Organic Biomakers And The Sedimentation Of Shell Bar Section From Qaidam Basin

The arid central Asia is the biggest non-zonal aid area in temperate zone on earth. The area has not only a fragile eco-environmental system with a changeable climate, but also is sensitive to the human induced global changes. Basin, which is situated at the northeastern Tibetan Plateau, is in the central part of this arid area, and is the conjunction area among the Westerly jet, southeastern Asia Monsoon and Southwestern Asia Monsoon that influence the climate of China and therefore, it is very sensitive to the regional and global changes, at the same time, the study results already show that the land surface changes in the area and huge amount of dust inputs into the atmospheric system not only influences isolation equilibrium and the marine life productivity, further influence the content of atmospheric CO2, the area is one of the most sensitive area that strongly response to human induced global warming. Detailed researches can promote our understandings of the climate change in the central Asia to the global changes and the mechanisms behind these changes. The study on the climate change history in the arid-cold area of the northeastern Tibetan Plateau is of great theoretical and applied importance.For these reasons, this thesis concentrate our attention on the Shell Bar section at the southeastern Cha'erhan salt lake, Qaidam Basin, based on the detailed discussions of the dating and chronology establishment, through analyses on the biomarkers of the organic matters, sedimentological characteristics and geochemical analyses, with the reference of the eco-climatic backgrounds of the living cubicula, ostrucod assemblages in the samples, the climate-environmental change history is reconstructed and the main results can be summarize as follows:1. the alkanes of the Shell Bar section show that between 39.6ka BP and 35.5kaBP, the maturity of the organic matter is high, the alkenes originated from the bacterial and alga, and water living creature are abundant, which implies that the main organic matters in this part of the section comes from the old materials with a high maturity organisms, and together with some lake organic matters. It indicates a main material input into the lake from the outside. Between 35.5 and 17.1kaBP, the maturity of the organic mater is low, implying that the degrading of the vegetations in the surrounding area of the lake that dominated by the grasses and the organism in the sediments originated mainly from lake itself. The Pr/Ph ratio is reduced from the lower part of the section to the upper part, which shows the reduction in the lake system increased and the saltification of the lake water in the later of the lake evolution history.2. from samples of the shell bar section, the branched aliphatic alkenes with quaternary substituted carbon atom (BAQCs) is identificated. It includes 5,5-diethylakanes of series A, 6,6-diethylakanes of series B and 5-butyl, 5-ethylakanes of series C;among them, A and C are the odd carbon number distribution pattern while B is of even carbon number distribution pattern and their abundant is A>B>C. The high value district of the series A corresponds to the high value of the B and C, indicating they share a similar source. Because the abundant of a series is lower in the earlier of the palaolake evolution than the late period, it can deduced that these three series originated most likely from kinds of bacteria and/or alga living in thewarm mesohaline waters. This is of great importance to the palaoenvironmental reconstruction because such kind of bacteria and alga are extremely sensitive to the environments. At the same time, A25/nC25 ratios change systematically in the section and possibly bearing the environmental information of the lake evolution process.3.the sedimentary and geochemical indices of the samples from the Shell Bar section show the palaolake evolution processes, that are from 39.6 to 35.5ka BP is the period of lake formation, from 35.5-33.3ka BP the lake level was high and stable, temperature in the area was more warm-humid than that of today. The lake level reached it maximum between 33.3 and 27.1ka BP, both temperature and precipitation were higher than today. The living creatures were blooming and the spices are abundant. Between 27.1 and 18.1ka BP, the climate was still warm, but the lake level started to lowering, most likely is because of the reduction of the precipitation amount and the deterioration of the environments. Start from 18.1ka BP, the lake retreated abruptly with a very strong increase of the evaporation, resulted in the salt formation. Then the lake shrank further and never reached the level again.4. Based on the existence of the fossil cubicula and their living climate backgrounds, it can be deduced that between 35.5 and 17.5 ka BP the paleoclimate recorded by the Shell Bar section from the Cha'erhan was 5- 11°C higher and the precipitation was 600-800mm more than that of today, it means that the climate was completely different from that of today. The ostracod assemblages reveal that salt degree was higher in earliest stage of the lake formation and then reduced very much along with the fresh water input. The lake level increased continuously along with the increase of the water input amount from both the precipitation and river water from the surrounding mountains.5.Based on the regional correlations between elevated high Qaidam Basin and lower Tengger desert, it show a similar evolution patter because in both areas the palaolake level started to increase and lake areas started to advance. The climate reconstructed in both areas show that it was warm and humid between 35 and 21ka BP, with a substantial increase of the temperature and precipitation, especially precipitation, and resulted in high lake levels in both areas. The study results from the separated geological evidences prove that palaolake in such different areas not only posses a similar evolution pattern but also started synchronously, imply the climate change resulted from the regional changes which links to global changes, providing new data for understanding the mechanisms of the climate change.