The Superficial Geochemical Fractionations Of The Elements And Its Environmental Significance In Lacustrine Deposition

Qaidam Basin, situated at the northeastern Tibetan Plateau, is in the central part ofthe arid Asia, which is the biggest non-zonal arid area in temperate zone on the earth,and is the conjunction area among the Westerly jet, southeastern Asia Monsoon andsouthwestern Asia Monsoon that influence the climate of China. Therefore, it is verysensitive to the regional and global changes. Detailed researches can promote ourunderstandings of the climate change in the central Asia to the global changes and themechanisms behind these changes. The study on the climate change history in thearid-cold area of the northeastern Tibetan Plateau is of great theoretical and appliedimportance.Shell Bar is the only one which has been discovered at Qaidam Basin so far, it hasan important significance. In the stratigraphy there are four layers containing largeamount fossil shells of Corbicula fluminea m(u|¨)ller and Corbicula lorgillierti philippi,with both taxa often kept together and standing in situ. Based on the spatial distributionpattern of the shell bar, sedimentary characteristics of the deposits, assemblages ofmicrofossils such as Ostracods, the appearance of fossil shell Corbicula and pollen ofPediastrum, all of them only could survive under certain water depth, it could beconcluded that the shell bar section is a continuously deposited sequence under thewater condition and possibly one of the most suitable geological records of thepaleoclimate change history. Based on the analytical results of the elements of the acidsoluble (AS) and residual (AR) fractions of the Shell Bar section and the correlationsbetween the related elements and their ratios, the geochemical fractionations inpaleolake deposits and their sedimentary environmental significance were studied.According to the results, the paleoenvironment change history between 43.5 cal ka B. P.and 22.4 cal ka B. P. (39.7～17.5 ka B. P. ¹⁴C) was reconstructed.1. According to the correlations between the elements (major, trace and rare earthelements) in AS and AR, which are been seen as the climatic proxies, indicated that thelacustrine deposition have different material sources, experienced varying geochemicalprocedures and the different response in the evolution pattern of environment. The AS fractions of the sediments have been utilized as a sensitive geochemical andenvironmental indicator in lake area, and AR fractions were always bearing thegeochemical characteristics of source area and information of weathering processes.2. It is not only necessary, but should separate AS fractions from the AR fractionswhen elements was used to study the paleoclimate and environmental evolution historyrecorded in the lacustrine deposits, especially during the high resolution paleoclimateand lake level reconstruction. In this way, we could not only avoid interfering amongvarious proxies and drawing misleading conclusions, but also improve ourunderstanding on natural processes and reliability of the information about materialsources, climate changes in the catchment and lake areas.3. Based on the sediment proxies of geochemical and other results of the section,the environmental change history was reconstructed between 43.9 cal. ka BP and 22.4cal. ka BP. During 43.9～43.5 cal ka BP, the paleolake has been formed and reached thestudy section. From 43.5 to 39.9 cal. ka BP, it is the development of the paleolake andindicated a reductive sedimentary environment, with the precipitation of the catchmentincreasing, the scope of paleolake had been expanding, the climate became warm-humidalong with the paleolake level increased and had strong chemical weathering of sourcearea. From 39.9 to 31.9 cal. ka BP, it is reveal a reductive sedimentary environment, theclimate during this period was the warmest and the paleolake reached it fully developedperiod because of the abundant precipitation and strong chemical weathering. From 31.9to 27.0 cal. ka BP, it was the transitional period of the environment recorded by theShell bar section, all proxies implied strong decrease of the lake level even thetemperature was high, it means that the climate shifted from humid-worm to warm-drythat resulted in an oxidative sedimentary environment. During the period between 27.0and 22.4 cal. ka BP, it is indicated an abrupt temperature decrease and strong lake levelfluctuations during the period, showing that the climate changed from a warm-dry into acold-dry condition. Further temperature decrease and a sudden lowering of lake levelsand shrank of the water body related to the strong evaporation resulted in the formationof the salt layer in the top of the section. Till then, the paleolake retreated quickly alongwith the further climate deterioration, the high paleolake level evolution history ended and never recovered again.4. According to the regional correlations between elevated high Qaidam Basinand lower Tengger desert, it shows a similar evolution patter. The study results from theseparated geological evidences prove that palaolake in such different areas not onlyposses a similar evolution pattern but also started synchronously, imply the climatechange resulted from the regional changes which links to global changes, providing newdata for understanding the mechanisms of the climate change. All of these can help usbetter to understand the climate change and the characteristics of the process duringLate Pleistocene. Thereby, we can explore the mechanism of climate change and itsimpact on human beings and the biosphere.5. This study suggested that the variations of elements contents, the ratiosbetween different components and their indices revealed the changes of the sedimentaryenvironments. A comprehensive test and detailed analyses on the same kind of deposits,such as lacustrine deposits using a similar technique with the same standard, is crucialto explore the regional climate changes and their responding to the global changes.