Vegetational And Climatice Changes During The Last Glacial Maximum And Deglaciation In The Catchment Of Lugu Lake,Northwest Yunnan

Most of available studies on the climate of the last glacial maximum and deglaciation are quanlitive and semi-quantitative reconstructions in Yunnan.Quantitative reconstructions of climate are thus urgently needed for the study of Southwest monsoon history in Yunnan,especially those high-resolution quantitative climatic records.In this study,pollen data from surface samples of 67 lakes in Northwest Yunnan and 332 fossil samples of the LGH-2 core?449-780 cm?in Lugu Lake,as well as observational climate data?1961-2011?were analyzed to determine climatic parameters controlling modern pollen rain using the canonical correspondence analysis?CCA?,then to establish modern pollen-climate transfer functions using the weighted average of the partial least squares?WA-PLS?model,and finally to reconstruct paleoclimate during the last glacial maximum and deglaciation in the catchment of Lugu Lake using those transfer functions.Following conclusions are drawn:?1?The results of pollen analysis on surface samples from 67 lakes in Northwest Yunnan show that Pinus,Alnus,evergreen Quercus and Poaceae are four pollen taxa with the highest pecentages in modern pollen spectra;modern pollen spectra are dominated by arboreal pollen?73.3%?,in which dominant pollen taxa are Pinus,evergreen Quercus,and Alnus,accompanied by some pollen taxa such as Picea/Abies,deciduous Quercus,Betula,Rosaceae,Hippophae and Juglans;herbaceous pollen taxa are mainly Poaceae,Chenopodiaceae,Aretmisia,Cyperaceae,Cruciferae,Polygonaceae,Asteraceae,and Thalictrum.Modern pollen spectra from 67 lakes reflect the overall characteristics of regional vegetation in Northwest Yunnan.?2?The result of CCA on modern pollen and climate data shows that mean annual temperature(T_ann),mean annual precipitation(P_ann)and mean precipitation in dry season(P_dry)are three climatic parameters controlling the variation of modern pollen rain in Northwest Yunnan.Among them,T_ann is the most important climatic parameter.?3?WA-PLS model and modern analogue technique?MAT?were used to establish modern pollen-climate transfer functions,and both methods were evaluated by the leave-one-out cross-validation method.The results show that the maximum biases?Max-bias?in MAT are always higher than that in WA-PLS model,and the determination coefficient?R²?in MAT is always lower than that in WA-PLS model.Therefore,WA-PLS model is finally selected to establish modern pollen-climate transfer functions in Northwest Yunnan.?4?Paleoclimate during the last glacial maximum and deglaciation in the catchment of Lugu Lake was quantitatively reconstructed using modern pollen-climate transfer functions derived from WA-PLS model and fossil pollen data from Lugu Lake.It shows that the variation trends of T_ann and P_ann were in reverse phase with P_dry.Cold and dry climatic conditions occurred during the period of22.17-17.68 ka BP,when T_ann was about 4.8?lower than the present,and P_ann was about 51 mm lower than that at present.T_ann and P_ann exhibited an increase trend from17.5 to 11.7 ka BP,showing a warm and humid climate.However,three cooling events and three precipitation revesals occurred in the catchment of Lugu Lake during the last glacial maximum and deglaciation.Three cooling events happened at17.68-17.5,14.2-13.85 and 11.7-10.93 ka BP,when T_ann dropped by 3.8,2.8 and1.8?,respeectively.Three precipitation reversals occurred at 17.68-17.0,14.0-13.3and 11.7-10.93 ka BP,when P_annnn decreased by 137,96 and 109 mm.