Geochemical mass-balance, weathering and evolution of soils formed on a Quaternaryage basaltic toposequences

Abstract. The purpose of this research is to assess the geochemical mass-balance and weathering intensity of Typic Haplustert and Lithic Ustorthent soils represented by four profiles that developed in a Quaternary-age basaltic toposequence under semi-humid conditions in the central Black Sea region of Turkey. The researchers employed mass-balance analysis with a view to measuring elemental gains and losses along with alterations concerning the soils formed on the basaltic parent material. For 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, CIW, bases/R2O3, WIP, P and PIA. According to the results, CIA, CIW, PIA, P, WIP and bases/R2O3 index values of all soils varied between 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 spite of similar weathering rates, the soils were classified under different groups as a result of erosion. This showed that the 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.