Study On Climatic And Environmental Changes Of The Yunnan Cheng Hai Region Recorded By Lake Sediments Since 1800

The climate change of recent 200 years plays an important role for understanding the climate change law before and after the industrial revolution and the conflicts between modern civilization and natural environment.It has become a hotspot and focus in the modern climate research,enjoying an irreplaceable role for predicting the future climate change direction and coordinating the man-land relationships.Due to its geographical location and topographic features,the Yunnan-Guizhou plateau is sensitive to climate change,its numerous lakes are good carriers for recording paleoclimatic information.This paper targets to Cheng Hai,which located in the northwest of Yunnan province enjoying its nature of the dry and hot valley of Jinsha river.The climate of it obviously influenced by southwest monsoon and regional topography.This study was carried out from the extraction of 47cm sediment cores with the water depth of 35m from the deepest part of Cheng Hai,measuring indicators in the laboratory for grain size composition,carbonate content,organic matter content,carbon and nitrogen isotope and elements of XRF scan and so on,and using ²¹⁰ Pb-¹³⁷ Cs dating to create a reliable age ruler to reconstruct the lake change variety and climate evolution history of Cheng Hai region since 1800 AD,from which some conclusions drawn as follows:1.The paleo-climate reconstruction in Cheng Hai region since 1800.In general,the climate of Cheng Hai region has been characterized by cold-wet,warm-dry,hot-dry combination since 1800.From 1800 to 1880,the climate was total cold-wet,and the vegetation coverage in the lake basin was relatively better,and the primary productivity of the lakes was relatively lower.From 1880 to 1950,the climate was total warm-dry,basin precipitation just has a nuance change,evaporation increased gradually than last stage,which is the root of the lake area's continuing narrow,the water level's sustained lowering,and the slow rise of lake's primary productivity level,meanwhile,the watershed vegetation coverage reduced relatively with an increased exposed surface area.From 1950 to now,the climate has been in total hot-dry.So far,the temperature of this region still climbs up slowly;the precipitation cuts down relatively;the accidental intense drought event happens in a quiet often frequency;even human activity intensity is higher than all historical periods;furthermore,the lake vegetation coverage decreased dramatically.2.Significance of sediments'each indication on the study of environmental climate change.From the sediments of Cheng Hai,the change of grain size composition content can clearly reflect lakes hydrodynamic conditions;the change of carbonate content can effectively reflect the lake level's variety and the change of lake's temperature;the change of the combination of grain size composition and carbonate content can beautifully indicate the regional dry-wet change;the change of organic matter content combined with C/N can clearly reflect the source of organic matter,indicating lake basin surface's runoff intensity at the same time to reflect the rainfall intensity of the lake region or the condition of surface vegetation coverage.3.Response of element content changes to climate changes in sediments.The content of Fe in sediments is positively correlated with the precipitation in the area,so the change of Fe can indicate the change of precipitation intensity and the change of weathering erosion intensity of this area.Rubidium strontium ratio?Rb/Sr?and carbonate content are negatively correlated.In future studies,Rubidium strontium ratio and carbonate content can verify each other by this relationship.