LATE TERTIARY DRY AND HOT PALEOCLIMATE IN CENTRAL CHINA: REVELATION FROM AUTHIGENIC NANO-PHASE MINERALS IN THE RED CLAY LAYERS OF CHINESE LOESS

Loess deposits in central China can provide a good opportunity to reconstruct the long-term monsoon changes in China and East Asia. The eolian record in the Chinese Loess Plateau can be extended from 2.6 Ma back to about 7.0 Ma, because it has been demonstrated that the late Tertiary red clay deposits underlying the Pleistocene loess and paleosol sequence in the Chinese Loess Plateau are also of eolian origin. Many important nanophase minerals such as hematite, dolomite, calcite nano-fibers, smectite and palygorskite., which can act as indicators of the paleoclimate and paleoenvironment changes, were found in the red clay of loess plateau of China. Results from X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM) indicate that large amount of nano-phase and low crystalline hematite was formed due to chemical weathering of Fe-bearing silicates. Dolomite and palygorskite were crystallized from the interstitial solution of soils and/or transformed from calcite and smectite-illite, respectively. We propose that the calcite nano-fibers resulted from crystallization in bioorganic-bearing solutions. Dolomite and palygorskite assemblage indicates dry and hot environment, because it is widely known to occur in modern desert soils. Preservation of calcite nano-fibers also indicates a dry environment. All characteristics of authigenic nano-phase minerals occurring in the red clay indicate a paleo-environment with strong evaporation and chemical weathering, and much drier and hotter than the overlying Pleistocene loess and paleosol. Low magnetic susceptibility (MS) in the red clay layers is resulted from the strong oxidation of eolian magnetite as a combined result of strong chemical weathering and low authigenic magnetite formation due to weak microbial activity. Based on our results, it can be inferred that strong chemical weathering and weak microbial activity under dry and hot climate is the main reason for low magnetic susceptibility in the red clay layers, which is different from the paleoclimate for the overlying Pleistocene loess (indicating cold and dry paleoclimate) and palesol (indicating warm and humid paleoclimate) formations.