| As the problem of global warming intensifies,the global carbon cycle is increasingly becoming a key concern for global climate change research.Among them,the carbon cycle of terrestrial ecosystems is the part most affected by human activities,and lakes are an important part of it,so it is crucial to study the carbon cycle of lakes.And because of their stable depositional environment,lake sediments record a wealth of carbon information and are an important vehicle for studying the lake carbon cycle.Fuxian Lake is a typical highland deep-water lake in Yunnan,China,and the increasingly intense human activities around it in recent years have had an impact on the deposition of carbon in the lake.Studying the changes in the deposition of carbon fractions in it is important for understanding the carbon cycle in the lake ecosystem.Black carbon(BC)and polycyclic aromatic hydrocarbons(PAHs),both products of fuel combustion,are widely present in soils and sediments,and have received much attention from researchers over the years due to their high aromaticity and inertness,which significantly distinguishes them from endogenous and terrestrial sources of organic carbon.Therefore,this paper collected surface sediments and sediment columns from Fuxian Lake,comprehensively analysed the content and distribution characteristics of BC and PAHs in Fuxian Lake under different spatial and temporal conditions,studied and resolved the sources of BC and PAHs,and explored the natural and social factors that may influence the deposition of BC and PAHs.The main results obtained are as follows.(1)The contents of BC,TOC and PAHs in the surface sediments of Fuxian Lake ranged from 1.83 to 3.63 mg g-1 for BC,with a mean value of 2.60±0.66 mg g-1;the contents of PAHs ranged from 162.82 to 469.48 ng g-1,with a mean value of 264.32±106.65 ng g-1 The content of TOC ranged from 13.48 to 25.03 mg g-1,with a mean value of 21.00±4.59 mg g-1.The main component of BC was char,accounting for more than80%on average;the main component of PAHs was low ring number PAHs,accounting for 49.21%on average.As a whole,the contents of BCs,PAHs and TOCs in the surface sediments of Fuxian Lake all showed a trend of high in the north and south and low in the centre.Inferences based on the geography of Fuxian Lake suggest that the high northern part may be due to terrestrial carbon input from rivers in the north;the high southern part may be due to southwesterly winds transporting carbon generated by human activities within Yuxi to the lake for deposition;and the low central part may be due to the proximity to the outlet rivers,resulting in the loss of each carbon fraction.(2)The sedimentary characteristics of BC,TOC and PAHs in Fuxian Lake over the last 140 years were studied and analysed based on the results of 210Pb dating.The BC content in the sediment column of Fuxian Lake ranged from 0.11?4.18 mg g-1,the TOC content ranged from 2.90?29.39 mg g-1 and the PAHs content ranged from 63.99?579.24 ng g-1.Based on the temporal trends of the deposition fluxes of BC and PAHs,and with reference to the timing of major historical events in China,the deposition time of BC and PAHs in the sediment column of Fuxian Lake was divided into four stages.Stage 1:Before the founding of New China,when productivity was backward,the deposition fluxes were relatively stable and the values were small.The peak around1960 coincided with the Great Leap Forward Movement of 1958-1960.Stage 3:After the reform and opening up,the country entered a period of rapid development with a rapid increase in energy consumption,leading to a large increase in pollutant emissions.Sedimentation fluxes rose rapidly.Stage 4:Around the 21st century the economy entered a phase of rapid and steady development and the country began to adopt a series of environmental protection measures,causing sedimentation fluxes to show a constant or decreasing trend.The changes in the contribution of BC to TOC over the last 140years were derived from the BC/TOC values,and it was inferred that these changes were mainly influenced by human activities and changes in energy mix.A correlation analysis of the various carbon fractions revealed that BC and its fractions in Fuxian Lake sediments were significantly correlated with high ring number PAHs.(3)Using the char/soot ratio method to analyse the sources of BC,the ratio gradually decreases with time,indicating that the energy structure in China is changing from low-temperature to high-temperature combustion,and the fuel is changing from biomass to fossil fuels;using the isomer ratio method and the PMF model analysis method to analyse the sources of PAHs,the main sources include biomass combustion sources,coal combustion sources and industrial and transportation The analysis of the effects of different sources of PAHs on BC illustrates that the historical changes of each source are closely related to the local human activities and economic development of Fuxian Lake,showing the gradual transformation of China from an agricultural to an industrial economy.The internal and external sources of TOC were also analysed using n-alkanes to analyse the effects of different sources of organic carbon on BC concentrations,showing that BC is mainly influenced by exogenous organic carbon.(4)A correlation study with natural and socio-economic data from Yuxi found that,among the natural factors,temperature changes had a greater influence on the BC and TOC contents in Fuxian Lake,and that BC was more sensitive to temperature changes than PAHs;the effect of forest fires on BC deposition may have a lag.Among the socio-economic factors,the contribution of agricultural activities to carbon growth was stable and showed a significant positive correlation with char;while the rapid growth of industrial activities was responsible for the significant increase in carbon content,among which the rapid development of secondary industries and the increase in civilian car ownership could significantly influence the change in soot content. |
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