The reconstruction of palaeoenvironment during development of the fourth palaeosol in the southern Loess Plateau of China

The fourth palaeosol (S₄) of the Brunhes Epoch (780,000-0years B.P.) at Shuangzhu, Tianjiapo, and Hejiacun in the Guanzhong Plain developed over about 40ka and varies in thickness from 3.6 to 4.0m. It consists of four horizons: the first (uppermost) is a well developed red-brown clay horizon (Bts), the second is a dark yellowish-brown weathering-cracked loess horizon (Cs) with red ferruginous argillans, the third is a yellowish-brown weathering-cracked loess horizon (Cl) without red ferruginous argillans, and the fourth consists of CaCO₃ nodules (Ck). The two weathering-cracked loess horizons, which are a major feature differentiating a pedocomplex from a palaeosol, have not been found previously in Chinese palaeosol. The ferruginous clay argillans are composed of montmorillonite-illite with some kaolinite minerals. The pedocomplex suggests that a moist subtropical climate prevailed between 420,000 and 360,000years in the southern parts of the Chinese Loess Plateau. Its profile is Bts-Cs-Cl-Ck-Co. When the pedocomplex was developed, the mean annual temperature and precipitation were about 16°C and 1000mm respectively, about 3°C higher and 400mm more than at present in the Guanzhong Plain. The removal depth of CaCO₃ and the weathering-cracked loess layer indicate that the gravitational water reached a depth of at least 3.2m, and the moisture content in S₄ would have been sufficient to sustain forest development at that time. In the typical interglacial in which S₄ was developed, the Qinling Mountains lost their function as the boundary between the subtropical and temperate zones of China, and water-bearing air masses resulting in rich rainfall could frequently reach the southern Loess Plateau.