| Lake evolution and hydrological cycle during the late Quaternary in NW China has been a focus of recent studies on the natural state of the Earth’s climatic systems. Hence, this study not only provides the basic information for further research of the hydrologic cycle and water resources in arid region, but also provides new evidence for environment evolution and its mechanisms. This doctoral dissertation selected Badain Jaran Desert and its marginal area as the object of study, lake levels fluctuations were recovered during the characteristic periods in Late Quaternary. Based on parameters being debugged according to the field position observation and the water level observation data, Basin precipitation and lower reaches runoff in the Heihe River and precipitation in deep desert were calculated during the characteristic periods by the water and energy balance equations. At the same time, the lake hydrologic cycle evolutions respond to climate change were researched by high lake levels. Besides, the different mechanisms between lake groups in deep desert and close lakes in arid area were discussed. All of the doctoral dissertation works are below.1Field investigations including lake shoreline measurement and14C chronology have been carried out on the Alashan Plateau to better understand the formation and evolution of the high lake levels in northwest China during the late Pleistocene. Comprehensive field investigations show that there are at least ten shorelines at different elevations at Juyan Lake, at least four on the northwest side of the Jilantai Salt Lake, and nine shorelines and one terrace on the northeast margin of Zhuye Lake. Stratigraphic correlations and chronological evidence indicate that the paleo-Iakes have the highest lake levels in Marine Isotope Stage (MIS)3. Though the optically stimulated luminescence (OSL) results which have been reported by some scholars don’t support this view, the fact that the paleo-lakes have the highest lake levels in Marine Isotope Stage (MIS)3can’t be denied. Differences between OSL and14C results from this work might indicate that high lake levels existed on the Alashan Plateau during two different periods. It also suggests that the high lake levels may exist in both MIS5and early MIS3. This paper also provides the basic hydrological information for further water resources research in this arid region.2Deep deserts are lack of chronology materials, which are always a hot and difficult point in palaeoclimate and palaeoenviromenet chorology research. Hence, selected suitable dating materials is the key factor of solving this question. In this doctoral dissertation, total30calcareous roots samples which were located in Alxa Plateau were collected for14C dating, and then their palaeoenviromenet significances were discussed. The14C dating results showed that most of16calcareous roots samples in Tengger Desert were formed during the Mid Holocene. In Badain Jaran Desert,9samples were formed during the MIS3a,2samples were formed during the MIS2and other3samples were formed during the Mid Holocene. Statistically, as the inorganic matter, calcareous roots were affected by hard water effect and others factors mostly as same as shells, which indicated that the calcareous roots are suit for14C dating. At the same time, the dating results showed that they were reliable in millennium scales during the Holocene, but the century scales results improvement were needed due to other factors. On time scale of104years, calcareous roots are suit for14C dating. However, during the MIS3a, because of14C dating accuracy and measurement limits for14C dating, veracity are lower than the Holocene results in millennium scales. Besides, most of calcareous roots on Alxa Plateau were formed during the MIS3a and Mid Holocene, which indicated that humid environment was suit for their formation; it’s the same in reverse.3The research on how to reconstruct the runoff of a river with a proper method has been the crucial part of paleo-climatology and paleo-hydrology. With a research object being from the lower reach of Heihe River to the terminal lake, using the measured data and the improved water and energy balance equations, this paper has managed to reconstruct the annual average runoff of zhengyi Gorges hydrological station in Heihe River lower reaches during a period from2002to2008. The result of it indicates that there exists an error between the reconstructed annual runoff and the measured values. However, the error between the reconstructed average runoff of several years and the measured value is within10percent, that means the water and energy balance equations are successfully used to reconstruct the runoff in arid regions. What is noted is that because of region difference, parameters must be debugged when applying water and energy balance equations, or it will result in great error. In addition, it is found that the latent heat of the evaporation of lakes in arid area is greater than their net radiation according to the calculation of the latent heat and the sensible heat, which demonstrates the existence of cold island effect.4We examined the vestigial evidence of the extent of paleo-lakes, and using reasoning outline above, reconstructed the paleo-lakes. GPS co-ordinates measured on-site, and the paleo-lakes levels fluctuation plotted onto1:50,000scale topographic maps. The results showed that the area of terminal lake of the Heihe River was over7430km2during MIS3a. More specifically, there was the largest lake area during34cal ka, and then it retreated to5400km2 during26cal ka. Using water and energy balance equations, quantitative reconstruction of the lower paleo-runoff and basin precipitation in Heihe River showed that the basin precipitation was about180mm which was30%higher than today, beside the evaporation accounted for30%of today. During34cal ka, the annual lower runoff was more than61.95×108m3, which was1.7times than today. Moreover, during32～26cal ka the annual lower runoff was little difference from recent runoff of the river from the mountain areas, but3.5～4times than lower runoff. In that case, high basin precipitation and low evaporation may cause the high lake levels formation, the lake levels fluctuated on different time scales during MIS3a may influenced by the precipitation distributing variety in Heihe River Basin.5There were many high lake levels evidence such as geomorphology and sedimentology in deep Badain Jaran Desert. Based on lake areas reconstructed during the characteristic periods in Late Quaternary, lake evaporation in the desert were calculated during31cal ka and7.4cal ka by water and energy balance equations. The results indicated that the evaporations were697mm and616mm respectively. They were both50%lower than pan evaporation in recent desert. Besides, the precipitations were260mm and200～250mm respectively, higher than modern precipitation in this region, which confirmed that effective precipitation increased during MIS3a and humid environment during the Mid Holocene. Hence, effective precipitation increasing is one of the key factors that high lake levels formation in deep desert. However, there were different mechanisms about lake evolution between deep desert and other inland area. The former’s lake evolution wasn’t only depended on climate change due to the lots of ground water recharge. It was controlled by the lake water budget and total amount of water.6Palaoe wind direction during the cold periods in Last glacial was recovered by palaeo sand dune trends counting (total108groups). The results showed that prevailing wind direction was northwest during cold periods in the Last glacial, which was different from prevailing west wind today. It indicated that it was controlled by high atmospheric pressure from Mongolia and East Asian winter monsoon intensity increasing. Besides, the West wind located to the south. |
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