The Pollen Records From Lake Sediments And Climate & Lake Model In The Marginal Area Of The Asian Monsoon

Through a variety of proxies, such as pollen, grain size, magnetic susceptibility, carbonate content, total organic carbon and organic carbon isotopes, to study the lake sediments, is widely used in reconstructing the past climatic and environmental changes. In response to the climatic and environmental changes, in the lake sediments, the proxies reflect a wide range of information from the lake basin and watershed. Studying the relationship among various proxies, allows us to better understand the significances of the proxies, and get a more accurate reconstruction of the past environment. This doctoral dissertation contributes a comparative study of the relationship between the pollen records and other proxies.Zhuye Lake, the terminal lake of the Shiyang River drainage basin, is located between the Badain Jaran Desert and Tengger Desert in Minqin Basin, the arid region of northwest China. In the marginal area of the Asian monsoon, this lake is sensitive to the evolution of the Asian monsoon, simultaneously, which is also affected by the westerlies, so it is an ideal place to study the Asian monsoon history and the interaction between the Asian monsoon and the westerlies. Since the 20th century, the ecological environment deteriorated sharply, so reconstructing the Late Glacial and Holocene environmental changes and understanding the geomorphologic history of Zhuye Lake are of great significances for the guidance of the ecological restoration and reconstruction in Minqin Basin and the Shiyang River basin.Late Glacial and Holocene environmental records were obtained from the QTH01 and QTH02 sections in Zhuye Lake, which lie in the dry lake basin, at 1309m after sea level, and fourteen 14C dates were sampled from the two sections, in which there are 8 conventional 14C dates and 6 AMS 14C dates. The fluctuations in pollen, grain-size, magnetic susceptibility, carbonate content, total organic carbon content, C/N ratios and 813C record notable environmental variation. Previous work has recorded little about the relationship between pollen records and other proxies in lake sediments, and our approach in this work is to combine information from these proxies in order to clarify some past environmental changes in Zhuye Lake. According to the pollen records, the vegetation history can be divided into 6 parts in the section, from the bottom of the section to-13 cal ka BP, with a relatively high percentage of tree pollen, and the total pollen concentration is very low. Between-13 cal ka BP and-7.7 cal ka BP, the tree pollen is rich, Picea pollen occupies the maximum score in the tree pollen, and the pollen concentration is still low; during-7.7--7.4 cal ka BP, a sharp decline happen to the tree pollen, and the average of herbaceous pollen reaches about 94%, the percentages of Ephedra pollen increase obviously. During-7.4--4.7 cal ka BP, although Artemisia and Chenopodiaceae accounted for most of the pollen, in contrast to the small percentages of the tree and shrub pollen, however, a large number types of pollen happen in this phase, and a sharp increase in total pollen concentration, which reaches the maximum level in the section. Between-4.7 and-1.5 cal ka BP, Artemisia begins to decline and Chenopodiaceae pollen begins to increase, and so on, Ephedra and Nitraria increase. During-1.5 cal ka BP-0, the content of Artemisia continues to decline, meanwhile, Chenopodiaceae continues to grow, and as drought-resistant vegetation, a large number of Ephedra, Nitraria, Rosaceae and Compositae pollen emerge.By analyzing the relationship between the pollen records and grain-size, a certain degree of relevance emerges between some of the pollen and grain-size components, such as the percentages of Betula, Quercus, Urtica pollen have a positive correlation with the coarse components. Picea pollen is positively correlated with the three components of Result below 1.490μm, Result 1.49μm-21.50μm, Result 21.50μm-89.31μm, and negatively correlated with the components of the Result 89.31μm-691.73μm, Result above 691.725μm. Some pollen, for example, Artemisia and Pinus are not significantly correlated with grain-size. Totally, the relationship between the pollen and grain-size is mainly controlled by the pollen spread, the lake area, the hydrodynamic conditions of the river, and the effective surface moisture in the whole drainage basin.The values of the magnetic susceptibility perform the main sources of the Zhuye Lake sediments. During the Late Glacial and early Holocene, the