The Holocene Juyanze Paleolake Evolution Process And Its Possible Forcing Mechanisms

The process of lake evolution contains abundant geographical environmental information,which is one of the important medium in studying the global climate and environment change.However,lake evolution is very complex,which is not only affected by climate change,but also affected by tectonic movement,river channel change,human activities and other non-climatic factors.Therefore,a thorough comprehension of the lake evolution process and its influencing mechanism is helpful to improve the reliability of using lake evolution as the basis for studying paleoclimatic and environmental changes.The Juyanze paleolake,located in the arid area of northwest China,was once one of the terminal secondary lakes of Heihe river,and the region is sensitive to climate change.There have been a lot of studies on the lake evolution and regional climatic and environmental changes in the Juyanze paleolake,but the understanding of its evolution process,mechanism and climate change signals during the Holocene were controversial,in which when the lake highstand formed and whether the lake evolution is suitable for the climate change research.The dispute arises from the different study methods and data frameworks.Therefore,in this study,?1?we used high-precisional Differential GPS to systematically measure the well-preserved series of paleoshoreline of the Juyanze paleolake,and analyzed the elevation and classification;used SRTM DEM data to extract contours,and restored the lake area according to the corresponding paleoshoreline elevation;a more perfect data framework was provided by accelerator mass spectrometry?AMS?¹⁴C with aquatic gastropod shells.?2?The Holocene evolution history of the Juyanze paleolake was reconstructed on the basis of the shorelines'elevation,corresponding lake areas and ages.?3?The hydrological characteristics of four characteristic periods of the basin are calculated by using the water balance model in combination with the paleoshorelines elevation and the corresponding ages.The main factors affecting the lake evolution in the middle to late Holocene of the Juyanze paleolake was analyzed,and the paleoenvironmental significance of the lake evolution was further discussed.In the field investigation,28 paleoshorelines were measured in different parts of the Juyanze paleolake,which were distributed nearly horizontal.All the shorelines could be divided to 12 classification based on the elevation,namely 912,914,916,917,918,920,921,922,923,924,925 and 927 m respectively.The AMS ¹⁴C dating obtained19 dates data and the results showed that all the surveyed paleoshorelines were formed in the middle and late Holocene?6.0-1.6 cal kyr BP?.Chronological reliability analysis shows that there is a centuries-old carbon influence when dating by AMS ¹⁴C use the Juyanze paleolake snail shells,which is suitable for the study on the millennium scale.Based on the field investigation and relevant experimental results,the evolution process of Juyanze paleolake was reconstructed during the Holocene,which could be divided into three stages according the elevation.1)before 6.0 cal kyr BP,there was no paleoshoreline be found,indicating that the lake water level was unstable.2)6.0-1.6cal kyr BP,a highstand stage of the Juyanze paleolake,and its lake level was on the rise.This stage also can be divided into three sub-stages:6.0-4.6 cal kyr BP,the lake began to develop steadily and the water level rose from 912 m?6.0 cal kyr BP?to 916 m?4.6cal kyr BP?with the area of Juyanze paleolake expanded from 720 km² to 969 km²;4.6-3.0 cal kyr BP,the lake continued to be in a highstand state and relatively stable,about3.5 cal kyr BP,the water level rose to 918 m with an area about 1154 km²;3.0-1.6 cal kyr BP,the water level continues to rise.About 2.9 cal kyr BP,the water level rose to920 m,and about 2.0 cal kyr BP,the water level rose to 923 m with an area of 1553km².3)After 1.6 cal kyr BP,no paleoshoreline was found,indicating the lake may be in an unstable state of continuous shrinking until dry.Compared with other sedimentary records of previous studies,it was found that paleoshoreline results had a correspondence with the profile results of the Juyanze paleolake,which all recorded that tow unstable water level stage before 6.0 and after 1.6 cal kyr BP,a gradual rise water level stage during 6.0 to1.6 cal kyr BP.Although the paleoshoreline could not record some details of water level fluctuation reflected in the sedimentary profile,it could record the information of highstand quantitatively,indicating that the wettest period occurred in the late Holocene?3.0-1.6 cal kyr BP?rather than the middle Holocene.According to the change of water level in the Juyanze paleolake,the hydrological conditions of the basin were calculated by using the water balance model in four characteristic periods,which is 6.0 cal kyr BP?912 m?,4.6 cal kyr BP?916 m?,3.5 cal kyr BP?918 m?and 2.0 cal kyr BP?923 m?.The results show that the mean precipitation in the four periods is 185.6,184.8,188.3 and 191.0 mm respectively;the water surface evaporation is 1144.1,1128.3,1145.1 and 1129.7 mm respectively;the runoff into the lake is 20.69,21.89,24.60 and 29.33×10⁸ m³ respectively.Compared with the average conditions of modern basins,each stage is characterized by higher precipitation,lower evaporation,higher effective humidity and higher runoff into the lake.This study indicated that the Holocene evolution of the Juyanze paleolake recorded by paleoshorelines did not impact by the neotectonic movements except the north of the Swan Lake,where might had a limited affection of the neotectonic movements.During 6.0 to 4.6 cal kyr BP,the evolution process of the Gaxun Nur basin was inconsistent with the Juyanze paleolake,which may have been influenced by the shift of the downstream channels.By calculating the runoff into the two secondary basins in6.0 and 4.6 cal kyr BP respectively,it is found that the channel shift has great influence on Gaxun Nur and little influence on the Juyanze paleolake during this period.The highstand trend of the Juyanze paleolake during the period of 6.0-1.6 cal kyr BP was coincided with the increases of the runoff and effective humidity in the catchment,indicating climate factors mainly influence its evolution.Moreover,human activities in the last two thousand years might have accelerated the drying up of the Juyanze paleolake.Therefore,during 6.0-1.6 cal kyr BP,climate change was the main factor affecting the evolution of Juyanze paleolake,and its evolutionary process could be applied to the study of paleoclimatic and paleoenvironment in the region.The evolution history of the Juyanze paleolake during 6.0 to 1.6 cal kyr BP could effectually reveal the reginal climate change,and the increased lake level indicates increased effective humidity.Moreover,the change of the effective humidity recorded in Juyanze paleolake is consistent with the effective humidity change pattern in the westerly region,so its climate change is mainly affected by the westerly circulation.During the mid-late Holocene,the enhancement of westerly circulation and the balance of solar radiation decreased in winter and summer in the northern hemisphere increased the regional effective humidity,thus forming the highstand of the Juyanze paleolake.In this study,the paleoshorelines of Juyanze paleoplake were systematically investigated,and the lake evolution process of the Holocene was quantitatively defined from the geomorphic evidence.Furthermore,the influence mechanism and regional climate change were analyzed,which could provide reference for the study of climatic and environmental changes in large regions.