青藏高原东北部更尕海沉积记录的全新世湖泊水位与气候变化

The objective of this research is to reconstruct the Holocene history of climatic changes in the northeastern Qinghai-Tibetan Plateau, using sediments from Genggahai Lake. The lake is small and shallow, and is fed mainly by groundwater and occupied by abundant aquatic plants at present days. Sediment cores will be retrieved at five sites along a transection of the lake. Chronological frames of the cores can be established through multi-dating methods of AMS C-14, Pb-210 and Cs-137. Analyses of climatic proxies, including stable isotopic compositions of authigenic carbonates and organic matter, carbonate content, total organic carbon (TOC) content, total nitrogen (TN) content, and grain size, will be conducted on samples of the sediments. Biologic macrofossils, namely aquatic plant remains, stem ecrustations, and mollusk shells, will be identified and quantified along the sequential sediments, and then associating them with the specific living species in the lake. Meanwhile, the relationship between the individual living submerged plants and water depth will be quantified, which makes a quantitive or semi-quantitive assessment on lake-level fluctuations over the Holocene, on a basis of the variability of biostratigraphical macrofossils. The carbonates are composed of fine-grained component directly precipitated in water body, stem encrustations attached to submerged plants, and mollusk shells. Paleoclimatic significance of stable isotopes of the authigenic carbonates will be tested detailedly through modern monitoring on isotopic fractionations between the various carbonate components formed within different seasons. In combination with the lake-level history and the climatic changes recorded by other proxies, variations in oxygen isotope of autogenic carbonate are employed to demonstrate the possible changes in atmospheric circulation patterns during the Holocene, which could give rise in a profound influence on the Holocene climatic changes in the study area. In addition, by comparing the results of this study with the published work from different regions, the paleoclimatic connections between them will be investigated, especially for arid Central Asia and regions dominated by Asian monsoon. This research will provide some important information for studies on climatic changes in the marginal areas affected by Asian monsoon. Furthermore, it will play an important role in further understanding of the temporal and spatial differentiations and the potential forcing mechanisms for the Holocene climatic changes across Central and Eastern Asia.

项目选择青藏高原东北部更尕海沉积重建全新世气候变化历史。沿湖泊断面钻取系列沉积岩芯，采用AMS C-14、Pb-210和Cs-137等测年手段建立沉积岩芯的年代序列，分析沉积物碳酸盐含量、自生碳酸盐同位素、有机质碳同位素、TOC/TN、粒度组成等代用指标，鉴定、统计沉积物中植物大化石、软体动物壳体等。考察湖泊现生植物分布与水深的关系，利用植物大化石地层变化，定量－半定量重建全新世湖泊水位变化历史；理解不同成因碳酸盐的氧同位素分馏过程与机理，结合湖泊水位变化与其他代用指标揭示的气候变化事实，明确其气候指示意义，探讨全新世水汽来源变化及其与大气环流格局的关系。与不同地区的气候记录比较，探讨季风边缘区全新世气候变化的特点及其与典型季风区、中亚干旱区等区域气候变化的内在联系。项目的实施对于加深理解中东亚地区全新世气候变化模式的时空分异和驱动机制具有重要意义。

全新世气候环境的研究不但有助于理解气候变化的规律及其与人类文明演化的关系，而且有助于客观地认识和应对正在发生的全球变化。然而，有关全新世气候变化时空演变及其驱动机制的认识存在较大的分歧，因此需要更多的具有可靠定年且代用指标环境意义明确的气候变化序列。项目的主要研究内容包括：1）通过分析更尕海岩芯GGH-A的生物大化石、有机质碳同位素、自生碳酸盐氧同位素、粒度组成等，重建了更尕海16 ka以来湖泊水位、水汽来源（区域大气环流格局）、风沙活动等的变化历史；2）针对更尕海湖水、地下水开展了历时4年的夏季定位监测，同时分析不同水体、自生碳酸盐、沉水植物植株等的稳定同位素，理解碳酸盐碳氧同位素的气候环境意义；3）通过分析更尕海不同地点沉积岩芯岩性的变化，结合绝对测年和代用指标，理解湖泊充填过程与湖泊演化历史；4）通过比较更尕海沉积与研究区风成沉积记录的风沙活动历史，探讨了控制风沙活动关键因子的时空分异过程与机制。项目取得的重要结果：1）更尕海重建的水位、水汽来源以及风沙活动等序列反映了研究区气候变化的不同方面。尽管过程彼此独立，然而相互支持的事实表明代用指标解释与气候变化重建结果的可靠性。结果显示，早中全新世（11.4－6.3 ka）研究区亚洲季风强盛，水汽主要由季风输入，流域风沙活动减弱。晚冰期和晚全新世季风环流减弱，阶段性西风输入的水汽显著，风沙活动增强；2）更尕海不同地点沉积地层、生物大化石、粒度组成显示，晚冰期以来湖泊沉积中心存在自西向东迁移的特点，可能与湖泊上风向风沙物质的输入有关。项目的实施为理解季风边缘区晚冰期以来气候变化历史与驱动机制提供了可靠的证据。此外，湖泊沉积体系现代过程的工作对于推动利用湖泊沉积重建气候环境变化的研究具有重要意义。项目执行中课题组成员参加了11次国内外相关学术会议，培养了博士研究生1名和硕士研究生4名，目前已发表学术论文10篇，其中SCI论文6篇。