Environment Change And Human Activity In Fuxian Lake Catchment Since Mid-late 933Holocene Based On LRCE And CEMMA Model

As a typical plateau fault depression deep lake which had continuous and stable deposition process,Fuxian Lake is a good research object to study regional environmental changes.Therefore,relevant mathematical models were used to quantitatively analyze traditional elements and grain size data in deep lake sediments,and this paper also discussed the environmental changes and human activity characteristics of the lake catchment by using other environmental proxies such as median grain size,fractal dimension,total organic carbon?TOC?,total nitrogen?TN?,C/N ratio?C/N?and low-frequency magnetic susceptibility(?_lf),and finally provided some suggestions for studying regional environmental changes process in the future.Based on log-ratio calibration equation?LRCE?and clustering end-member modeling analysis?CEMMA?,this paper analyzed the element,grain size and other environmental proxies of sedimentary cores of Fuxian Lake,a deep water lake in the Indian Ocean monsoon region,and discussed the change characteristics of elements and grain size,and the catchment erosion and human activity characteristics in the past 5000 years.The results indicated that:1.Results of model analysis:LRCE model was used to calibration the X-ray fluorescence?XRF?elements.The factor loading of ln?Rb/Sr?,ln?Ti/Sr?,ln?Zr/Sr?,ln?Fe/Mn?,ln?Ca/Fe?which indicated the lake catchment environmental changes,were exceed 0.79.The grain size data in Fuxian Lake sediments were analyzed by using the CEMMA model,and the four best end-members?CME1,CEM 2,CEM 3,and CEM 4?were extracted.The four end-members all showed obvious unimodal distribution,and CEM 1 corresponded to clay components,CEM2,CEM3,and CEM4 corresponded to silt components.2.Results of elements:The results showed that the characteristics of XRF core-scanning elements by the cluster analysis could be divided into 4 stages:150-115 cm?5000-3300 cal.a BP?,115-75 cm?3300-1800 cal.a BP?,75-25 cm?1800-300 cal.a BP?,and 25-0 cm?300 cal.a BP-2018 AD?.The change trend of elements showed that ln?Rb/Sr?,ln?Ti/Sr?,ln?Zr/Sr?and ln?Fe/Mn?decreasing and increasing alternately,and finally tend to decreasing.Ln?Ca/Fe?had the opposite trend.3.Results of grain size:CEM 4,median grain size and fractal dimension were high before 1800 cal.a BP,indicated that more coarse components entered the lake in this period,and then fine components became the major material in the lake.In the second stage,115-75 cm?3300-1800 cal.a BP?,CEM 4,median grain size and fractal dimension all showed obvious downward trend,indicated that the substances entering the lake in this period changed from coarse components to fine components.The particle size 25-0 cm?300 cal.a BP-2018 AD?entering the lake in the fourth stage showed a certain thickening trend,but still dominated by fine particles.4.Results of TOC,TN,C/N,and low frequency magnetic susceptibility:TOC,TN,and C/N in Fuxian Lake sediments in recent 5000 years had similar change characteristics,and generally showed a downward trend,which had a certain increase in recent years.The decreasing trend of 150-75 cm?5000-1800 cal.a BP?was more obvious in the first two stages,and 75-25 cm?1800-300 cal.a BP?,was relatively stable.The variation characteristics of low-frequency magnetic susceptibility had a good correspondence with human activities in lake catchment,the increasing value of low-frequency magnetic susceptibility usually meant the enhancement of human activities.Therefore,the low-frequency magnetic susceptibility in the first stage?150-115 cm,5000-3300 cal.a BP?was relatively low during the period of weak human activities,and the low-frequency magnetic susceptibility showed an obvious upward trend since then,only in recent years?25-0cm,300 cal.a BP-2018 AD?showed a certain decline.5.Environmental changes and human activity characteristics of Fuxian Lake in recent 5,000 years:the environmental changes in the basin before 1800 cal.a BP were less affected by human activities.During this period,the substances entering the lake were mainly coarse components,and the water level of the lake was generally low,which was the lowest period in recent 5,000 years.The water level of the lake only rose in the later period.The climate in the basin was generally warm and wet in the early period,and the erosion level was weak.In the later period,there was a certain drying trend,and the erosion level of the basin also rose obviously.At this time,the intensity of human activities in Fuxian Lake basin also increased.Since1800 cal.a BP,the intensity of human activities in the basin continued to increase.During this period,the erosion level of the basin and water level of lakes were high,the climate of the basin was wet,and the environment was more suitable.especially in the past 300 years,the environmental changes in the basin had been obviously influenced by human activities,and the changes of various environmental proxy indicators had a good corresponding with the human activities in the basin.