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

143Nd/144Nd and 87Sr/86Sr 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 87Sr/86Sr ratios of the Lingtai profile can be divided into two stages. From ∼ 7 Ma B.P. to 2.5 Ma B.P., the acid-insoluble residues in the Red Clay (RC) Formation, are characterized by higher 87Sr/86Sr ratios with an average of 0.7230. From 2.5 Ma B.P. 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 87Sr/86Sr ratios and display an overall descending trend from 0.7223 at ∼ 2.5 Ma B.P. to 0.7182 at the present. Among three possible interpretations for the variations of 87Sr/86Sr 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 ∼ 7 Ma B.P. to ∼ 2.5 Ma B.P., but became continuously enhanced from ∼ 2.5 Ma B.P. to the present. All the red clay and the overlying loess-paleosols in the Lingtai profile have generally identical εNd (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.