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

• Four horizons of the fourth palaeosol are firstly discovered.
• The fourth palaeosol was determined to be a pedocomplex.
• Soil water content in loess is firstly studied.
• The Qinling Mountains lost their function as the climatic boundary at that time.

The fourth palaeosol (S4) of the Brunhes Epoch (780,000–0 years B.P.) at Shuangzhu, Tianjiapo, and Hejiacun in the Guanzhong Plain developed over about 40 ka and varies in thickness from 3.6 to 4.0 m. 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 360,000 years 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 1000 mm respectively, about 3 °C higher and 400 mm 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 3.2 m, 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.