兰州大学机构库 >资源环境学院
古居延泽全新世湖泊演化过程及其影响机制
Alternative TitleTHE HOLOCENE JUYANZE PALEOLAKE EVOLUTION PROCESS AND ITS POSSIBLE FORCING MECHANISMS
魏志巧
Subtype硕士
Thesis Advisor李卓仑
2019-06-03
Degree Grantor兰州大学
Place of Conferral兰州
Degree Name硕士
Degree Discipline地球系统科学
Keyword全新世 湖泊演化 气候变化 古湖岸堤 AMS 14C年代 水量平衡
Abstract湖泊演化过程记录着丰富的地理环境信息,是全球气候环境变化研究的重要载体。但是,湖泊演化过程非常复杂,除了受气候变化的影响外,还会受到构造运动、河道变化、人类活动等非气候因子的影响。因此,深入理解湖泊演化过程及其影响机制,有助于提高以湖泊演化为基础研究古气候环境变化的可靠性。古居延泽位于我国西北干旱区,曾是黑河终端湖的次级湖盆之一,区域气候环境十分复杂。前人对古居延泽湖泊演化和区域气候环境变化做了大量的研究,但对其演化过程与气候环境指示意义的认识仍存在矛盾,主要争议点在于其高湖面形成时间及其湖泊演化是否适用于气候变化研究。以往研究结果的矛盾可能缘于研究方法和年代框架的不一致,因此,本研究(1)利用高精度差分GPS系统地调查了古居延泽保存良好的系列古湖岸堤,分析了古居延泽古湖岸堤的级数与高程;利用SRTM DEM数据提取等高线,进而恢复了古湖泊面积;利用岸堤上的古生物壳体进行加速器碳十四(AMS 14C)测年,提供了一套更为完善的年代框架。(2)在岸堤高程、湖泊面积和年代的基础上重建了古居延泽中晚全新世的演化历史。(3)利用水量平衡模型结合岸堤高程和相应年代,计算了流域四个特征时期的水文特征,结合区域相关研究分析了古居延泽全新世湖泊演化的主要影响因子,并进一步探讨了古居延泽中晚全新世湖泊演化的古环境意义。野外调查中,在古居延泽不同位置共测量了28道古湖岸堤,其分布近乎水平。根据岸堤高程可划分为12级,海拔分别为912,914,916,917,918,920,921,922,923,924,925,927 m。利用岸堤上采集的螺壳进行AMS 14C测年,共获得19个年代数据,结果表明调查的古湖岸堤均形成于全新世。年代可靠性分析发现古居延泽螺壳的AMS 14C测年结果存在几百年的老碳影响,适用于千年尺度的研究。基于岸堤高程和年代,本文重建了古居延泽全新世的演化过程,根据湖面高程变化情况可以将古居延泽在全新世的演化划分为三个阶段。1)6.0 cal kyr BP之前,未发现古湖岸堤,表明湖泊水位不稳定。2)6.0-1.6 cal kyr BP,发育高湖面,且水位逐渐增加,具体又可分为三个次级阶段:6.0-4.6 cal kyr BP,开始发育稳定高湖面,水位从6.0 cal kyr BP的912 m上涨至4.6 cal kyr BP的916 m,古居延泽湖域面积从720 km2扩张至969 km2;4.6-3.0 cal kyr BP,湖泊继续处于高水位状态且较稳定,3.5 cal kyr BP时水位上升至918 m,古居延泽湖域面积约1154 km2;3.0-1.6 cal kyr BP,湖泊水位继续上升,2.0 cal kyr BP时水位923 m,对应湖域面积达1553 km2。3)1.6 cal kyr BP之后,未发现古湖岸堤,湖泊可能处于不断退缩至干涸的不稳定状态。岸堤结果与区域其他沉积记录的湖泊水位变化具有较好的一致性,都记录了古居延泽在6.0 cal kyr BP之前和1.6 cal kyr BP之后水位不稳定而在6.0-1.6 cal kyr BP期间形成高湖面。岸堤虽不能很好地记录沉积剖面所体现的一些水位波动变化细节,但更能定量化地记录高湖面信息,指示湖泊水位最高时期在3.0-1.6 cal kyr BP期间。根据古居延泽湖泊水位的变化情况,本文利用水量平衡模型分别计算了四个特征时期的流域水文条件,即6.0 cal kyr BP(水位912 m),4.6 cal kyr BP(水位916 m),3.5 cal kyr BP(水位918 m)和2.0 cal kyr BP(水位923 m)。计算结果显示,四个时期的流域平均降水分别为185.6,184.8,188.3和191.0 mm;水面蒸发分别为1144.1,1128.3,1145.1和1129.7 mm;入湖径流量分别为20.69,21.89,24.60和29.33×108 m3。各阶段相比于现代流域平均条件,均表现出高降水量、低蒸发量、高有效湿度和高入湖径流量的特点。本文认为,除天鹅湖北部可能受到有限的新构造运动影响外,整体上古湖岸堤记录的古居延泽全新世的演化几乎不受新构造运动的影响。6.0-4.6 cal kyr BP期间,嘎顺淖尔湖盆与古居延泽的演化过程不一致,可能受到了下游河道摆动的影响。通过分别计算6.0和4.6 cal kyr BP两个次级湖盆各自的入湖径流量发现,河道摆动在这一时期对嘎顺淖尔的影响较大而对古居延泽湖泊演化的影响很小。古居延泽在6.0-1.6 cal kyr BP期间的水位增加与入湖径流和有效湿度的增加趋势一致,表明其主要受气候因子的影响。另外,近一千多年来流域的人类活动可能加速了古居延泽的干涸而不能有效反映区域的气候变化。所以,6.0-1.6 cal kyr BP期间,气候变化是影响古居延泽湖泊演化的主要因子,其湖泊演化过程可以用于区域的古气候变化研究。古居延泽6.0-1.6 cal kyr BP期间的湖泊演化过程能有效地揭示区域的气候干湿变化,水位的增加指示区域有效湿度的增加。并且,古居延泽记录的有效湿度的变化与西风区有效湿度变化模式一致,其气候干湿变化主要受西风环流的影响。中晚全新世期间,西风环流的增强以及北半球冬、夏季太阳辐射差额变小使得区域有效湿度增加,从而形成了古居延泽的高湖面。本文通过系统调查古居延泽的古湖岸堤,从地貌证据上量化地明确了其全新世的湖泊演化过程,并进一步分析了其影响机制和区域气候变化,有助于为大区域的气候环境变化研究提供参考。
Other AbstractThe 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 obtained 19 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) 6.0-1.6 cal 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 (4.6 cal 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, about 3.5 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 to 920 m, and about 2.0 cal kyr BP, the water level rose to 923 m with an area of 1553 km2. 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 in 6.0 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.
Pages72
URL查看原文
Language中文
Document Type学位论文
Identifierhttps://ir.lzu.edu.cn/handle/262010/344254
Collection资源环境学院
Affiliation
资源环境学院
First Author AffilicationCollege of Earth Environmental Sciences
Recommended Citation
GB/T 7714
魏志巧. 古居延泽全新世湖泊演化过程及其影响机制[D]. 兰州. 兰州大学,2019.
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