Characteristics of the fifth paleosol complex (S-5) in the southernmost part of the Chinese Loess Plateau and its paleo-environmental significance

摘要

The most prominent paleosol unit in the Chinese Loess Plateau (CLP) is the fifth paleosol complex (S-5) with its well-developed very thick and dark colored pedons. To provide more insight in the formation of S-5 and its environmental significance, the pedogenesis and clay mineral transformation in the S-5 of the Wugong section (Shaanxi Province) on the southernmost CLP are analyzed. S-5 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 S-5 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 (L-5 and L-6)) and extremely high magnetic susceptibility in the S-5 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 S-5 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