Red soils with white net-like veins and their climate significance in south China
Research Highlights
►More quartz while less clay minerals in the white veins. ►Depletion of Fe2O3 resulted in difference between the white veins and the red matrix. ►The white veins developed from the initial uniform red soil. ►Groundwater exerted influence on the formation of white veins. ►The red soils formed under extremely warm and humid palaeoclimatic conditions.
Introduction
In the south of middle to lower reaches of the Yangtze River in China, a soil horizon enriched with net-like veins occurs widely along the laterite profiles in the areas of hills, terraces, coalesced alluvial pans, and parts of the piedmont belts (Fig. 1). Previous studies on this horizon were mainly focused on its age, distribution, chemical composition, and its indication of climatic records (e.g. Yang et al., 1996, Jiang et al., 1997, Hu et al., 1998). Chronological studies indicated that it was formed during the mid-Pleistocene under warm and humid climate conditions. However, mechanisms of its formation are still largely in debate. It was generally accepted that the formation was involved in groundwater during pedogenic processes (Zhu, 1988, Li & Gu, 1997). However, other studies suggested that it could be derived from the root systems of former plants (Xiong et al., 2000).
Analogous to the net-like red soil horizon in south China, a typically thick and continuous mottled horizon was also observed at the transition from the weathered bedrock up to the lateritic duricrust and lateritic gravel in laterite profiles in Africa, Australia, and India, which was produced by weathering under extremely warm and wet climate conditions and was an efficient indicator of climate during a weathering process (Nahon, 1991, Aleva, 1994, Beauvais, 1999). Laterite was considered as the results of intensive weathering under seasonally dry/humid tropical climatic conditions (Bourman, 1993), and the mottled horizon was produced due to Fe depletion during intense weathering processes (Rosolen et al., 2002). However, the white net-like veins formed in laterites of south China had a much higher density and larger size compared to the light-colored spots in those of Africa, Australia, and India, and therefore, were believed to form by different processes under different climate conditions (Zhao & Yang, 1995, Li & Gu, 1997, Yin & Guo, 2006).
Among the soil profiles in south China, the Xuancheng section was one of the most extensively studied. It is situated in Xuancheng city, Anhui Province. Previous investigations of the section focused mainly on stratigraphy, magnetostratigraphy, particle size distribution, and climate significance (Zhao & Yang, 1995, Qiao et al., 2003, Hu et al., 2003, Hong et al., 2010), with few soil-forming studies carried out (Yin and Guo, 2006). Iron content and magnetic susceptibility studies suggested that the materials of the profile were, in fact, the weathered aeolian deposits and could be stratigraphically correlated to the loess deposits in northwestern China (Zhao & Yang, 1995, Qiao et al., 2003). In contrast, Hu et al. (2003) measured the magnetic parameters of both the white net-like veins and the red matrix of the soil horizons and pointed out that magnetic susceptibility of the Xuancheng section was not controlled by the weathering and pedogenic processes. The formation of net-like veins in the soils was considered due to Fe depletion in the weathering process. However, mechanisms causing differences in chemical and mineralogical compositions, pedogenic stage, and degree of weathering between the net-like veins and the red matrix were still not well understood (Yin & Guo, 2006, Hong et al., 2009). Formation of net-like veins in the red soil horizon produced different mineral and geochemical volumes, making it difficult to decipher the environmental and climatic conditions of the layers due to lack of the pedo-stratigraphy of the soil profile.
The Xuancheng soil profile exhibits an apparent layered structure according to its distinctive color, texture, and composition. Better developed net-like veins occur from ~ 1 m deep to the bottom of the profile. In the soil layers net-like veins align in vertical, horizontal, and other random directions, take different shapes from layer to layer, and make up to ~ 40% by volume in some soil layers (Hong et al., 2010). These variations provide a chance for us to study the mechanisms of the formation of net-like veins and the differences in weathering between the net-like veins and the red matrix, and especially, the environmental and climatic significance of the net-like veins along the profile. The objectives of this investigation were to characterize the clay mineralogy and geochemistry of the net-like veins and the red matrix along the profile, and therefore, to shed more light on the formation of the horizon and pursue a better understanding of environmental and climatic significance of the soils.
Section snippets
Materials and methods
The Xuancheng section (118°51′E, 30°54′N) is located in the zone of middle to lower reaches of the Yangtze River, south China (Fig. 1), and situates on the second terrace of the Shuiyang River, a branch of the Yangtze River. The climate is warm and humid in summer and relatively cool and dry in winter, with a mean average temperature of 15–16 °C and a mean annual rainfall of 1100 to 1400 mm, mainly between April and October. The altitude ranges from 20 to 45 m above the sea level, with a landscape
Polarized light microscopy observations
Polarized light microscopy observation showed that the red-brown color was caused by Fe oxide. The red matrix contains more Fe oxide and therefore displays a brown color, while the net-like veins contain less Fe oxide and shows a light yellow color. A gradual change in color from the red matrix to the white veins is indicative of a gradual decrease in Fe oxide content (Fig. 3a). However, in the white veins, small blocks of relict red matrix with higher Fe oxide content surrounded by white veins
Bulk geochemical composition and degree of weathering
In tropical to subtropical climate conditions, intense chemical weathering will result in depletion of mobile elements and accumulation of immobile elements relative to the composition of the original unweathered material. The CIA index, used to characterize the degree of weathering within a weathered soil profile (Nesbitt & Young, 1982, Nedachi et al., 2005), reflected depletion of mobile elements CaO, K2O, and Na2O, and accumulation of the immobile element Al2O3. Overall, the CIA values of
Conclusions
There was more quartz while less clay minerals in the white veins relative to the red matrix. In the upper portion the much more abundant quartz in the white veins compared to the red matrix was probably derived from the groundwater hydrodynamic conditions. In particle size distribution patterns the more abundant 5–20 μm particle size component of the white veins also pointed to more quartz compared to the red matrix.
Discrepancy in the degree of weathering between the white veins and the red
Acknowledgments
This work was supported by the Natural Science Foundation of China (NSFC), allotment grant numbers 40872038 and 41072030. The authors wish to thank Dr. Han F. for the particle size analysis, Dr. Yu J. S. for XRD analysis and Dr. Zheng H. T. for XRF analysis, and especially to Prof. M. J. Vepraskas, the Editor, and G. J. Churchman and Mehmet Arslan for their insightful reviews, valuable comments, and suggestions.
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