Geochemistry of the Xining, Xifeng and Jixian sections, Loess Plateau of China: eolian dust provenance and paleosol evolution during the last 140 ka

Loess and paleosol samples from three distant sections (Xining, Xifeng, Jixian) of the Chinese Loess Plateau were analyzed for chemical and Nd-Sr isotopic compositions in order to obtain information about the paleoclimatic variation during the last 140 ka. These three sections represent three different climatic conditions, from arid in the west (Xining) to more humid in the east (Jixian) across the Loess Plateau. Isotopic analyses show that all the three sections display a restricted range of Nd-143/Nd-144 ratios (or epsilon (Nd) approximate to -10 +/- 0.5), indicating the dominance of relatively young and uniform upper crustal sources for the eolian dust. The loess and paleosol samples have typical upper crustal Sr-87/Sr-86 ratios (0.714-0.718), and their variation can partly be attributed to different carbonate/silicate ratios in the samples. REE data from the three sections are indistinguishable from each other; they show typical upper crust patterns characterized by (La/Yb)(N) approximate to 10 and Eu/Eu* approximate to 0.65. The high degree of chemical and isotopic homogeneity suggests that the dust source region must have undergone multiple recycling and thorough sedimentary mixing processes. Various chemical indicators have been used to monitor the pedogenetic intensity in these sections. Although results for the Jixian section agree reasonably well with the known climatic conditions on the Loess Plateau, the other two sections yield results not in agreement with some well-accepted proxies like magnetic susceptibility or grain-size distribution. Some chemical proxy indicators established in our previous work on the Luochuan section, such as the systematic U-depletion and negative Ce anomalies in REE patterns in soils, are not clearly recorded in the present case. The discrepancy between different climatic proxies could be related to the effective soil water budget, temperature, as well as the rate of development of each proxy. (C) 2001 Elsevier Science B.V. All rights reserved.