【作者】 李育；

【导师】 王乃昂；

【作者基本信息】 兰州大学 ， 地球系统科学， 2009， 博士

【摘要】 QTHO1、QTH02剖面位于猪野泽干涸湖盆,剖面的海拔高度为1309m。两剖面共采集14C年代样品14个(其中8个常规14C样品,6个AMS14C样品),并完成了孢粉、粒度、磁化率、碳酸盐含量、有机化学元素含量(TOC、TON、TOH)、有机碳稳定同位素(δ13C)等环境代用指标的测试,两剖面的沉积物记录了猪野泽地区及整个石羊河流域晚冰期以来的气候环境变化过程。本项研究从环境代用指标的对比研究入手,以孢粉记录为主,对比并探讨孢粉记录和其他指标之间的相互关系。根据猪野泽沉积物孢粉记录,该区域环境变化可以分为6个部分,从剖面底部到年代上限-13 cal ka BP,乔木花粉百分比相对较高,样品孢粉浓极低；-13 cal ka BP--7.7 cal ka BP期间,乔木百分含量较高,乔木花粉中以云杉(Picea)为主,本段花粉浓度仍较低；-7.7--7.4 cal ka BP期间,乔木花粉急剧减少,草本植物花粉平均94%左右,麻黄花粉(Ephedra)明显增多；-7.4--4.7 cal ka BP期间,花粉组合虽以蒿(Artemisia)、藜(Chenopodiaceae)为主,乔木及灌木花粉较少,但花粉植物种类较多,且花粉浓度急剧增加,为整个剖面孢粉浓度最大值；-4.7--1.5cal ka BP期间,藜科花粉(Chenopodiaceae)开始增加,蒿花粉(Artemisia)开始减少,麻黄(Ephedra),白刺(Nitraria)等花粉增加；-1.5 cal ka BP-0期间,蒿花粉(Artemisia)含量继续减少,藜科花粉(Chenopodiaceae)含量继续增加,麻黄(Ephedra)、白刺(Ni traria)、蔷薇科(Rosaceae)、菊科(Compositae)等干旱区的耐旱草本及灌木花粉含量继续增多。通过分析不同花粉指标与粒度指标之间的相关性关系,发现部分花粉指标和粒度组分之间相关性较好。较为显著的是,Betula、Quercus、Urtica的花粉百分比与较粗的粒度组分存在正相关关系,Picea花粉与Result Below 1.490μm、Result 1.49μm-21.50μm, Result 21.50μm-89.31μm三个粒度范围的组分存在正相关关系与Result 89.31μm-691.73μm、Result Above 691.725μm两个粒度范围的组分存在负相关关系。Artemisia与Pinus花粉与各个粒度敏感组分相关性不显著。孢粉记录与粒度组合之间的相关性主要是由花粉传播过程、湖泊面积、河流和湖泊水动力条件及局部植被、地貌特点共同决定。猪野泽沉积物磁化率值的变化主要表现了沉积物来源的变化,晚冰期早全新世,云杉(Picea)、桦(Betula)、栎(Quercus)等乔木花粉百分比相对含量较高,总花粉浓度较低时,该阶段流域中下游地区有效水分少,植被较为稀疏,地表物质较易迁移,成为终端湖湖泊沉积物的主要贡献者,沉积物磁化率值相对中全新世较高；中晚全新世,孢粉浓度较高,全流域植被发育较好,地表物质迁移受到植被覆盖的影响,流域中只有部分较易迁移的物质才能最终在终端湖沉积下来,这样的沉积特点和物质来源,决定了该段沉积物较低的磁化率。-1.5 cal ka BP以来,陆生风成沉积物的形成,导致了磁化率的急剧增加。猪野泽沉积物碳酸盐含量与总花粉浓度值有正相关的关系,高碳酸盐含量对应了全流域有效水分条件较好的阶段,同时总花粉浓度也较高,表明流域性植被覆盖度较高。猪野泽沉积物TOC、C/N和δ13C指标主要代表了,湖泊周围及流域的初级生产力及植物种类,沉积物中乔木花粉含量相对较高,花粉浓度低的时期,对应了,TOC、C/N较低的时期,期间δ13C(‰)平均为—25.31；中晚全新世花粉浓度较高,植被茂盛的时期,TOC、C/N均达到了全新世阶段的最高值,δ13C(‰)平均为-26.68相对较低；-1.1 cal ka BP以来,花粉浓度相对中全新世较少,干旱植物发育,TOC、C/N均下降,δ13C(‰)范围及平均值与剖面底部相近。猪野泽地区及石羊河流域晚冰期以来环境变化过程可以划分为6个阶段：猪野泽剖面底部一-13.0 cal ka BP,该阶段剖面位置处于湖泊边缘,石羊河流域中下游地区及湖泊周围植被覆盖度低,流域性有效水分较低,气候环境条件受整个中国北方及蒙古国干旱区大气候背景的影响,气候相对干旱,同时也说明这一时期东亚夏季风强度较弱,西风带带来的降水也较少；-13.0一-7.7 cal ka BP,该阶段湖泊面积扩张,流域有效水分条件增加,中下游地区植被相对稀疏,这可能与地球轨道尺度的太阳辐射量增加有关。从12000年前开始,增加的太阳辐射量加强了海陆热力差异,同时导致了东亚地区的季风强度；-7.7一-7.4 cal kaBP,该阶段湖泊面积突然缩小,流域中下游及湖盆周围植被覆盖度在增加,是流域性的环境突变事件,在未找到邻近区域及整个东亚季风区类似的气候事件的情况下,可能是流域内部水文循环变化和湖泊本身地貌演化的原因诱发了这次突变；-7.4—-4.7 cal ka BP,该阶段处于“全新世适宜期”,流域植被覆盖度高,湖泊面积达到全新世最大,形成了典型的干旱区盐湖,湖泊中浮游生物发育,全流域初级生产力较高,石羊河流域中全新世气候变化过程与西风带和东亚季风区大量研究结果相符,“中全新世适宜期”建立在较强的东亚季风和较为湿润的西风环流的基础上,可能受到季风与西风带环流的共同影响；-4.7一-1.5 cal ka BP,该阶段湖泊开始退缩,剖面上泥炭层和湖相沉积物交替沉积,流域植被覆盖度减小,一些旱生植物增加,根据对比其他区域的气候记录,该地区“全新世适宜期”的结束可能与东亚季风的整体退缩有关,但是要稍早于典型东亚季风区的气候记录,这可能是季风边缘区对气候变化的敏感性所致,也可能由于终端湖地区极端干旱的气候状况所致；-1.5一0 cal ka BP,该阶段湖泊基本退出剖面位置,早生植物含量在该阶段继续增加,植被覆盖度减小,这次进一步干旱化,可能与西风气流变干有关。最后,基于CCSM 3.0系统和湖泊能量、水量平衡模型,模拟了东、中亚地区全新世期间,湖泊水位变化的过程和原因。根据对比模拟结果和猪野泽全新世湖泊水位演化过程,发现猪野泽受到季风区和中亚干旱区影响湖泊水位变化因素的共同作用,其湖泊水位变化的特点既体现了季风区湖泊的部分特征,也表现了中亚干旱区湖泊的部分特征,由此进一步说明季风边缘区湖泊演变和气候变化的复杂性。更多还原

【Abstract】 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.更多还原