Formation of plinthite mediated by redox fluctuations and chemical weathering intensity in a Quaternary red soil, southern China

2021 
Abstract Widely distributed Quaternary red soils (QRSs) in southern China represent the product of intense chemical weathering under a warm and humid climate. Soil containing plinthite is an important part of China’s QRSs, yet the soil environment and mechanism of formation of QRS with plinthite have been insufficiently studied. By comparing iron and clay mineralogy, magnetic properties, visible- and near-infrared (VNIR) spectral behavior, and chemical weathering and redox conditions between the plinthic soil and the overlying homogeneous soil, we determined the environmental conditions and mechanism by which plinthic soil forms. Weathering intensity scale (WIS)-related plots, as well as vertical profiles and ANOVA plots of several mineralogical parameters related to chemical weathering, indicate generally more intense weathering in the plinthic horizon (PH) compared to the homogeneous horizon (HH). Soil VNIR spectral parameters differed for the PH and HH mainly due to differences in mineralogical composition related to horizon-specific pedogenic processes. Horizon-specific pedogenic dynamics also resulted in differences in rare earth element + yttrium (REY) distributions. Based on our integrated geochemical and mineralogic dataset, we infer that the PH formed in a relatively warmer and periodically water-saturated environment with seasonal redox fluctuations, whereas the HH formed in a cooler/drier environment with less rainfall and a lower water table, reflecting transient aridification of local climate during glacial stages. Based on environmental analysis, we propose a model for the formation/development mechanism of plinthic QRS, in which soil pedogenic environment plays a key role in pedogenesis in particular iron mineral transformation. Our findings are of importance for understanding pedogenesis in tropical/subtropical regions and iron cycling and mineral transformation in the Earth’s critical zones.
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