Geochemical mass balance applied to the study of weathering and evolution of soils

Abstract

Soil is viewed as an open system with additions, losses, translocations and transformations of materials. The purpose of this research is to assess the geochemical mass balance and weathering intensity of Vertisols (Typic Haplusterts) and Entisols (Lithic Ustorthents) developed in a Quaternary-age basaltic toposequence under semi-humid conditions in the central Black Sea region of Turkey. We used mass-balance analysis with a view to measuring elemental gains and losses along with alterations concerning soil forming processes. To this end, geochemical properties, elemental mass-balance changes and certain physicochemical features were identified to benchmark the weathering levels of the profiles. Lithic Ustorthents are distinguished by having a rough texture along with a low organic substance ingredient, whereas Typic Haplusterts have a high clay texture with low bulk density and slickenside features. X-ray diffraction showed that smectites were the prevailing minerals inside the Typic Haplusterts, while a significant amount of kaolinite and illite was observed in the Lithic Ustorthents. Mass-balance computations indicated that massive mineral weathering resulted in substantial Si losses through leaching as well as an exchange of cations, such as Na+,K+, Ca2+ and Mg2+, particularly from the upper horizons. The study also took into account other features such as the pedogenic evolution of soils using weathering indices such as CIA (chemical index of alteration), CIW (chemical index of weathering), bases/R2O3, WIP (weathering index of Parker), P (product index), PIA (plagioclase index of alteration). According to the results, CIA, CIW, PIA, P, WIP and bases/R2O3 (Fe2O3 + Al2O3) index values of all soils varied from 42.33 to 73.83, 44.46 to 80.43, 37.53 to 65.63, 75.39 to 84.31 and 0.45 to 1.27 respectively, to solum depth. This result indicated that soils classified as Entisol and Vertisol have similar pedochemical properties in terms of weathering indices. In spite of similar weathering rates, the soils were classified under different groups as a result of erosion. This showed that environmental conditions for soil development in the studied area had a far more impact on weathering and elemental loss than the parent material on the site.