Late Quaternary Lake Evolution Of Juyanze Basin,Gobi Desert,NW China,Revealed By Quartz And K-Feldspar Luminescence Dating Of Palaeoshorelines And A Drill Core

The Alxa Plateau is located on the northwestern margin of modern East Asian summer monsoon(EASM)region and is characterized by arid climate and by gobi-desert landscape.Endorheic river systems,such as those of the Heihe and Shiyang rivers,flow hundreds of kilometers from their headwaters on Qilian Mountains on the northeastern Tibetan Plateau to oases and closed-basin lakes where they terminate in the Gobi desert region of northwestern China.These now dry lake basins produce extensive amounts of dust that is lofted high into the atmosphere and deposited throughout the Northern Hemisphere.Studying late Quaternary environmental change in this region and clarifying the interrelationship between gobi-desert landforms and inland lake high stands is therefore important in understanding modern environmental processes and how they both respond to and impact global climate change.The Juyanze basin is the eastern sub-basin of the larger Ejina Basin,a major interior drainage basin on the western edge of the Alxa Plateau.The Ejina Basin is fed by Heihe River,which under modern conditions alternately feeds the Juyanze basin and two other sub-basins(Gaxun Nur and Sogo Nur)located in the central and western Ejina Basin.Juyanze itself consists of two smaller sub-basins,East Juyanze and West Juyanze.We collected a 27 m-long drill core(JYZ11A)from the lowest part of the East Juyanze sub-basin.The lower portion of the core is comprised of reddish brown sandy gravels,while the upper 10 m consists of eolian-lacustrine deposits.Optically stimulated luminescence(OSL)dating was conducted to the core for chronology.Then we measured proxies including grain-size,low-frequency susceptibility and element content and acquired palaeoenvironmental records of Juyanze Basin.We used a differential GPS system to measure the elevations of well-preserved palaeoshoreline features at multiple locations around the Juyanze basin and mapped those shorelines using remote sensing imagery.Detailed lithological descriptions and sedimentary analyses were obtained from typical localities along these shorelines.Extensive OSL studies conducted in the laboratory to establish reliable protocols for quartz OSL and K-feldspar post-IR infrared stimulated luminescence(p IRIR)dating.These protocols were then used to date the shorelines systematically.By combining the elevations and chronologies of the palaeoshorelines with climate proxy records taken from the JYZ11A drill core,it is possible to reconstruct lake evolution processes during late Quaternary and to discuss possible mechanisms for climate change on multiple time scales.Juyanze palaeolake surface areas at different periods were estimated using a modern Digital Elevation Model(DEM),allowing the climatic implications for these lake area variations during the Holocene in Ejina Basin to be interpreted.The basic conclusions of this dissertation are:1,Eight distinct palaeoshorelines with surface elevations of 944,940,927,924,920,918,914and 912 m were identified,mapped,and described.These are above the central basin floor(895 m elevation)with water depths between 49 and 17 m,indicating the palaeolake varied from 49 to 17m deep in the past.2,Results of a dose recovery test for the quartz OSL signal suggest that the quartz single-aliquot regenerative dose protocol(SAR)is reliable to date lacustrine and palaeoshoreline deposits in Gobi Desert region after?60 ka.Results of both sunshine bleach tests and modern analog residual dose tests indicate that both the coarse-grained quartz OSL and the K-feldspar p IRIR signals from the eolian-lacustrine sediments in the drill core and shorelines are well-bleached and were reset before burial.Results of anomalous fading tests(g value measurements)and dependence tests between equivalent dose and prior IR stimulation temperature results imply that K-feldspar p IRIR signals are stable,taking into account minor age underestimations.K-feldspar p IRIR signal characteristics indicate that K-feldspar p IRIR dating could be used to establish the chronology of eolian and beach sediments from palaeoshorelines dating up to?330 ka.3,Dating of the palaeoshorelines suggest that the Juyanze Basin contained stable lakes at>330,?314-303,?220-181,?122-76 and?3-1 ka,corresponding to MIS 11 or earlier,MIS 9,MIS 7,MIS 5 and the Holocene.This sequence suggests lakes formed on distinct?100 ka cycles.During MIS 9 and MIS 7 the mega-lake reached to at least a depth of 49 m and covered Juyanze,Gaxun Nur and Sogo Nur,forming a 10,000 km~2 palaeolake that covered much of the lager Ejina Basin.During MIS 5 and the Holocene lakes reaching a depth of 32 m and covering up to 1800km~2 of surface area formed in the Juyanze basin.4,Multiple cycles of eolian-beach sandy gravels in the 927 m shoreline sediments suggest there were several hydrological cycles of mega-lake Juyanze with marked fluctuations during MIS5.Neither drill core nor shoreline sediments contain any MIS 3 deposits,possibly indicating no stable lakes formed in the Ejina Basin during MIS 3.5,Comparison of drill core and shoreline records suggests that the Juyanze basin experienced remarkable shifts in desert-lake conditions during the Holocene.Before 8 ka no drill core deposits or palaeoshorelines were identified,possibly reflecting an extraordinarily arid climate and deflation.During?8-6 ka no stable lake was present in the basin and we hypothesize that eolian sands covered the basin floor.A lake then began to develop and rose to a high level of 918 m at?5ka.After 5 ka lake levels dropped,but rose again at?3.3 ka.During?3-1 ka there were at least four distinct lake level fluctuations at?2.9,?2.0,?1.5 and?1.0 ka,corresponding to water depths of 32-29 m.After?1 ka the lake rapidly regressed.6,Stacked lake-level and lake area results from the Juyanze basin and Gaxun Nur-Sogo Nur basins suggest flow in the Heihe River shifted between the basins several times during the Holocene.Variations in the total Ejina Basin lake area during the Holocene indicate dry conditions during the early Holocene(?11-8 ka),a medium-sized lake area in mid-Holocene(?8-5 ka),a gradually expanding lake area during late Holocene(?5-1 ka)with a maximum during a?3-1 ka period,and a rapid deterioration over the past?1 ka.This study,combined with previous archaeological and documentary research,indicates the Heihe River shifted across its distributary delta several times during the historical period,directly influencing migrations of its terminal lakes and the vicissitudes of oasis civilization.EASM precipitation changes induced by shifts in insolation and glacial-interglacial boundary conditions appear to dominate the Juyanze basin lake history on cycles of?100 ka.The shoreline features of mega-lake Juyanze,together with palaeoenvironmental records from adjacent areas suggest favorable climatic conditions with extensive precipitation during MIS 5.During the Holocene,the pattern of lake area variation in the Ejina Basin was much different from that in adjacent areas.We hypothesize the diversity of hydrological changes across this broad area of northwestern China was caused by the interrelationships of monsoon and westerly circulation in the region.Frequent shifts in the Heihe River distributary system during the Holocene and human activities in historical times are also important factors impacting hydrological changes in the terminal lakes of the Ejina Basin.