Red soils with white net-like veins and their climate significance in south China

Abstract

The microstructure, particle size distribution, mineral and chemical compositions of both the white net-like veins and the red matrix of the Xuancheng laterite soil section in Anhui, south China, in the middle to lower reaches of the Yangtze River, were investigated using X-ray diffraction (XRD), X-ray fluorescence spectrometry (XRF), polarized light microscopy observation, and particle size distribution analysis. There was more quartz while less clay minerals in the white veins relative to the red matrix. In particle size distribution patterns the more abundant 5–20 μm particle size component of the white veins resulted from their higher quartz content. Discrepancy in degree of weathering between the white veins and the red matrix was derived from the depletion of Fe 2O 3 (total) and MnO 2 and the relative accumulation of SiO 2 in the white veins. White veins across the lithological layers and inclusions of relict red matrix material in the white veins indicated that the net-like soils originated from an initial uniform red soil. The white spots or veins were derived from the long-term modification of the pre-existing soil or concurrent weathering soils. Hydrodynamic conditions of the groundwater exerted influence on the formation of white veins. The formation process of the white net-like veins was different from that of the red matrix, suggesting that the white vein materials reflected a hydrological imprint upon the result of these conditions. The net-like soils in south China were compatible to the mottled soils in Africa, Australia, India, and Brazil. However, much more intense depletion of Fe oxide and much better developed white veins in the net-like soils of south China suggest probably even warmer and more humid palaeoclimatic conditions relative to those in Africa, Australia, India, and Brazil during the period.