Caracterização de perfis de regolito e formação de carbonatos em saprolitos derivados de gnaisse no semiárido de Pernambuco

Abstract

The regolith, which encompasses soil and saprolite, plays an ecological role in the relationship between plant species, groundwater cycle and geochemical processes relevant to the ecosystem, an important component for the study of the critical zone. CO2 emissions into the atmosphere are a matter of concern for society, and regolith is a recognized sink. The formation of carbonates “mineral carbonation” occurs naturally in regoliths through reaction of CO2 with mineral dissolution products, potentially acting on carbon sequestration. In the literature, the formation of carbonates by the weathering of basaltic rocks is well known. However, in this study, our hypothesis is: Regolith profiles derived from acidic metamorphic rocks located under a semiarid climate in the state of Pernambuco have geochemical, mineralogical and structural characteristics that favor the process of carbonate formation in the saprolite. The study focused on three equidistant regolith profiles along 900 meters in the municipality of Jataúba, inserted in the geomorphological domain of the Borborema Complex, state of Pernambuco. Soils and saprolites were classified, morphologically described, structurally evaluated and collected. In the deformed and undisturbed samples collected from autochthonous horizons, chemical and physical analyzes, mineralogical analyzes by X-ray diffractometry (XRD), geochemical analyzes by X-ray fluorescence (FRX) and micromorphological analyzes by scanning electron microscopy (SEM) associated with energy dispersive X-ray spectroscopy (EDS) were performed. Field tests showed a strong reaction of saprolite to HCl and electron micromorphology showed calcium precipitates around altered feldspars. The compact structures of the horizon overlying the saprolite and the virtually preserved rock underlying the saprolite must have favored the formation of carbonates by keeping the H2CO3 solution in contact with the altered minerals. In addition, the biotite and plagioclase-rich melanosomes inherited from the gneiss by the saprolite have mineralogical and geochemical characteristics similar to those observed in basic rock saprolites. We infer that the increase in the levels of Ca2+, Na+, Mg2+ and H2CO3 in the solution is favored by the regime of rainfall + evapotranspiration and that the origin of the CO2 necessary for the formation of carbonates in saprolites is atmospheric and by respiration of microorganisms. The present work serves as a reference for future studies to estimate the contribution of carbonate formation in saprolites from the northeastern semi-arid region to CO2 sequestration.