The Sr and Nd isotopic variations of the Chinese Loess Plateau during the past 7 Ma: Implications for the East Asian winter monsoon and source areas of loess

Nd-143/Nd-144 and Sr-87/Sr-86 ratios of the acid-insoluble residues of the red clay and overlying loess-paleosols from the Lingtai profile of the Loess Plateau, China, were investigated. The results show that variations of the Sr-87/Sr-86 ratios of the Linglai profile can be divided into two stages. From similar to 7 Ma B.P. to 2.5 Ma B.P., the acid-insoluble residues in the Red Clay (RC) Formation, are characterized by higher Sr-87/Sr-86 ratios with an average of 0.7230. From 2.5 Ma BY. to the present, the acid-insoluble residues in the Wuchen Loess (WL4-WS1), Lishi Loess (L15-S1), Malan Loess (L1) and Holocene Loess (S0) have relatively lower (8S)sr/Sr-96 ratios and display an overall descending trend from 0.7223 at similar to 2.5 Ma B.P. to 0.7182 at the present. Among three possible interpretations for the variations of Sr-87/Sr-86 ratios of acid-insoluble residues of the loess. (provenance change, chemical weathering change, or grain-size distribution change), only grain-size distribution change provides a satisfactory interpretation. This implies that the East Asian winter monsoon strength was weak and relatively stable from similar to 7 Ma B.P. to similar to 2.5 Ma B.P., but became continuously enhanced from similar to 2.5 Ma B.P. to the present. All the red clay and the overlying loess-paleosols in the Lingtai profile have generally identical epsilon(Nd) (0), implying that the source regions of these Eolian deposits of the Chinese Loess Plateau may be relatively stable and has not changed since 7 Ma B.P. (c) 2007 Elsevier B.V. All rights reserved.