干旱区生态环境知识资源中心

The Lanzhou Basin, located in the convergence zone of monsoon region of southeast China and arid region of northwest China, has the potential to reveal the onset time of aridification in Central Asia and the evolutionary history of East Asia monsoon. Its well developed mammalian fossil-bearing Cenozoic sequences provide valuable material to understand the chronology of the strata and the environmental evolution of the NE Tibetan Plateau. Here we present detailed rock magnetic results from the Duitinggou section in the Lanzhou Basin to explore the relationship between the magnetic minerals and sedimentary environment.

Environmental magnetic parameters, e.g., low field magnetic susceptibility (chi(1f)), percentage of frequency dependent magnetic susceptibility (chi(fd)%) , susceptibility of the anhysteretic remanent (chi(ARM)), saturation isothermal remanent magnetization (SIRM) and high coercivity (hard) remanent magnetization (HIRM*) were measured to identify the content of the magnetic minerals and the variation of these magnetic minerals in different lithologies. Rock magnetic measurements (e.g. temperature dependence of magnetic susceptibility (chi-T), isothermal remanent magnetization (IRM) and hysteresis loops) were carried out to explore the type of the magnetic minerals. Moreover, non-magnetic methods (e.g., X-ray diffraction (XRD) and diffuse reflection spectrum (DRS)) were employed to discriminate the magnetic minerals. The grain-size analysis helps to distinguish different sedimentary environment.

The values of chi(lf,) chi(fd)%, chi(ARM) SIRM and HIRM* of the sandstone are much lower than that of mudstone, which indicates the relatively low concentration of the total magnetic minerals in the sandstone. The chi(fd)% is less than 5% and varies with the content of the magnetic minerals, suggesting the low concentration of the superparamagnetic particles (SP) in the section. The sharp drop of chi at 500 similar to 600 degrees C in the heating curves indicates the presence of magnetite. All samples have an increased chi during cooling after heated to 700 degrees C, which may result from the neoformation of the fine-grained magnetic minerals. Furthermore, the XRD results show that chlorite provides iron source (Fe2+) or reducing agent for newly formed fine-grained ferrimagnetic minerals, which leads to increased chi. The component analyses of IRM acquisition curves suggest that both low-coercivity component and high-coercivity component contribute to the acquired IRM, in which the high-coercivity component accounts for more than 69%. Consistent with these rock magnetic results, the hysteresis loops of the mudstone samples is wasp-waisted and not closed at 800 mT, which indicates that both high and low coercivity components exist in the mudstone. But the hysteresis loops of the sandstone are thin and less wasp-waisted, indicating the dominance of high coercivity component. The second derivative spectra of DRS display a higher content of hematite and goethite in the mudstone than in the sandstone. The grain-size parameters imply-a fluvial sedimentary environment for the sandstone and lacustrine facies for the mudstone.

The concentration of magnetic minerals in the sandstone is low, and dominated by high-coercivity component (goethite and hematite). The magnetic minerals in the sandstone is related to a fluvial reduction condition, which are dissolved or converted to weak magnetic minerals, leading to the low concentration of magnetic minerals. The mudstone contains more magnetic minerals, including low coercivity (magnetite) and high-coercivity (hematite and goethite) component. We speculate that the mudstone is lacustrine facies according to the field investigation and grain-size analysis. The semiarid environment in the Lanzhou Basin gives rise to the formation of the hematite in the surface soil and enhances the erosion of the surface soil (with extremely weak pedogenesis), thus providing the lake with detrital deposit rich in hematite.