Pollen Record Of Vegetation And Environmental Changes From Lakes' Sediment In Eastern Xinjiang China, During The Holocene

Presently, global climatic change is a hot topic around the world. Paleoclimatic studies are greatly important to understand the potential characteristics and trends of further climatic change. Chinese northwestern region is a key part of the arid central Asia. The climate there is dry, with a frangible ecology which is easy influenced by climatic change. Hence, I selected both lakes Balikun and Tuolekule which are located in eastern Xinjiang, to carry out the studies of vegetation and environmental changes during the Holocene. Both lakes are typical inland salt lakes without an outlet, and are ideal study sites to understand vegetation and environmental changes in this area.In March 2006, we collected several parallel sediment cores from both lakes. In June 2010, we extracted another 81-cm (TLKL10) sediment core, located near the center of Tuolekule Lake. The sedimentary record from both lakes covers the late Quaternary. This dissertation, however, solely focuses on the records during the Holocene.We selected terrestrial plant remains, bulk sediment, and pollen concentrate as the materials for 14C dating by conventional and accelerator mass spectrometry (AMS). As a result, we have21 age dates (14 in BLK06 core, seven in TLKL06 and TLKL10 cores) from both lakes. I analyzed fossil pollen samples from both lakes Balikun (205 samples, ca.55 years/sample) and Tuolekule (74 samples, ca.160 years/sample), as well as the modern pollen samples from both lakes (Balikun-46 samples; Tuolekule-23 samples). The overall purpose of this study is to reconstruct the vegetation and environmental changes in eastern Xinjiang, China, during the Holocene.It was a relatively dry environment, with the typical vegetation types of desert, steppe or desert steppe mainly consisting of Artemisia, Chenopodiaceae, Gramineae. and Compositae, as well as patches of birch forest in certain periods over the Balikun Lake region during the Holocene.The desert consists of Chenopodiaceae and Compositae. with low sedimentation rate indicating it was a dry environment between 11600-9400 cal yr BP over the lake region. The vegetation type changed from desert to desert steppe with main vegetation plants of Artemisia, Gramineae, and Chenopodiaceae between 9400-7900 cal yr BP. It was an interim period that the climate changed from dry, possibly cold to warm-wet.Since about 7900 cal yr BP, the steppe composition was further increasing, while the desert vegetation, such as Chenopodiaceae was abruptly decreasing, and Betula was quickly rising. The vegetation type was typical steppe with patches of birth forest. It was a warm and humid environment between 7900-4200 cal yr BP around the lake.However, percentages of Artemisia and Betula pollen were quickly decreasing. Conversely, proportion of Chenopodiaceae pollen was markedly increasing, indicating that the vegetation changed from typical steppe to desert. It was a typical centennial-scale climatic event between 4200-3800 cal yr BP.Since 3800 cal yr BP, the steppe vegetation, such as Artemisia, was quickly expanding and the local meadow pollen, such as the percentages of Cyperaceae and Thalictrum were also increasing. However, the percentage of Chenopodiaceae pollen was markedly decreasing during this period, while percentage of Betula pollen was still low. All of these signifying that it was a desert steppe or meadow steppe around the lake in the late Holocene, and the environment was also relatively warm and wet. However, comparing with the middle Holocene (7900-4200 cal yr BP), the environment had some deterioration.During the relatively warm and wet phase, there was a deterioration period of 2500-2300 cal yr BP. Artemisia pollen was reducing, but Compositae pollen, indicating the dry environment was increasing.The pollen diagram for Tuolekule Lake mainly consists of Compositae, Ephedra, Artemisia, Chenopodiaceae, and Caryophyllaceae. All of these major pollen types indicate a relatively dry environment around the lake during the Holocene. According to the pollen assemblages, the vegetation and environmental changes can be dividing into two main stages in the Holocene.In the early Holocene (11600-8000 cal yr BP), the pollen assemblages mainly consist of Compositae, Ephedra, and Caryophyllaceae, indicating a gravel desert over the lake region. At the same time, the low sedimentation rate indicates the weak weathering around the lake's catchment, as well as the low productivity of the lake due to the extremely dry and perhaps cold climate during this period.However, since 8000 cal yr BP, the vegetation composition had greatly changed. The gravel desert was markedly decreasing, while desert steppe or steppe plants, such as Artemisia, Chenopodiaceae, and Gramineae were quickly expanding around the lake. These changes denote that the climate became warm and wet in comparison to the early Holocene (11600-8000 cal yr BP). Similar to Balikun Lake, it also records a centennial-scale dry event between 4500-3800 cal yr BP. During this short period, the percentage of Chenopodiaceae pollen was quickly increasing, while percentages of Gramineae, Artemisia, and Cyperaceae pollen were apparently decreasing.Since 3800 cal yr BP, percentages of Gramineae, and Chenopodiaceae pollen were gradually decreasing, with their proportions eventually stabilizing. At the same time, Artemisia pollen was gradually increasing. All of these indicate that the climate was recovering, and became warm comparing with last stage (4500-3800 cal yr BP). The modern environment gradually formed since that time, especially since 2650 cal yr BP, when the vegetation and environment were relatively stable.The vegetation and environmental changes by pollen records from both lakes are coincident with each other in the Holocene. Dividing it into four major stages, it was dry and perhaps cold in the early Holocene (11600-8000 cal yr BP); warm and humid in the middle Holocene (8000-4500/4200 cal yr BP); and relatively warm and wet climate since 3800 cal yr BP, with a typical centennial-scale climatic event during 4500/4200-3800 cal yr BP in both lakes. Correspondingly, gravel desert or desert were the main types around both lakes in the early Holocene, and the typical steppe appeared in the middle Holocene, as well as the desert steppe with some local meadow in the late Holocene in this region.The characters of vegetation and environmental changes from the records in both lakes are consistent with the other records in Xinjiang and arid central Asia, even with records around the North Atlantic during the Holocene. However, the pollen results in both lakes are markedly different from the records in the Asian summer monsoon region, north margin of the Tibetan Plateau, and even the Altai Mountains region especially in the early Holocene, and pollen records in both lakes existing 2000-3000 years lag compared with the records in above region during the early Holocene.