The Sedimentary Environmental Evolution Since The Holocene In The Maqu Plateau Recorded By Langqu Section

The sediments of desertified grassland in alpine region represent the significant changes of sedimentary external force with the climate change of warm-wet and cold-dry, but also record the forward and adverse process of desertification and the change of desertified strength during the geological history, so the sediments can serve as the information carrier to recovery the evolution of ancient sedimentary environment. A variety of indicators of elements, particle size, magnetic susceptibility and organic matter contain abundant environmental information in sediments, which can preferably reveal ancient sedimentary environmental evolution. Maqu Plateau is located in the east of Tibetan Plateau, this paper selected a typical paleosoil-fluvial sediment profile of Langqu in the southeast of Maqu Plateau, and utilized the 14 C and OSL, according to the comprehensive analysis of the environmental proxies of geochemical elements, grain size, magnetic susceptibility and organic matter, and compared with typical area, and rebuit the sedimentary environmental evolution process of Maqu Plateau since the Holocene.Research results showed that: major elements oxides of Langqu section were followed by SiO₂>Al₂O₃>Fe₂O₃>CaO>K₂O>Na₂O>MgO, the major elements of Ca, Na and Si were migrated and leached during the chemical weathering process, but the elements of Mg and Fe were obviously enriched, the activity and migration of the major elements were followed by Ca>Na>Si>Al>K>Mg>Fe. Most of the trace elements were enriched except the elements of Ba, Zr and Sr were obviously migrated and leached during the chemical weathering process, the migration of the typical trace elements were followed by Ba>Zr>Sr>Cu> Pb>Co>V>Co>Zn>P>As>Mn, from left to right, the trace elements relatively increased in the degree of enrichment. The SiO₂/Al₂O₃ ratio, Na₂O/K₂O ratio,?K₂O+Na₂O+CaO?/Al₂O₃ ratio, Cu/Mn ratio, Sr/Cu ratio and Sr/Ba ratio in fluvial deposition were higher than paleosoil, on the contrary, the chemical weathering indexes?CIA?, Al₂O₃+Fe₂O₃/RO+R₂O ratio, Rb/Sr ratio, Sr/Ba ratio, P/Mn ratio, and Zn/Mn ratio in paleosoil were higher than fluvial deposition. The migration features of the major elements and the A-CN-K diagram revealed that Langqu profile had completed the primary process of chemical weathering characterized by leaching of Ca and Na. The sand content in fluvial deposition was higher than paleosoil, on the contrary, clay, Md, Mz, SC/D, magnetic susceptibility and organic matter content in paleosoil were higher than fluvial deposition, which refered to the warm-wet environment in paleosoil, cool- humid environment in fluvial deposition.Through the analysis of geochemical elements, grain size, magnetic susceptibility and organic matter of Langqu profile in Maqu Plateau, and combined with 14 C and OSL, the environmental evolution in Maqu Plateau since the Holocene could be divided into 4 stages: intense aeolian activities and cool-humid environment before 9.4 ka B.P.; aeolian activities weakened and warm-humid environment during 9.4⁶.4 ka B.P.; intense aeolian activities and cool-humid environment during 6.4².3 ka B.P.; 2.3 ka B.P. to present, weaker aeolian activities and warm-wet environment.Though contrasted with the environmental information recorded by the northeast of Tibetan Plateau, Qinghai lake?QDST profile?, and the pollen of Gonghe Basin, found that the recording of sedimentary environment and Aeolian activities in Langqu profile was obviously consistent with Tibetan Plateau and around area since the Holocene, and the recording of sedimentary environment and Aeolian activities in Langqu profile was similar to MO and OL profile in Maqu Plateau since middle-late Holocene, the results showed that the sedimentary environment evolution in Maqu Plateau has the very good synchronicity with Tibetan Plateau and around area.