Magnetic parameters indicate the intensity of chemical weathering developed on igneous rocks in China

Abstract

Continental weathering plays a key role in earth surface processes by linking Earth's spheres and producing material for cycling. Previous studies have proposed numerous weathering indices for the evaluation of chemical weathering intensity. However, few research efforts are devoted to the changes of sediment physical properties during weathering processes and on the correlations between weathering and mineralogical/chemical alteration. This study investigates mineral transformation and magnetic properties of two weathering profiles developed on granodiorite and basalt in China. The notable differences in magnetic properties between the two profiles, i.e., magnetic susceptibility (χ), percentage frequency-dependent susceptibility (χfd%), anhysteretic remanent magnetization (χARM) and saturation isothermal remnant magnetization (SIRM), are ascribed to the combined effects of parent rock lithology and climate conditions on the formation and alteration of magnetic components during weathering/pedogenesis. Parent rock lithology (e.g., Fe-bearing basalts and relatively Fe-poor granites) determines the initial magnetic susceptibilities. Warm and humid climate favors rock weathering and thus enhances the magnetic intensity, whereas changes in redox conditions lead to inhomogeneous degree of chemical weathering, and thus alter the spatial distribution of magnetic minerals. This study further suggests that the weathering intensity in geologic profiles can be evaluated by magnetic indicators such as χfd% and χARM/SIRM based on their positive correlations with the chemical index of alteration (CIA). The research findings may shed new light on the constraints on continental weathering and detrital sediment cycling in the East Asian continental margin.