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
Abstract 143 Nd/ 144 Nd and 87 Sr/ 86 Sr 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 87 Sr/ 86 Sr 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 87 Sr/ 86 Sr 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 87 Sr/ 86 Sr 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 87 Sr/ 86 Sr 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 e 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.