The magnetic mechanism of paleosol S5 in the Baoji section of the southern Chinese Loess Plateau

摘要

In the central Chinese Loess Plateau (CLP), the magnetic susceptibility has the highest value in the S5 paleosol unit. However, at the southern of the CLP, such as in Baoji, the magnetic susceptibility value in the S5 paleosol unit is on the lower range of the top 6 paleosols (S0, S1...S5). The studies of rock magnetism and geochemistry showed that the Baoji S5 experienced a stronger pedogenesis and chemical weathering than the S3. The paleosol S3 was formed under an oxidizing environment where the maghemite and hematite are formed simultaneously. Maghemite is the main contributor to magnetic enhancement. However, the concentrations of maghemite in the S5 are lower than in the S3. With the development of pedogenesis, the proportion of the maghemite component decreased in the S5, while the concentrations of anti-ferromagnetic minerals (mainly goethite) increased. In the field, measurable amounts of dark brown iron-manganese cutans can be observed on the cranny surface of S5. These indicate that the iron-manganese elements in the S5 were assembled, and pedogenesis occurred intermittently between wet and dry, and strong leaching soil conditions. This may have led to finegrained magnetite, maghemite and hematite, which were gradually converted into weak magnetic goethite, therefore resulting in the lower magnetic susceptibility of S5.