Magnetic susceptibility as a pedogenic proxy for grouping of geochemical transects in landscapes

Cervi, E.C. and Maher, B. and Poliseli, P.C. and de Souza Junior, I.G. and da Costa, A.C.S. (2019) Magnetic susceptibility as a pedogenic proxy for grouping of geochemical transects in landscapes. Journal of Applied Geophysics, 169. pp. 109-117. ISSN 0926-9851

Abstract

Magnetic susceptibility (χ lf) is a sensitive, fast, and accurate technique to detect soil pedogenic processes and features, and can improve understanding of soil-forming factors. The aim of this study was to determine catenary groupings of geochemical segments by identifying the relationship between magnetic susceptibility and the soil-forming factors within and along the landscape. Soil physical and chemical properties, as well as soil mineralogy were evaluated in twenty-two topsoil samples from three different landscapes in southern Brazil. The bulk χ lf and frequency-dependent susceptibility (χ fd) were also measured, represented by i) air-dried fine earth (< n μm, χ lf ADFE) ii) after dithionite-citrate-bicarbonate treatment (χ lf CBD), and iii) the parent material (χ lf ROCK). Geochemical segments were grouped using multivariate analysis. Our results indicate that soil parent material and climate, are first order factors for the pedogenic enhancement of magnetic minerals. Magnetic susceptibility of air-dried fine earth (χ lf ADFE) is widely variable, from 11.00 to 2.075 × 10 −8 m 3 kg −1. χ lf was higher in soils developed on basalt, and much lower for soils developed on sandstone. χ lf ADFE also reveals a strong positive linear correlation with χ lf CBD (r = 0.96) and a moderate positive linear correlation with “free” and total Fe content (r = 0.49 and 0.69, respectively). The χ fd (< 2%) indicates samples in which either non-SP grains dominate the assemblage or where the SP fraction is <0.03 nm. After CBD dissolution there is an increase in χ lf values. The rate of iron weathering and iron release, driven by hydrolysis and oxidation, and reductive dissolution of magnetite affect the formation and/or persistence of secondary iron oxides in some of the soil transects. Both principal component analysis (PCA) and cluster analysis group soils from a) crest and upper slope locations, b) down slope and sandstone areas, and c) valley floors, indicating that magnetic susceptibility is an effective proxy for soil properties for grouping and identifying pedogenic/geochemical segments along a landscape of 360 km 2.