Characteristics of the fifth paleosol complex (S5) in the southernmost part of the Chinese Loess Plateau and its paleo-environmental significance

• Combined data on pedogenesis, geochemistry and mineralogy of the S5 are presented.
• Three well-developed pedons implies three glacial-interglacial climatic fluctuations.
• Completely decalcification and clay transformation and translocation occur in the S5.
• Chemical alteration of phyllosilicate minerals mainly are is weak.
• Mineral weathering is restrained by calcic horizon, argillic horizon, flat terrain.

The most prominent paleosol unit in the Chinese Loess Plateau (CLP) is the fifth paleosol complex (S5) with its well-developed very thick and dark colored pedons. To provide more insight in the formation of S5 and its environmental significance, the pedogenesis and clay mineral transformation in the S5 of the Wugong section (Shaanxi Province) on the southernmost CLP are analyzed. S5 at the Wugong section is essentially composed of three well-developed reddish pedons (i.e., S5 -1, S5 -2, S5 -3) which signify three glacial–interglacial climatic fluctuations during its formation. Complete decalcification in each pedon and a calcic horizon of only 30–50 cm in thickness beneath each of the three pedons suggests that after deposition the pedons developed with a relatively stable surface in a sustained warm and humid climate. Clay formation in the S5 includes neogenesis of clay materials by in situ post-depositional weathering and mechanical migration of the fine fraction after complete decalcification.Complete leaching of CaCO3, intensive clay formation (with 60–100% higher clay content than that in the overlying and underlying loess (L5 and L6)) and extremely high magnetic susceptibility in the S5 pedons reflected a warmer, more humid climate and soil environment for pedogenesis than in the ‘optimum’ Holocene. However, the chemical alteration of the phyllosilicate minerals was weak and restrained by the hard calcic horizon, the compact argillic horizon and the flat terrain. The major clay mineral weathering processes during the formation of the S5 pedons at the Wugong section were depotassication, hydrolysis of primary minerals and degradation of chlorite. The pedogenesis in a loess–paleosol sequence and its pedogenic environment can best be deduced from combined data on pedogenic properties, and geochemical and mineralogical characteristics.