Mid-Late Holocene Environmental Change Based On Lacastrine Sediment Of Fuxian Lake

The study of environmental changes in the middle and late Holocene is important for predicting the significance of future environmental changes.Fuxian Lake is in the southwest monsoon area in Yunnan Guizhou Plateau,its formation age is long,it is the lake deposition process is continuous,it is the study of sediment paleoenvironmental changes in China's southwest monsoon region of the ideal material.There is a lack of research on the change of Fuxian Lake on millennial scale and high resolution paleoenvironmental aspects.This paper establishes the age sequence of sediments of Fuxian Lake,and carried out a detailed analysis the grain size and the organic carbon isotope(?13Corg),total organic carbon content(TOC)and carbon nitrogen ratio(C/N)of environmental indicators of Fuxian Lake.The results showed that in the 5000-2300 Cal yr BP stage,Fuxian lake delta sediments ?13Corg,TOC,TN and C/N index is relatively stable,shows the source pattern of organic matter in Fuxian Lake is the source model of organic terrigenous and endogenous common source,different particle in sediment in this stage the size composition is very stable,sorting coefficient and median grain size value is almost constant,the lake sediment deposition rate is relatively low,that of Fuxian Lake watershed erosion degree is not high and stable.In 2300-2000 Cal yr BP,13 Corg delta sediments have the obvious change,TOC,TN and C/N are significantly reduced,they indicate the source pattern of organic matter in Fuxian Lake has changed obviously.The characteristics of sediment grain size variation in this stage is very obvious,the percent content of clay grain size and fine size coarse sand are increased in this stage,the sorting coefficient and median size change is larger,and the rate of deposition of the sediments in the Fuxian Lake the increase is,they show that the degree of erosion of the surface watershed of Fuxian Lake is increased.Since 2000 Cal yr BP since the Fuxian lake delta sediments ?13Corg overall change greatly,TOC,TN and C/N value in general is reduced,they indicate the source of organic matter in sediments of the Fuxian Lake is the dominant endogenous organic matter.The change in the grain sizeof the sediments is large,the clay content is increased,and the total content of sand is decreased.The rate of sediment deposition is generally higher,and the magnitude of its decrease is greater.They show that the input of Fuxian Lake material grain size difference is obvious,but also indicates the degree of erosion of the surface watershed of Fuxian Lake is relatively high.According to the characteristics of the sediment environment index of Fuxian Lake to the reconstruction of the history of the environmental changes in the Fuxian Lake watershed since the middle and late Holocene,influence and change of basin Fuxian Lake Environment in 5000 Cal yr BP to the time may have the climate and human activities has carried on the discussion,this paper draws the following conclusions:1.In 5000-2300 Cal yr BP stage,the degree of surface coverage of the vegetation in the watershed of Fuxian Lake Basin is relatively high,the environment is relatively stable,changes of climate change and human activities on the environment of the Fuxian Lake is not obvious.2.In 2300-2000 Cal yr BP phase,the surface of the Fuxian Lake watershed vegetation coverage is significantly reduced,the watershed of Fuxian lake environment has changed significantly in 300 years,has a certain influence of human activities on the environment of the watershed,but climate change may be the controlling factor the main result in Fuxian Lake Environment change.3.Since 2000,Cal yr BP,the surface of the Fuxian Lake watershed vegetation coverage is relatively low,the change of basin environment is very significant,the human activity may have significant effects on the change of basin Fuxian Lake environment.4.Since the middle and late Holocene changes in the watershed of Fuxian lake environment are likely to be affected by the southwest monsoon weakened,in addition,the interference of watershed of Fuxian lake environment changes at each stage of the basin may also be affected by different levels of human activities.