Vegetational And Environmental Changes During Last Glacial Maximum(LGM)in Lugu Lake

The research of past climate and environment change during last glacier maximal(LGM)have been the concerns of the academics.Especially the relevant report in the region of Northwest in Yunnan province is rare.The past vegetation and environment changes during the last glacier maximal(LGM),are reconstructed using high-resolution pollen and charcoal analyses on the lake sediments at the depth of681-780 cm from core LGH-2 in Lugu Lake,together with the measurement of TOC and Ca CO3.The age-depth model of the core LGH-2 is established from AMS14 C dates.Pollen analysis shows that the pollen concentrations are much lower during the LGM than the modern lake surface mud,suggesting that the vegetation covers of the LGM are much less dense than the present.Pollen spectra is dominated by tree pollen with an average pollen percentage of 74%,of which pollen percentages of Pinus are high.Vegetations in the watershed of Lugu Lake experienced four succession phases,i.e.,fir/spruce forest+pine forest,fir/spruce+expanded pine forest,fir/spruce forest+shrinked pine forest,and fir/spruce forest+expanded pine forest during the LGM,indicating climatic conditions from cold-dry to warm-wet to cold-wet,and to warm-wet,and lake level from low to high.(1)Phase I,22770~21860 a B.P.: pollen spectra are dominated by tree pollen consisting of fir/spruce and pine pollen,together with herbaceous pollen mainly including Poaceae,Artemisia,and Rannuculaceae.Pollen assemblages reflect the occurrence of fir/spruce forests and pine forests,indicating colder and drier climatic conditions than the present in the watershed of Lugu Lake.Relatively low charcoal influxes indicate relatively low frequent forest fires in this climatic conditions.The high percentages of Ca CO3 suggest low water levels of Lugu Lake.A peak of charcoal influx and Ca CO3% around 21800 a B.P.implies the occurrence of a centennial dry event.(2)Phase II,21860~19910a B.P.: it is characterized by a decrease of herbaceous pollen and an increase of arboreal pollen,especially pine pollen.Pollen spectraindicate expanded pine forests,a rise in temperature and humidity in the watershed of Lugu Lake.Charcoal influxes are still low just like at the first stage,suggesting that there is low frequent fires in forest.TOC contents show no significant changes;Ca CO3% decreases gradually,suggesting the rise of lake water lever,however,a reversal occurring at ca.20500 a B.P.indicates a centennial-scale event with a major drop in lake water lever.Principle component analysis(PCA)of pollen spectra reveals an increase trend of humidity in the watershed of the Lugu lake during the LGM.This phase is the transition from dry to wet with a shift timing around 20000 a B.P..(3)Phase III,19910~18180a B.P.: Pollen spectra show a decrease in pine pollen,no significant changes in other pollen.Pollen assemblages indicate the shrink of pine forest,suggesting a drop in temperature.PCA result of pollen spectra shows that climatic conditions in this phase are as wet as the above phase.Charcoal influxes increase,suggesting no significant changes forest fires compared with Phase II.The content of Ca CO3 continues to decrease until it reaches its minimum during the LGM,probably indicating high lake level,even the existence of external flow.(4)Phase IV,18180a~17590a B.P.: it is characterized by an increase of pine pollen,and a decrease of herbaceous pollen.Fir/spruce pollen decreases,and pollen percentages of some pollen types indicating dry conditions such as evergreen oak and Cyclobanopsis decrease to reach their minima.Pollen spectra indicate warmer and wetter climatic conditions in this phase than the previous phase.Charcoal influxes increased,implying more forest fires.The content of Ca CO3 is higher than that at the previous phase,indicating a gradual drop of lake level.A peak of charcoal influx and Ca CO3 content occur around 17700 a B.P.,implying another centennial-scale dry event with increased forest fires and major drop in lake level.