Study On The Evolution Process Of The Sedimentary Environment Since Later Pleistocene In Heihai Lake, Northern Tibet Plateau

Tibet Plateau is a large tectonic geomorphic unit in the world, because its formation and exist can generate dynamic action and thermodynamic activity, so that not only it change the climate pattern in the Eurasia, but also effect the northern hemisphere even global environment facility in the long time scale and large space range. Our study area Heihai Lake which locates on located on Kunlun rive-wild ox ditch fault valley in the east Kunlun Mountain of the northern of Tibet plateau is a cold area. The research area is high altitude and is lightly interfered by human activity which takes on nearly original natural state. Therefore, the lake sediments has good continuity which can reflect sensitively the change of the sedimentary environment in geological history. So Heihai Lake is an ideal region to investigate sedimentary environment evolution.In this study, we took the C2 lake material core and seven seismic profilers included 2ew、3we、4we。6ew。2ns。3s。4ns in the Heihai Lake. We analyzed the end member of the sediment by EMMA and the parameters of the grain size such as the mean grain size, sorting, skewness, kurtosis and component, and revealed the compositions of the lake sediment and typical sedimentary dynamic. Furthermore, based on the principle of the seismic interpretation, the seismic profiler was interpret, in combined with the sedimentary proxies such as the grain size, carbonate and total organic carbon, the spatial distribution of the sedimentary strata and sources of the sediment in the research area were revealed, and reconstructed the history of evolution in the environment since the later Pleistocene. The main conclusions of the dissertation are as follows:(1) According to the parameters of the grain size and EMMA analysis, the result show the sedimentary process was controlled by current, wind and glaciations transport dynamic, the sediment component includes clay, silt and sand composition, which represent weak current sedimentation, Aeolian sedimentation and glaciolfluvial deposit separately in the Heihai Lake. And during 16.7-15.9 KaBP, the fine sediment, more silt and clay showed the sediment is the alluvial deposit which is clayey silt; then during the 15.9-14.6 KaBP, the sediment is very coarse, less clay, more sand and gravel indicating glaciolfluvial deposit and sandy gravel; from 14.6KaBP to 10.7 KaBP the current and wind effect the transport process together, so the sediment contain alluvial deposit and Aeolian deposit, which is sandy silt; during 10.7-7.12 KaBP the sediment is more sand and silt, represent a Aeolian deposit; during7.12-3.42 KaBP the sediment contain alluvial deposit and Aeolian deposit, but more clay and less silt, represent a clayey silt; during 3.42-0 KaBP the sediment is more clay and less silt and sand, which is alluvial deposit, represent a silty clay.(2) According to the analysis of seismic profiler, the result indicating Since the late Pleistocene the sedimentary strata has evolved five stages from bottom to top as follows in the heihai lake:U5 is the permafrost that developed during Last Glacial Maximum—U4 is the lacustrine deposit in proglacial lake and glaciolfluvial deposit developed in the last stage of the later Pleistocene-U3 is the lacustrine deposit in deeper water lake and glaciolfluvial deposit developed during the last stage of the later Pleistocene to the early Holocene-U2 is the lacustrine deposit developed in the shallow water lake which is a weak dynamic environment during the mid-Holocene-U1 is the lacustrine deposit in deeper water lake developed during late Holocene.(3) the landform of the lake basin and the thickness distribution characteristics of strata sequences, showing that the sediment source was glaciolfluvial deposit of the northern and eastern low mountains during 22.0-14.6KaBP and the melt water deposit of the east Kunlun mountain during 14.6-OKaBP.(4) The result of the grain size, carbonate, total organic carbon and the seismic profiler, showing that the water level had two low periods and three high level periods since the late Pleistocene in the Heihai Lake. During 22.0-14.6KaBP the alluvial deposit and current dynamic indicated high water level happened when the climate is humidity and cold; about 14.6KaBP the erosion surface and coarse sediment in the U4 showed Heihai Lake appear a low water level, but the onlap, toplap and fine sediment in the U3 indicated the water level rose rapidly from 14.6 KaBP to 7.12 KaBP; during 7.12-3.42KaBP the lake level fell down again, but after 3.42 KaBP the lake level rose again. Furthermore, In this study, we speculate that the water possibly outflowed in 7120 aBP, based on the content of carbonate and total organic carbon abrupt change, and the lake level fell down to lowest water level during Holocene.(5) According to the result of the grain size, carbonate, total organic carbon and seismic stratigraphy, showing that the history of paleoclimate change was reconstructed since the later Pleistocene in the study area as follow:colder and dryer-warm and humidity-more warm and more humidity-cold and dry.