The fifth paleosol layer in the southern part of China's Loess Plateau and its environmental significance

Through field investigations, dark red–brown lumpy ferruginous argillans (LFAs) and an underlying weathered loess layer under the fifth layer (S5 paleosol) were discovered in the Guanzhong Plain in the southern part of the Loess Plateau in northwest China's Shaanxi Province. The ultramicrotexture of the LFAs and their mineral and chemical compositions were analyzed. Experimental results showed that mixed illite–smectite minerals comprise about 90% of the LFAs in mineral composition, with 40% SiO2 content and high Fe2O3 (about 10.0%) and Al2O3 (about 22.5%) contents in chemical composition. Neo-formed microcrystalline grains of clay minerals dominated the LFAs. The micromineral crystal grains in the LFAs were mainly plates with some degree of alignment, giving the argillans crystalline optical properties. Red LFAs and weathered loess layers under the S5, and the depth to which Fe2O3 and CaCO3 from the S5 have been removed, indicate that chemical weathering was much stronger when the paleosol layers formed than that at present. While the S5 paleosol was developed, a subtropical climate prevailed in the southern Loess Plateau, with a mean annual temperature and rainfall about 4 °C and 500 mm higher respectively than that at present. The adequacy in moisture in at least 4.2 m of the gravitational water zone during the formation of S5 was suitable for forest growth. During the warmest and moistest climate span of growth for 2.5 million years when S5 was formed, the Qinling Mountains lost its function as a climatic boundary between the subtropical and temperate zones of East Asia, when the summer air masses crossed the Qinling Mountains and reached the Guanzhong Plain, bringing rich precipitation.