Weathering of soils originated from diabase in the Paraná Basin, São Paulo State, Brazil

Abstract

Weathering of the parent material is the initial process that plays an essential role in soil fertility. The study of this process has become crucial to better understand soil dynamics. Most studies on weathering are based on indexes that calculate the feldspar dissolution rate (for example, CIA, CIW, and PIA) which not always refl ect the complexity in the soil profi le. Other methodologies have been designed to improve these approaches, such as geochemical balance, difference of elemental contents (DE), and the W index. This work aim to assess the effi ciency of these methods to identify variations in recent soil profi les developed from diabase as compared to the CIA, CIW, and PIA indexes. Data on the main oxides were used to calculate the geochemical balance, DE, CIA, CIW, PIA, and W indexes to analyze weathering intensity in two profi les originated from diabase in the São Paulo State, Brazil. The Ki and Kr indexes and Fe o /Fe d ratio were also factored and we analyzed the mineralogy of soil horizons and the source rock. The W index and DE presented greater weathering variation than the CIA, CIW, and PIA in both profi les. The geochemical balance, mineralogy, and the Ki, Kr and Fe o /Fe d ratios explained the variations between the horizons. The DE and geochemical balance together generated new information for the analysis of the weathering process than the isolated indexes.

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“…Low Ki values indicate processes of monosiallitization and alitization, leading to the formation of 1:1 clay minerals and aluminum oxides (Lima et al, 2022). On the other hand, higher Ki values (greater than 2.2) indicate the occurrence of bissiallitization processes with a higher presence of 2:1 clay minerals, indicating a lower degree of weathering (Guimarães et al, 2021).…”

“…On the other hand, for SiO2 and Al2O3 oxides, the differences were more pronounced, with median contents in the 0-20 cm layer of 118 and 100 g kg -1 , and in the 80-100 cm layer of 163 and 154 g kg -1 , respectively. The surface layers are in greater contact with the weathering agents and causes a more intense loss of elements than in the deeper layer, especially in tropical regions, resulting in lower quantities of oxides in the surface layers (Guimarães et al, 2021).…”