Weathering of the Ethiopian volcanic province: A new weathering index to characterize and compare soils

Mohammed Umer,Tadesse Berhanu,Javier Cuadros,Jennifer S. Le Blond,Yordanos B. Molla,Peter J. Baxter,Gail Davey

doi:10.2138/am-2015-5168ccby

Abstract

Soil formation occurs through numerous physical and chemical weathering processes acting to alter the parent rock on the Earth’s surface. Samples of surface soils were collected over a range of elevations (2000–3600 m) from profiles directly overlying basaltic to more felsic parent rocks, over a region in NW Ethiopia. The soils were investigated to determine their chemical composition and X-ray diffraction was used to identify and quantify individual mineral phases. The data set was analyzed using non-parametric statistics (Spearman’s Rank and Mann-Whitney U tests) to compare the soils forming over the two parent rocks. Principal component analysis (PCA) was used to identify the mineral alteration assemblage and formation during pedogenesis. The extent of alteration was quantified using several chemical weathering indices (Chemical Index of Alteration = CIA; Chemical Index of Weathering = CIW), including an index calculated by multivariate analyses of the soil chemical composition data (weathering “ W ” index). Further to this we devised and tested a new weathering index ( W min) using multivariate analysis of the soil mineralogy, to estimate the extent of weathering and physico-chemical proprieties of the parent rock from which the soil formed. The soils present a fair to advanced stage of alteration, with abundant iron (Fe) oxides (up to 40 wt%) and phyllosilicates (up to 57 wt%), including kaolinite-smectite (K-S) mixed-layer phases. The K-S was composed of either 30–50% kaolinite or 94–98% kaolinite layers. Discrete kaolinite was also present. The bimodal K-S mineralogical composition is likely due to two precursor phases: feldspar for the kaolinite-rich K-S and volcanic glass for the smectite-rich K-S. K-S with intermediate composition (50–94% kaolinite) was rare, due to its instability. Statistical analysis showed significant differences between the chemical compositions of the soils developed on the two different parent volcanic compositions. The soils overlying the more felsic parent rocks were less altered than those overlying the flood basalt. When comparing the weathering indices calculated in this study, we conclude that while the CIA and CIW may be more readily determined, the W and W min indices can elucidate information on the composition of the original rock from which they formed. The W index is more sensitive to certain variables when compared with the newly derived mineralogical W min index; however the W min index takes into account mineral phases within the sample, which provides a more detailed interpretation of weathering rates than chemistry alone. In addition the W min index correlated with meteorological variables, such as elevation (and consequently temperature and precipitation), known to influence the degree of pedogenesis. The W min index can be used to enhance our understanding of the processes that occur during weathering processes to supplement information gained from traditional chemical weathering indices.

Highlights

The formation of soils is a process by which weathering alters constituents within the parent deposit through the loss of more mobile elements, concurrent en‐ richment of less mobile elements, combined with alteration and formation of new secondary minerals and accumulation of organic matter
The values for the major element chemical analyses and quan‐ tification of the mineral phases within the soil are shown in Table 1, and Spearman’s Rank correlations between the data are shown in Appendix Table 21
The correlations observed in the surface soils approximately reflect the chemical characteristics of the volcanic parent rock, where the proportion of elements more commonly found in mafic rocks (Fe, Mg, Ca, Ti) were generally negatively correlated with those found in more felsic rocks (Si, Na, K)

Summary

Introduction

The formation of soils (pedogenesis) is a process by which weathering alters constituents within the parent deposit through the loss of more mobile (i.e., soluble) elements, concurrent en‐ richment of less mobile elements, combined with alteration and formation of new secondary minerals and accumulation of organic matter. Basaltic rocks are composed of primary minerals, mesostasis, and glass that, over time, develop into soils with comparably more stable mineral phases at surface conditions. Geochemical weathering of basalt begins with the loss of nonhydrolyzing cations (such as calcium, magnesium, and sodium) and the concomitant enrichment of silicon, aluminum, and iron ions in both crystalline and non-crystalline phases (Chorover et al 2004). In terms of mineralogy, weathering typically removes the primary silicate phases (and glass), and replaces them with poorly crystalline aluminosilicates. Weathering can eventually produce iron oxide and oxyhydroxide phases that evolve toward more crystalline varieties (e.g., Rasmussen et al 2010; Dahlgren et al 2004; Righi et al 1999)

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