干旱区生态环境知识资源中心

In the Loess Plateau in northern China, the Quaternary loess-soil sequences, the Hipparion Red Earth of eolian origin (Red Clay), and the Miocene loess-soil sequences constitute a near-continuous terrestrial record of paleoclimates for the past 22 Ma. In this study, Miocene loess and paleosol samples from Qinan (QA-I) were analyzed for their major, trace, and rare earth element chemistry and compared with the Plio-Pleistocene samples from Xifeng with emphasis on their provenance and paleoclimatic implications. The results show similar geochemical signatures for the eolian deposits of different ages, and they are also comparable to the average composition of the upper continental crust. These suggest that the dust materials were all derived from well-mixed sedimentary protoliths which had undergone numerous upper crustal recycling processes. They also support the notion of broadly similar source areas and dust-transporting trajectories for different periods since the early Neogene. The slightly higher K2O, Fe2O3, and MgO concentrations and loss on ignition values and the lower Na2O content in the Miocene loess samples compared to their Quaternary counterparts are attributable to the finer grain size of the Miocene loess associated with weaker dust-carrying winds. In comparison with some loess in Europe and America with less extensive sources, eolian deposits from northern China show higher Cs and lower Zr and Hf content. This is attributable to the sorting processes from remoter sources during transportation and could be regarded as an indication of the desert origin of the loess deposits. Miocene paleosol samples show higher chemical index of alteration values and lower CaO, MgO, and Na2O concentrations than does the intervening loess, indicating stronger weathering of the paleosols. However, the moderate chemical weathering of the paleosol samples indicates a constant semiarid and subhumid climatic range in northern China since the early Miocene.