Geochemical indices of soil development on basalt rocks in arid to sub-humid climosequence of Central Iran

Abstract

Quantitative weathering indices are efficient tools in determining the soil development from the underlying rocks. In order to evaluate the effects of climate on the soils developed under an arid to sub-humid climosequence in central Iran, twenty soil-development indices were compared. Twenty-four samples from six pedons were analyzed for routine physico-chemical and geochemical analyses using X-ray fluorescence (XRF). The lowest and highest calculated soil development indices were observed in the arid and sub-humid regions respectively. Among the studied indices, fifteen indices showed similar trends concerning the intensity of weathering. The consistency of such trends reveals the feasibility of using these indices to evaluate basalt weathering rate and soil development in arid, semi-arid and sub-humid regions. This study demonstrated that, among the evaluated weathering indices, the Weathering Index of Parker (WIP) and mass transfer coefficient (τ) were the most proper indices for predicting basalt weathering intensity. This is due to the fact that these indices use the highly mobile earth elements which are the most sensitive elements in basalt chemical weathering. The calculated indices were subdivided into three subgroups including the ratios of mobile/immobile, mobile/mobile and immobile/mobile elements based on the elements used in their calculation. The state of soil evolution was more accurately predicted compared to the other subgroups using the mobile/immobile subgroup of indices. Overall, the weathering indices calculated using the mobile elements are better indicators of weathering intensity, soil formation and the exogenous processes across the arid to sub-humid climosequence.