Element Geochemistry Evidence For Paleoclimate Evolution From The Redclay Over Xiaoshuizi Planation Surface

There remain some planation surfaces sporadically in the northeast of Tibetan plateau on the west of Liupan moutain.The deposition on the planation surface is very important not only for getting knowledge of the development or history of landform process and the age of landformformation,but also for reconstruction of the paleoclimate which is closely linked with thelandform process.In this paper,the deposition on the Xiaoshuizi planation surface is studied and after getting accurate chronology,grain size,carbonate content and elementary composition were measured.According to the integrated analysis of experimental datas and lithologic characters,some preliminary conclutions about the cause and transport agent of deposition,appropriateweathering indices and paleoclimate changes were drawn:(1)The deposition on planation surface is eolian deposition.The red-clay deposition may becarried by EAWM(East Asia Winter Mosoon)and westerly.The loess depositon may becarried by extremely strong ESAM or local circulation stronger than the typical EAWM.(2)Characteristic of changes in major element,correlation analysis,Principle componentanalysis synergistically indicate that K₂O,Ti O₂,Al₂O₃,Fe₂O₃,Si O₂,Rb,Ba,Zn,V and Zr areappropriate criteria elements applied to weathering indices.Ca O,Na₂O and Sr are sensitiveto climate change and can be mobile elements applied to weathering indices.In XSZ,thesensitivity of Na₂O to climate change is less than that of Na₂O and Sr.Mn O,Cl and P haveno indicator to environmental change.PIA,CIA and V are the most appropriate chemicalindices in XSZ while CPA and K₂O/Na₂O ratio are the most appropriate indices in SYT.(3)The paleoclimate change from late Miocene to Pliocene has been reconstructed by themultiple climatic proxy indices:during 6.7 to 6.2 Ma,climate is humid.The EAWM andwesterly circulation have the same trends and may be control by the global temperature.During 6.2 to 5.2Ma,paleoclimate is dry and stable.Weak EAM which enhanced with globalwarming,dominates the climate change on the planation surface.During 5.2Ma to 4.6Ma,paleoclimate is humid and the amount of precipitation,weathering and pedogenesis intensityincrease simultaneously.Enhanced EAM driven by expansion of the Antarctic ice sheet and westerly circulation moved southward when temperature decreases,put an equivalent impact on climate change of Xiaoshuizi planation surface.During 4.6Ma to 3.58Ma,precipitation amount,pedogenesis and weathering intensity decrease and the amplitude of dry-wetoscillation is larger.The EAM and westerly control the climate change of studied regiontogether.During 4.5 Ma to 4Ma,closure of the Panamanian Isthmus increased thermohaline circulation and enhanced the moisture supply to high latitudes.That enhanced moisture can be then transported to studied region via the westerly circulation which creates this extremely large amplitude dry-wet oscilation.There are 3 heavy precipitation events at about 4.25Ma,4.5Ma and 4.7Ma,which may result from global warming and circulation system(EAM and westerly circulation).(4)The red clay started to accumulate at about 7 Ma which indicate interia Asian aridification may start at that time.