Analysis Of Lake Sediment And Its Palaeoenvironment Reconstruction In The Badain Jaran Desert During The Holocene

It is sensitive to the impact of climate change in the arid and semi-arid area, and it plays an important role for reconstructing historical period of the climate change. The Badain Jaran Desert is the second large desert in China, characterized by its coexistence of tall sand and lakes, and these have an irreplaceable significance for reconstructing environmental change during the Holocene. The ancient lakes, Zhunzhahanjilin and Taoerlegetu are located in the hinterland of the Badain Jaran Desert. Because of less human intervention, the two lakes can reflect the regional environmental evolution process more accurately.In this paper, we have analyzed two profiles of ancient lakes in the desert hinterland. We obtain the trend of different development stages of grain size composition and percentage of different grading, different layer of soluble salt content, carbonate content and organic geochemical index and other data for measuring and analyzing the samples from the two profiles. We also obtain the accurate peat age relatively through the AMS14C dating for large piece of plant residues at the bottom of two profiles in the peat. It reflect that the characteristics of climate change is complicated based on the trend of each index, the result of measuring and counting date. The main conclusions are as follows:1 The date of peat at the bottom of two profiles shows that the peat beds are formed in the early Holocene. The peat in the Zhunzhahanjilin is formed in 10.9?10.4 cal ka BP, the peat in the Taoerlegetu is formed in 9.4?8.8 cal ka BP, and the formation period corresponds to the sudden fall in temperature, these show that the two profiles are the product of the cold. It is indicate that the peat in the two lakes is the conduct of the combination cold and humid climate, combination of peat condition for water environment.2 There is a big difference between Zhunzhahanjilin and Taoerlegetu of organic content underlying the peat at the bottom of two profiles. The average of organic content underlying the peat bed is 0.75% in Zhunzhahanjilin and 2.82% in Taoerlegetu. It is instruct that the desert environment is more conducive to the production and preservation of the organic matter about 9.0 cal ka BP.3 According to deposition rate calculate the Taoerlegetu lake level is much higher in 7.8 cal ka BP. And we research group discovery that Zhunzhahanjilin had formed high lake level at about 7.4 cal ka BP, based on the field survey, the discovery of the high lake shorelines and the dating of biological shells. The above results show that the two lakes during the period of high lake level differ to 400 years in millennial scale. Under the condition of inconsistent lake deposition rate, the date calculation of high lake level period in Taoerlegetu is ignored, so the value exists of error inevitallly. It is indicate the lake level is much higher in hinterland Badain Jaran Desert about 7.5 cal ka BP.4 The climate is unstable during the Holocene, the volatility is also severe relatively, and the two lakes have different developmental stage. The grain size, carbonate and organic geochemical index show that it exist the evolution of lake and marshes- lake- dry lake basin in Zhunzhahanjilin and Taoerlegetu, and its form of evolution is more complicated. Together with those kinds of index curves, the lake evolution can be roughly divided into three stages in the desert hinterland:(1) 11?8.9 cal ka BP. The climate environment was cold and wet, with less water into the lakes and low lakes level. The peat was benefit to the accumulation and preservation under the condition of cold and wet.(2) 8.9^7.5 cal ka BP. The climate environment was warm and wet, with the development of lakes from shallow to deep, the water quantity increasing and lake level rising gradually. In the later stage, the lake achieved the highest water level during the stages of lakes, and the terrestrial vegetation coverage was also higher around the lake basin.(3) 6.0?0 cal ka BP. The climate environment turned into drought gradually, with less water into the lakes. The lake level dropped to dry up finally, the terrestrial vegetation coverage was lower around the lake basin and organic matter was not easy to save.