Element Geochemistry Characteristics And Climate Evolution Of Cenozoic Paleosol In Lanzhou Basin

The Lanzhou Basin is located at the intersection of the Tibetan Plateau,eastern monsoon region,and northwestern arid region of China,so is sensitive to the paleoclimatic changes.In this paper,the red sediments of Duitinggou profile in Lanzhou Basin are studied,and we recognize the following soil types,Eutric Fluvisol,Calcie Xerosol,Gypsic Cambisol,Yermosol,Eutric Cambisol and Orthic Luvisol from evidence of root traces,soil horizons and soil structures existing in Xiliugou Formation,Yehucheng Formation and Xianshuihe Formation.R-mode factor analysis is used to confirm our paleosol classification.On this basis,the mean annual precipitiation(MAP)and mean annual temperature(MAT)of Lanzhou Basin from early Eocene to middle Miocene are semi-quantitatively estimated by using chemical index of alteration(CIA).Supporting data from Rb/Sr and Si O₂/(Al₂O₃+Fe₂O₃)ratios in paleosols show that paleoclimate was semi-arid,arid to sub-humid from the Early Eocene to the Middle Miocene,also revealed transient climatic events.The conclusions are as follows:(1)At the beginning of early Eocene(?55-54 Ma),the decreasing Si O₂/(Al₂O₃+Fe₂O₃)ratio and the increasing of Rb/Sr ratio indicate the increasing pedogenesis.The high MAP(786-602 mm)and MAT(?11.0?)consistent with the warm climate conditions at the beginning of Early Eocene Climatic Optimum(EECO).Eutric Fluvisol and Calcie Xerosol are developed in the study area.(2)From the middle Eocene to the early Oligocene(?46-33 Ma),the increasing Si O₂/(Al₂O₃+Fe₂O₃)ratio,decreasing Rb/Sr ratio,low MAP(412 mm)and low MAT(8.4?)show a cold and dry paleoclimate.During the Oi-1 Glaciationat(?33.5 Ma),the Gypsic Cambisol and Yermosol developed.(3)At the end of late Oligocene(?29-26 Ma),the Si O₂/(Al₂O₃+Fe₂O₃)ratio decreased gradually while the Rb/Sr ratio increased.The increasing MAT and MAP(720-643 mm,10.5-9.9?)indicate warm-drought paleoclimate.During the Late Oligocene Warming,the Eutric Fluvisol developed.(4)At the beginning of early Miocene(?24-23 Ma),the increasing Si O₂/(Al₂O₃+Fe₂O₃)ratio and decreasing Rb/Sr ratio correspond to the Miocene-1Glaciation(Mi-1).At the middle of early Miocene(?20 Ma),the Si O₂/(Al₂O₃+Fe₂O₃)ratio decreased and the Rb/Sr ratio increased gradually.The MAT increased to 8.8?,and MAP increased to 615 mm.(5)During the middle Miocene(MMCO)(16-15 Ma),Si O₂/(Al₂O₃+Fe₂O₃)and Rb/Sr ratio reached to the peak indicate that the paleoclimate became warm and humid with the highest values of MAT and MAP(13.0?,961 mm).The Orthic Luvisol developed during the Middle Miocene Climate Optimum(MMCO).After that the paleoclimate became dry and the calcareous arid soil was formed.Finally,the impacts of the uplift of the Tibetan Plateau,regression of the Paratethys sea,and the change of planetary wind systems on the paleoclimate of Lanzhou Basin are discussed,which provide a new perspective for understanding Asian inland aridification.