lake are dominated by aquatic vascular plants and C3 plants（Houguan Lake:31.32%and 27.04%,Chen Lake:34.54%and 31.76%）,and the sediments of Tangxun Lake are dominated by phytoplankton and aquatic vascular plant sources after 1983a（28.27%and 36.40%）,with the three lakes having a high hydrodynamic effect in 1960a,The significant increase in phytoplankton contribution in the three lakes around 1959a and 1963 corresponds to the large-scale dredge net farming and paddock farming events in Wuhan in the 1950s.3.The ratios of light and heavy rare earths in the sediment cores of Houguan Lake,Tangxun Lake and Chen Lake all show enrichment of light rare earths and loss of heavy rare earths（ΣLREE/ΣHREE:8.09,9.04 and 8.42）and slight negative anomalies of Eu,which are consistent with the background lithology of Wuhan and the distribution of rare earth elements in the Yangtze sediments.The chemical weathering indices（CIA）of the principal elements in the sediments of Houguan Lake,Tangxun Lake and Chen Lake（71.43,76.02 and 70.97）are also close to those of the Yangtze River sediments（72.78）,indicating that the sediments of the three lakes are influenced not only by the weathering of rocks in the local background area,but also by the lithological background in the Yangtze River and the watershed through which it passes.4.The values of ²⁰⁶Pb/²⁰⁷Pb and ²⁰⁸Pb/²⁰⁶Pb in the sediment cores of Houguan Lake,Tangxun Lake and Chen Lake（1.180-1.201 and 2.078-2.105 for Houguan Lake,1.181-1.195 and 2.082-2.097 for Tangxun Lake and 1.177-1.196 and 2.078-2.097 for Chen Lake）,indicating that these lakes may have been primarily affected by industrial activities such as coal burning and ore mining;The overall negativeδ²⁰²Hg is related to industrial waste gas discharge,indicating that Pb and Hg are mainly related to industrial sources.The anthropogenic contribution of Pb appears to be high in 1960-1969,1949 and 1974,corresponding to the rapid rise of industry in Wuhan after the First Five-Year Plan.5.The combination of particle size,biogenic elements（C,N）and major and heavy elements in the principal component analysis（PCA）shows that PC1（agricultural source+natural source）in Houguan Lake and PC2（agricultural source）in Chen Lake both show high values between 1950 and 1965,corresponding to agricultural activities such as land reclamation and dredge net farming in Wuhan;PC2（industrial mining/smelting source）in Houguan Lake and PC1（industrial source+agricultural source）in Tangxun Lake show high values between 1950 and 1965.PC2（industrial mining/smelting source）in Houguan Lake and PC1（industrial source+agricultural source）in Tangxun Lake,which have relatively high values in 1960-1973,correspond to the industrial evolution history of Wuhan.6.The stable isotopes and composite indicators of PCA analysis are good indicators of human activity,and the Anthropocene time indicated in the sedimentary core of urban lakes in Wuhan is around 1950-1965a.