Nd-Sr isotopic and REE geochemical compositions of Late Quaternary deposits in the desert-loess transition, north-central China: Implications for their provenance and past wind systems

摘要

The Nd-Sr isotopic and REE geochemical compositions of the Dishaogouwan profile in the desert-loess transition between the Ordos Desert and the Chinese Loess Plateau were investigated to identify the provenance of the sediments and to reconstruct the regional wind systems of the past. The ＜75 μm fractions are characterized by ~(87)Sr/~(86)Sr ratios of 0.7158-0.7175, ε_(Nd)(0) values of-8.9 to-16.8, ∑REE values between 133 μg/g and 277 μg/g and roughly flat PAAS-normalized REE patterns, which differs from the ＞75 μm fractions in the Dishaogouwan profile. This is most likely due to mineral sorting and/or source features. Isotopic and geochemical comparisons of the Dishaogouwan profile with potential sources show that (1) coarse eolian deposits resulted from wind erosion of the highlands in the adjacent west-central Ordos, and the coarse fluvio-lacustrine deposits originated chiefly from water erosion of the underlying eolian deposits in the Dishaogouwan profile; (2) fine particles of eolian deposits with absolute Δε_(Nd)(0) values of ≤1 originated completely from the highland erosion in west-central Ordos and the fining of in situ coarse particles, but the fine particles of the eolian deposits with absolute Δε_(Nd)(0) values of ＞1 have more complex sources; (3) fine fluvio-lacustrine deposits were fed from the central Loess Plateau by water for the Chengchuan and middle-lower Salawusu formations, but for the upper Salawusu formation, they were significantly fed from the east-central Ordos desert by wind. According to the source identification of the Dishaogouwan profile, the wind systems have been reestablished over the Ordos and surrounding areas in the past 200 ka: (1) the northwesterly was the dominant wind for eolian deposits in the Dishaogouwan profile, and the northeasterly simultaneously affected fine eolian deposits in certain periods (the middle part of MIS 6, and the lower part of MIS 4); (2) the summer monsoon was a predominant wind in periods during which most of the fluvio-lacustrine deposits developed in the Salawusu River Valley but was thoroughly displaced by the cold-dry northeasterly wind during the transition between MIS 5 and 4. This study is helpful for further understanding the evolution of the desert-loess transition and the related climate change.