Geochemical Characteristics Of Loess Deposits In The Eastern Tibetan Plateau And Its Environmental Implications

Aeolian loess was closely related to the past atmospheric circulation and the environmental conditions.Loess is widely distributed in the eastern Tibetan Plateau?TP?,which is one of the ideal material for the paleoclimate reconstruction,the evolution of monsoon circulation system and the effects of the TP.The detailed investigation on the geochemical characteristics of the plateau loess is great helpful to understanding the sedimentary processes,the sources area and the paleo-environmental implications.This paper conducted a systematic element geochemical analysis on the loess from the eastern TP,and the provenance significances and the chemical weathering processes since the last glacial period are discussed.It can be concluded as follows:The geochemical analyses of the surface soils and loess deposits in the eastern TP show that the major elements SiO₂,Al₂O₃,Fe₂O₃ and CaO,are consistent with typical loess from the Chinese Loess Plateau?CLP?,with large variations of the SiO₂ and CaO.The elements of Rb,Sr,Sr,Ba,Zr,V take the most part of trace elements in all samples,while the concentrations of trace elements in loess and paleosol are quite different.The comparisons of the provenance proxies of the SiO₂/Al₂O₃?TiO₂/Al₂O₃?Zr/Al?Zr/Ti?K/Rb?K/Ba?Zr/Hf?Y/Nb and the major elements ternary?Ca-Mg-K,Ca-Mg-Na?between the loess from the eastern TP and that of the CLP and the Hexi Corridor,have shown that the eastern TP loess has different source with the loess from other regions.Moreover,the elemental characteristics of the loess and surface soil,fluvial sediments,aeolian sediments from the eastern TP are quite similar.Thus,we can infer that the loess deposit is mainly from the inland of the TP.The major and trace elements along the loess sequence from Ganzi have shown the increasing trends of the concentrations of the CaO and Na₂O at Ganzi and the decreasing trend of the Al₂O₃,Fe₂O₃,MgO and K₂O since the Last Glacial,which are similar with the loess from Jinchuan.The concentrations of the trace elements including Co,Cr,Cu,Ni,V,Zn,Rb,Sr,Ba increase along the depth at Ganzi,and those concentrations increase firstly with the depth then decrease at Jinchuan.The provenance proxies of the ratiosSiO₂/Al₂O₃,TiO₂/Al₂O₃,SiO₂/TiO₂,Zr/Al,Zr/Ti and Y/Al have no obvious change along the depth at Ganzi and Jinchuan,which indicate that there is no significant change in the loess provenance in the eastern TP.The CIA indexes of the Ganzi section vary from 56.73 to 68.53,indicating a slightly weathered stage,while the CIA indexes of the Jinchuan section vary from 61.34 to 71.37 showing a moderately weathered stage.The weathering strength of the Ganzi section shows a decreasing trend since the Last Glacial.The weathering strength of the Jinchuan section fluctuates continuously with the alternation of the loess and paleosol.The CIA and CPA are obviously lower during the Glacial and higher during the Interglacial.The ratios of CaO^*/MgO,Na/K and?Ca^*+Na+K?/Al decrease with depth at Ganzi and Jinchuan since L1,which indicates the gradually drying trend in the eastern TP since the Last Glacial.This study will further our understanding on the geochemical characteristics of the loess-paleosol sequence and its paleo-environmental implications in the eastern TP,as well as in understanding the sedimentary process,regional environmental changes.