Carbonate Mineral And Grainsize Changes Since 16 Kyr BP In Lake Lugu And Environmental Indication

In this paper,Lugu Lake located in the northwest of Yunnan-Guizhou plateau was selected as the research object.Carbonate content,mineral composition percentage,partial element percentage,particle size distribution and organic matter content in the core sediment samples of Lugu Lake?LGH-2?borehole were determined.Then combined with AMS¹⁴C to establish a reliable chronological sequence.This paper analyzes the various characteristics of various environmental indicators in core sediments and then reveals the environmental significance indicated by each environmental indicator.In addition,this paper also discussed the climate change and environmental evolution characteristics of the Lugu Lake basin since16,000 years ago,and preliminarily obtained the following conclusions:1.The response of carbonate content,calcite,Ca element and Sr/Rb in the sediments of Lugu Lake to climate changeThe carbonate content in the sediments of Lugu Lake decreased obviously in the early Holocene,even to zero.The carbonate content in the lake fluctuated and declined in the middle Holocene.At the end of the Holocene,it fluctuated greatly.There is a high correlation between the content of carbonate,XRF intensity of Ca and XRD intensity of calcite in the sediments of Lugu Lake.The change of element Ca is determined by the change of carbonate content,and carbonate basically exists in the form of calcite.Therefore,the change of carbonate content is positively correlated with the change of Lake water level,that is,the change of carbonate content shows the change process of Lake water level to some extent.The variation trend of Sr/Rb was generally consistent with that of carbonate content,Ca element and calcite signal strength.For the most part,the element Rb is not affected by biological and other conditions.Therefore,Sr/Rb in Lugu Lake can reflect the precipitation in the basin.2.The response of particle size,quartz and Si elements in the sediments of Lugu Lake to climate changeIn the early Holocene,the sand content of sediment particle size peaked.In the middle Holocene,silt content increased.At the end of the Holocene,clay content increased,indicating that the characteristics of particle size grouping were mainly affected by the amount and intensity of precipitation in the basin.The signal strength of quartz has little correlation with Si content.This suggests a problem that the sediments in lugu lake may come from quartz or clay,while quartz mainly comes from silt.Therefore,quartz strength and Si elements are used to indicate weathering strength and transport status.3.Climate change characteristics since the Last Glacial period in Lugu Lake basin.During the period of 16-14.3 cal ka BP,the Lugu Lake basin was in the early stage of drought,which was characterized by low precipitation,low water level,and weak wind power and weathering degree.Later,the climatic characteristics changed from drought to humid,with increasing precipitation,rising lake water level,water desalination,and gradually enhanced wind power and weathering degree.During the period of 14.3-5.6 cal ka BP,the climate of Lugu Lake basin was generally humid with large precipitation and elevated lake water level,and even Lugu Lake became an outflow lake at that time.In the period of cal ka BP,the early climate alternates between dry and humid,and the late climate continues to be dry and the lake water level decreases.During the 2.8-0 cal ka BP period,the early period was the driest period since the Holocene in the Lugu Lake basin,and the climatic characteristics were warm and dry.The temperature of the lake increased during this period,and the lake became salty.The late period was characterized by the alternation of dry and humid and finally became humid.Overall,Lugu Lake basin since 16000 to climate change is not only to present the characteristics of dry-wet-drought-wet,also present during a certain time of dry-humid alternate phenomenon,it also records the YD and 8.2 ka BP and 4.2 ka events such as the climate abnormal events,in addition,the little ice age to the medieval warm period and also has a good record.4.Analysis of the climatic driving mechanism of Lugu Lake basin since the last deglaciation periodSince the last deglaciation period in the lugu lake basin,the climate is mainly affected by solar radiation and the Indian monsoon,and may be affected by the amount of ice in the northern hemisphere at high latitudes.Climatic anomalies including YD,8.2 ka BP events and 4.2 ka events were recorded in the LGH-2borehole of Lugu Lake,indicating that the high latitude region of the northern hemisphere has an influence on the climate of the low latitude region.In the early Holocene,the Lugu Lake basin received the most precipitation,which was consistent with the Indian monsoon.In the middle Holocene,there was still a large amount of precipitation in the Lugu Lake basin,which was not consistent with the changing trend of solar radiation,which may be related to the location and terrain of Lugu Lake itself.At the end of the Holocene,precipitation in the Lugu Lake basin also decreased with the continuous decrease of solar radiation.