Geological Evidence Of High Lake Level In Baijian Lake Since 200 Ka And Its Denotation For High Lake In Tengger During Late Quaternary

Lakes in the arid and semi-arid area of western China,especially in the transitional zone of monsoon and westerlies are sensitive to evolution history of the monsoon and westerlies system.They also can clearly and objectively record the changing of the climate and environment in these region.Previous studies' dating results of shells in the shorelines from Yabrai Lake,Juyan Lake and Jilantai Salt Lake showed that climate in the western region of China was exceptional warm and wet and developed large scale high lakes during Marine Isotope Stage 3(MIS 3).Ice cores records from Tibet Plateau,sporopollen analysised results and ostracod assemblages in lacustrine strata from Tengger commonly supported the conclusion that climate in the western region of China was truly different from other regions on the earth in MIS3.This brought widely discussion in relevant academic field in early stage.However,other studies through optical stimulated luminescence(OSL)dating method holds the different opinion.Some scholars dated the ancient lake sediments from lakes like Qinghai Lake,Gahai Lake,Qarhan Salt Lake,"Jilantai-Hetao" megalake,which located in the modern edge area of East Asian Summer Monsoon and the periphery of Tibetan Plateau.They proposed those lakes were developed earlier than 60-50 ka.This results are obvious different from the former conclusion.In fact,the "Megalake Tengger",being a widely explored lake in western areas of Tengger during MIS 3,its age of development is still a open question.The predecessors investigated and dated multistage lake shorelines and lacustrine strata in western region in Tengger and inferred there may developed a "Megalake Tengger"with area more than 16,000 km2 in Tengger areas during MIS 3(38-26 ka).There even have "Megalake Tengger Period" with lakes covering bigger than 20,000 km2 and the highest lake level with altitude of 1320 m a.s.l.during?42 ka.Soon afterwards,some scholars proposed a high lake reaching altitude of 1325 m and covering 2130 km2 in the region of Baijian Lake during 1-0.64 ka.However,further studies by OSL method indicated the age of the shoreline with altitude of 1315 m a.s.l.is about 74-96 ka.Therefore,it still a open question of the altitude and age of the highest lake level of Baijian Lake since late Quaternary.But these problems are critical for understanding the question "whether the climate in Tengger area is really different from other regions on the earth in MIS 3?",Therefore,it is essential to clearly answer the age of highest lake level of Baijian Lake since Quaternary emerged during MIS 3 or MIS 5.Guiding by this scientific problem,we investigated stratums of profiles and outcrops in almost 200 km2 areas of Baijian Lake by method of 3S(GIS,RS,GPS)and tradition methods of Geology and Geomorphology.We also analysised eight hydrological boreholes in northeast region of Minqin Basin.The method of Granularity,Ostracod,OSL dating are all applied in the studying of these stratums.We draw the conclusion as follows:1.By interpreting remote sensing images of northwest of Baijian Lake,we find there are ten suspected lake shorelines with the altitudes ranging between 1308-1345 m a.s.l.in this region.Comprehensively analysis of field investigation,stratums of profiles and outcrops,results of granularity and sedimentary facies analysis commonly support that there not only exist a series of lake shoreline with altitudes ranging between 1308-1315 m a.s.l.as predecessors reported,there also developed higher lake shoreline in the region with altitudes of 1320-1350 m a.s.l.In fact there even have typical lacustrine strata in the altitudes of 1350-1389 m a.s.l.Ostracod analysising results of the lake shoreline in 1320 m a.s.l.,which was developed in the stage of Megalake Tengger Period,show that the ostracoda fossils in this shoreline are all lake species lives in shallow water like Cyprideis torosa,Limnocythere inopinata and Darwinula stevensoni.Combining the sedimentary facies distinguished results of the representative profiles from this shoreline,we infer that the main sedimentary environment in Baijian Lake region is lake environment.2.OSL dating result of the lake shoreline at the altitude of 1310 m a.s.l.is ranging between 68-73 ka.It indicate the highest lake level at this elevation developed before MIS 3.The age of the shoreline with altitude of 1320 m a.s.l.is sometime prior to 193-154 ka.Sedimentary facies distinguished results and OSL dating result both support that there ever developed a highest lake level with altitude of 1310 m a.s.l.during sometime prior to 68-73 ka.The dating results of samples from the shoreline at altitude of 1320 m a.s.l.show this shoreline was developed about 155-193 ka.Comprehensive consideration those dating results,we infer the controversial highest lake level with altitudes between 1315-1310 m a.s.l.since late Quaternary is older than former opinion of MIS 3(20-40 ka).Its' earliest possible age is more than155-193 ka and no later than 68-73 ka.3.This series lake shorelines developed in the region of Baijian Lake is the relic of the biggest lake in the highest elevation of this area.With water reducing gradually,the lake scale became smaller and developed different lake shorelines at different elevation.Topographic profile analysis based on DEM show that it is continuous and have no mutation in the area with altitudes lower than 1350 m.a.s.l.Ostracoda fossils in the shoreline at altitude of 1320 m a.s.l.indicate salinity of the highest lake at this elevation is ranging between 2‰-15‰.This characteristic show that the lake may developed from a bigger freshwater lake in higher elevation and this concentration progress continued to nowadays and made the Baijian Lake become a salt lake with 1 m depth salt water.In addition,dating results by different methods show a characteristic that the ages become younger when the elevation gradually lowered.Changing of lakes state and evolution process based on GIS of the highest lake level with altitudes of 1327-1308 m a.s.l.show that evolution of the lakes in region of Baijian Lake is continuous,the lake retreat process has inherited as a hole.Synthesize the features of topography,salinity,OSL dating results and the simulated results of lakes evolution process,we conclude that on matter the highest lake level located at the altitude of 1320 m a.s.l.,or the highest lake level with altitudes between 1315-1310 m a.s.l.,all these lakes were just a fragment of the highest lake older than 155-193 ka during its long retreat evolution process.