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

Abstract

Magnetic susceptibility (χlf) 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 χlf and frequency-dependent susceptibility (χfd) were also measured, represented by i) air-dried fine earth (< n μm, χlf ADFE) ii) after dithionite-citrate-bicarbonate treatment (χlf CBD), and iii) the parent material (χlf 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 (χlf ADFE) is widely variable, from 11.00 to 2.075 × 10−8 m3 kg−1. χlf was higher in soils developed on basalt, and much lower for soils developed on sandstone. χlf ADFE also reveals a strong positive linear correlation with χlf CBD (r = 0.96) and a moderate positive linear correlation with “free” and total Fe content (r = 0.49 and 0.69, respectively). The χfd (< 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 χlf 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 km2.