content of tree pollen is relatively high, but the total pollen concentration is low, in the middle and lower reaches of the basin, the water conditions are less effective, and with sparse vegetation, surface materials are easier to move to the terminal lake, which becomes the main contributor of the lake sediments. During the mid-Holocene, the total pollen concentration is very high, so the lush vegetation of the basin limits the migration of surface materials, only the materials, which are smaller and easier to transfer, are able to move to the Terminal Lake. The values of magnetic susceptibility are relatively high in this phase. During the late Holocene, from-1.5 cal ka BP, terrestrial aeolian sediments are formed in the section, leading to a sharp increase in the magnetic susceptibility.The carbonate content in the Zhuye Lake sediments is positively related with the total pollen concentration. During the phase of the high total pollen concentration, the high effective moisture keeps the high lake-level, in this case, the expansion of the lake can dissolve a large number of carbonate in the surface of the basin, and the increase of the surface water also can bring much surface carbonate, both of which maintain the high carbonate content.The TOC, C/N andδ13C represent the major plant species and the primary production levels in the lake basin. During the Late Glacial and Early Holocene, the high tree pollen and low pollen concentration correspond the lower TOC and C/N, in this period,δ13C(%o) for an average of -25.31. During the high pollen concentration phase, the TOC and C/N reach the highest in the section and with relatively lowδ13C (%o). From-1.1 cal ka BP, the pollen concentration is relatively low, and drought-resistant plants develop, the values of TOC, C/N decreased, theδ13C (‰) values are similar to those at the bottom phase of the section.The environmental changes, since the Late Glacial in Shiyang River basin, can be divided into 6 phases. During the bottom of the section to-13.0 cal ka BP, the location of the section lied at the edge of the lake, the scene of low vegetation cover appeared in the middle and lower reaches of the basin. The effective moisture was low and the dry climate was affected by the background of the large-scale drought in the northern China and Mongolia, which was controlled by the weak East Asian summer monsoon and the dry Westerlies. Between -13.0 and-7.7 cal ka BP, the lake area began to expand, but in the middle and lower reaches of the basin, the vegetation was sparse. The relatively increasing moisture may be related to the strengthening Asian monsoon. From 12,000 years ago, the increase in the solar radiation, which was induced by the earth orbital changes, enhanced the thermal difference between land and sea, at the same time, led to the intensity of the East Asia monsoon. Then, a basin-wide abrupt environmental change, which was likely caused by some changes in the lake geomorphology and hydrological cycle, persisted for about 300 years. The Holocene Optimum was between -7.4 and -4.7 cal ka BP in this region. Pollen concentrations reached their highest values in the section. The abundant herb pollen reflected the high vegetation cover in the lower part of the watershed. The lake-level, primary productivity and effective moisture reached their highest in the Holocene. The Holocene Optimum benefited from the strong East Asian summer monsoon and the humid Westerlies. Between-4.7 and-1.5 cal ka BP the lake-level fell and silty peat was formed at the site. The vegetation cover reduced and a number of Xerophyte increased. The end of the Holocene Optimum might be related to the weakening of the East Asian monsoon, but which was earlier than other records in the typical East Asia monsoon domain, this due to the climatic sensitivity in the marginal area of the Asian monsoon. In the last 1500 years the lake shrank further, while terrestrial sediments were deposited at the site. In this phase, Chenopodiaceae, Nitraria and Ephedra reached their highest values in the section reflecting an arid environment. The intensive arid trend might be related to the dry westerlies.Then, based on the CCSM3, lake energy balance model and lake water balance model, we examined the Holocene lake level changes and their reasons in the monsoonal and arid central Asia. By comparing the model results and the lake level history in Zhuye Lake, we found the lake level changes in Zhuye Lake, in the marginal area of Asian monsoon, were both affected by the factors of influencing lake level changes in the arid central and monsoonal Asian.