Paleoenvironmental Evolution In The Neogene-Quaternary Period Of The Sanhu Depression, Qaidam Basin

Since the Cenozoic era,the uplift of the Tibetan Plateau has had a series of effects on the surrounding environment,forming a characteristic East Asian monsoon system,which has caused the arid environment in western China.The rapid uplift of the Tibet Plateau since the late Pliocene has increased the elevation of the plateau,the climate has become colder and drier,and the environment of ancient lakes has rapidly evolved.The Qaidam Basin is more arid,with lakes dried up,salt lakes,and even become dry lakes.The inner basin subsidence center of the basin migrated from west to east to the area of the Sanhu depression,and the basin has deposited a great thickness of Cenozoic sediments.The extraction of paleoenvironmental information from lake sediments in the basin can reflect the evolutionary process of the ancient ecological environment and paleoclimate,understand the process and characteristics of the uplift of the Tibetan Plateau,and the future development trend of the ecological environment within the basin.It is of great significance to study the global past environmental evolution mechanism.Since the Neogene in the Sanhu area of eastern Qaidam Basin,lake basins have deposited very thick lake sediments.The biomarker compounds of organic matter in lake sediments are of great important to the source of organic matter,vegetation types,and to the traceability of ancient climate and paleoenvironment.In this paper,we use gas chromatography-mass spectrometry to detect the samples of the normal biomarker alkanes for 30 lake samples within the depth of 2141 m⁴077 m in the Qijia-1well of the Dabson Lake in the Sanhu area.CPI,ACL,Paq,C₂₇/C₃₁,?C^-₂₁/?C⁺₂₂,isoprenoids and carbonates were analyzed,and the previous studies on sporopollen,paleostratigraphy,and paleo-vegetation were combined to obtain the following important understandings:?1?By analyzing the indexes of n-alkanes and isoprenoid alkanes,terpenoids,terpenoids,CPI,ACL,Paq,C₂₇/C₃₁,?C^-₂₁/?C⁺₂₂,and Pr/Ph,the results show thatQaidam basin neogene is given priority to with lake facies,organic matter is mainly from the aquatic plants,and less terrestrial herbs.?2?The depth of 4000 m from the Qijia-1 well was recorded from bottom to top in the Xiayoushashan Formation?N₂¹?,Shangyoushashan Formation?N₂²?,Shizigou Formation?N₂³?,and Qigequan Formation(Q₁₊₂).The evolutionary process reflects the evolution of the lake from wet to dry and from deep to shallow.At the same time,the water salinity gradually increased in the dry and cold environment since the Shangyoushashan Formation?N₂²?,limiting the growth of aquatic plants.?3?The comprehensive analysis of biomarkers shows that the early Pliocene lake in the Dabson lake area of Qaidam basin is relatively developed,with deep water and aquatic plants.Afterwards,the climate became dry and cold,the lake water became shallow,and the water plants and terrestrial plants increased their contribution to the organic matter in the lake sediments,characterized by grassland environment,and the forest was extremely underdeveloped.It indicates that the climate of Qaidam basin has cooled gradually since the Pliocene,and the rainfall has decreased,which is consistent with the global cooling background.