Geochemistry characteristics of loess deposits at northern foot of Tianshan Mountains

The continental loess-paleosol sequences of Loess in China have recorded the past climate and environment change of East Asia since Miocene. The loess on the northern pediment of the Tianshan Mountains is one important part of wind-blown deposits in the northwest of China and provides a good opportunity for studying the paleoenvironment variation in arid central Asia. The magnetic susceptibility of the loess has been widely used to probe the past climate change owing to its good correlation with marine oxygen isotope proxy. However,it is argumentative to use magnetic susceptibility as paleoclimate proxy in all areas for magnetic susceptibility is the comprehensive reflection of types,content and particle size of magnetic minerals,and influenced by the provenance,climate conditions of deposition area and other factors. A series of magnetic investigation of loess in recent years shows that the magnetic susceptibility enhancement model of the loess in the northwest of China is significantly different from the Loess Plateau,although we are not clear about the mechanism. So the research of magnetic properties of the loess and paleoclimate change in the northwest of China need to be re-examined by other parameters such as geochemistry,grain size characteristics. Most geochemistry investigation of loess in the northwest of China focused on Ili basin which is warmer and wetter than other places. So we carried out geochemistry investigations on the loess/paleosol sequence on the northern pediment of the Tianshan Mountains with few studies previously. Results show that:compared with the Loess Plateau,it shows a similar macroelement component but a significantly weaker chemical weathering intensity. Study on the geochemistry of loess on the Loess Plateau and lower reaches Changjiang River indicates migration ability of CaO is stronger than that of Na2 O. However,CaO plays a weaker mobility than Na2 O in Zhongliang profile,indicating Na has stronger migration ability than Ca in arid inland environment. Due to weak leaching of carbonate,the proxy Rb/Sr ratio which is able to reflect weathering intensity of the loess in the Loess Plateau have a poor corresponding relation with the strata in the Tianshan Mountains region. The chemical index of alteration(CIA)can reflect the weathering intensity and the strength of pedogenesis of the loess of interior arid area,which exempt from impact of differences of provenance and depositional environment.