Grain-size-dependent geochemical characteristics of Middle and Upper Pleistocene loess sequences from the Junggar Basin: Implications for the provenance of Chinese eolian deposits

Abstract

Thick loess-paleosol sequences are widely distributed in the arid inland basins of northwestern China. The geochemical composition of these sediments provides important constraints on the provenance of Chinese eolian deposits. However, grain-size-dependent geochemical characteristics of Early-Middle Pleistocene loess deposits from these arid inland basins have not been investigated. The paucity of such studies hinders a full understanding of provenance changes of loess deposits from the Chinese Loess Plateau (CLP). In the present study, we performed a detailed major, trace and rare earth element analysis of multiple grain-size fractions of Middle Pleistocene loess deposits from the Dongwan section (DW) in the Junggar Basin. We found that the major, trace and rare earth element concentrations of the DW loess are grain-size-dependent, with higher concentrations in the fine fraction. In addition, the <5 μm and 5–20 μm fractions are more homogeneous in composition than the coarse fractions, indicating that they were extremely well mixed prior to deposition. Compared to contemporaneous loess deposits from the CLP and surface loess samples from Tajikistan and Ili Basin in Xinjiang, the loess deposits of the DW section have higher major element ratios of TiO2/Al2O3 and K2O/Al2O3, and REE ratios of LaN/YbN, GdN/YbN and LaN/SmN, indicating that the loess deposits in the Junggar Basin have distinctive local or regional geochemical characteristics. These results support the view that the Junggar Basin is not a potential source region of the loess in the CLP, at least since the Middle Pleistocene. Furthermore, temporal variations in the major element and REE ratios of the coarse fractions (20–75 μm and >75 μm) from the DW section changed significantly at ~0.5 Ma, which may have arisen from intensified physical erosion or increased wind speeds in the northern Tianshan area.