Major element geochemistry and geomorphic relationships in Brazilian Cerrado soils

A thorough understanding of major and trace element geochemistry is important in assessing the impacts of rapid agriculturalization and increased human occupation of the Cerrado region of central Brazil, a region with some of the oldest soils on Earth. The objectives of this paper are to summarize the major element composition of Cerrado soils in the context of their morphology and geomorphology, to describe impacts of biogeochemical cycling on the chemical properties of Cerrado soils, and to use elemental data to provide insights on the possible origin of soil parent materials on the South American erosion surface and on the Serra Geral Basalts. We sampled three major subregions selected to be representative of the main areas of the Cerrado, namely: (i) eastern Goiás, (ii) northwestern Minas Gerais, and (iii) the Triângulo Mineiro area of western Minas Gerais. Five soils were sampled in each subregion, each represented by three pedons sampled at 0–0.2 and 0.8–1.0 m depths. Geomorphologically, the soils were on the South American, Velhas I, and Velhas II erosion surfaces. All sites were carefully selected to minimize potential anthropogenic contamination. General chemical characterization was by standard procedures and total major and trace elemental contents were quantified using wavelength-dispersive X-ray fluorescence spectroscopy. The soils ranged from loamy sands to clays, with Si (142–947 g kg−1 SiO2), Al (14–439 g kg−1 Al2O3), Fe (10–331 g kg−1 Fe2O3), and Ti (3–101 g kg−1 TiO2) contents closely following the clay contents. Organic C averaged 17 g kg−1 for the surface, and 7 g kg−1 for the subsurface horizons. The Na, Ca, Mg, and K contents were very low, tended to be higher in surface than in subsurface horizons because of recycling by vegetation and addition due to atmospheric deposition, and were higher in soils containing hard plinthite nodules. The Th/Zr ratios suggest that the parent materials of the clayey soils on the South American erosion surface have a common origin in sediments from rocks in the Central Plateau of Brazil that were transported eastwards to cover the underlying sandstone. Trace element signatures indicate that basalt-derived soils occurring at different elevations in the Triângulo Mineiro area formed from distinctly different magmas that may have been deposited in different basalt flow events.