Sr/Ca Ratio Of Ostracoda And Its Paleoenvironmental Significance In Cenozoic,Northwest Qaidam Basin

The Qaidam Basin is the largest inland basin on the northern margin of the Tibet Plateau,and its huge continuous sediments contain rich paleoclimate and paleoclimate information since the Cenozoic.The ostracoda in the basin can describe the environmental changes qualitatively from the perspective of paleontology,and quantitatively analyze the environmental evolution from the perspective of geochemistry,which has important research significance.Studying the paleoenvironment and paleoclimate of the Cenozoic in the basin can provide continuous and long-term geological evidence for the evolution of the paleoenvironment since the Cenozoic in the basin.In this paper,103 shell-shell fossils were collected from multiple drilling cores in the northwestern Qaidam Basin and distributed in all Cenozoic strata except the Lulehe Formation.From the ecological properties of fossil shells,the characteristics of fossil combination,the Sr/Ca ratio analysis of the shells,the simulation calculation of paleosalinity and the changes of water level fluctuation,etc.,combined with the results of previous studies,the Cenozoic in the northwestern Qaidam Basin Changes in paleoclimate and paleoenvironment:(1)Since the Cenozoic,the Sr,Ca content and Sr/Ca ratio of the shell have shown several cyclic fluctuations from low to high: upper section of Xiaganchaigou Formation(Sr: 3017.3ppb;Ca: 10827.6ppb;Sr/Ca: 0.5695).The upper middle segment(Sr: 732.5 ppb;Ca: 14501.4 ppb;Sr/Ca: 0.0548)and the Shangyoushashan group(Sr: 562.7 ppb;Ca: 8448.9 ppb;Sr/Ca: 0.0581)appeared high value,the corresponding paleo-salinity and water level rise and fall in a mirror-like manner.(2)According to the property characteristics of the scale fossils collected in the northwestern Qaidam Basin and their combined zoning,it can be seen that from the Xiaganchaigou Formation to the Qigequan Formation,the scale species of Austrocypris levis ? Ilyocypris errabundis ? Youshashania ? Hemicyprinotus ? Cyclocyprise ?Microlimnocythere sinensis?Cyprideis?Eucypris?Leucocythere?Leucocythere mirabilis,the indicated changes in the water environment: salt water-super salt watersalt water-semi-salt water-salt water-super salt water-salt water-semi saltwater..(3)Since the Cenozoic,the evolution of lake environment in the basin is roughly as follows: the arid climate prevails in the 40~31.5 Ma basin;during the period of 31.5~22.0 Ma,the basin has experienced the process of humid-arid-semi-humid-arid evolution;22.0~15.3 Ma During the period,the climate in the basin was relatively humid;during the period from 15.3~8.1 Ma,the basin began to show continuous drought characteristics,and the degree of drought continued to increase;during the period of 8.1 to 0.4 Ma,the drought in the west of the basin increased,the northern part of the basin is more humid.(4)During the Cenozoic,the basin experienced three most drought events.The influencing factors are different: the drought of the basin around 34 Ma may be affected by global cooling,the uplift of the Tibetan Plateau,and the Paratethys Sea retreat;around 25 Ma the drought may be affected by the uplift of the surrounding mountains and the global climate;the global climate change around 13 Ma and the uplift of the Tibet Plateau jointly affected the changes of the paleoenvironment in the basin.