Palaeoclimatic And Palaeoenvironmental Evolution Of Jilantai Salt Lake Over The Past 34 Ka

This study reconstructs the history of vegetation and regional palaeoclimatic and palaeoenvironmental evolution in Jilantai Salt Lake(Inner Mongolia,China)over the past 34ka based on pollen data,X-ray diffraction(XRD)data and lithology character from two sediment cores(JLT2 and JLT11-A).During the late of Marine isotope stage(MIS)3(34-28.8 cal ka BP),pollen data recorded desert steppe vegetation,dominated by Artemisia,Chenopodiaceae(Amaranthaceae)and Ephedra,with relative high pollen concentration and A/C value,thus indicating relatively wet and warm climate during this state.the main minerals of sediments were Quartz-and clay mineral-rich clastics(?47%)and Calcite-rich carbonates(?25%).In addition,a small amount of Halite and Gypsum,probably relating to enhanced evaporation resulting from higher temperature,could be found.This stage featured an enhanced East Asian summer monsoon and associated precipitation compared to the Last Glacial Maximum(LGM),which was also supported by other recoreds from neighboring regions.During MIS2(28.8-18 cal ka BP),the vegetation cover was replaced by desert dominated by Artemisia,Chenopodiaceae(Amaranthaceae)and Poaceae,with Picea and Pinus growing on the surround mountains.Pollen concentration and A/C value had reduced to minimum.The sediments contained about 85%of elastics minerals(mainly Quartz and Albite),which could be imported from land by strong winds related to winter monsoon.The content of carbonat(mainly calcite)and Halite was decreased to about 10%,which was probably due to the importing of clastic minerals.However,the lake still possessed some extent of salinity for the continuous existence of chloride and sulfate minerals(less than 5%).The minerals assemblage of Jilantai Lake was more associated with the input of terrestrial material,but was difficult to directly trace the lake water salinity during this stage.The lithology for this interval was characterized by clayey silt and fine sand.All of the above features suggested a weakened summer monsoon(dry)and strengthened winter monsoon(cold)during MIS2,especially during the LGM.These could also be confirmed by surrounding climate records from lakes,loess and climate simulations.During the last deglaciation(18-10.6 cal ka BP),pollen data showed that the land was covered by desert steppe dominated by Artemisia,Chenopodiaceae(Amaranthaceae)and Ephedra,with increased pollen concentration and A/C value,indication of a warm and wet climate during this period.XRD data suggested that the lake was transformed from a brackish-water lake to a salt-water lake.The evaporates and carbonates experienced a slight increse and frequent fluctuation,whereas clastics had decresed,reflecting climate instability during the last deglaciation.The increase in salinity was probably caused by enhanced evaporation resulting fom increasing insolation since 18 ka.During the early and middle Holocene(10.6-5.8 cal ka BP),the study region was occupied by steppe vegetation dominated by Artemisia(?80%)and Chenopodiaceae(<15%),with high concentration and A/C value,indication of a warm and humid climate.The primary mineral assemblage found in the sediment sequences was evaporates(up to 90%),composing of gypsum,halite,and thenardite.In contrast,this epoch was poor in clastics and carbonates,suggesting a salt lake stage with plenty of saline minerals crystallized.The crystallization of saline minerals was probably related to the insolation-induced warming,which remarkably enhanced the evaporation causing a reduction of the lake water in spite of the increased rainfall.Over the past 34ka,variation in mineral assemblage suggested that lake environment were strongly affected by summer insolation,global and regional temperature,and Asian summer monsoon intensity.The vegetation history probably recorded summer precipitation related to the summer monsoon and insolation variation.