Trace element geochemistry in Brazilian Cerrado soils

Abstract

The Cerrado Region on the Central Plateau of Brazil contains some of the oldest soils on Earth, with some soils weathering in place since the late Cretaceous. The region is currently undergoing rapid agriculturalization and industrialization. The objectives of this paper are: (1) to inventory the native trace element contents of pristine Cerrado soils, and (2) to better understand which trace elements accumulate and which are depleted in well-drained soils over extremely long periods of pedogenic weathering. We sampled the 0–20- and 80–100-cm layers of 45 pedons located in three widely separated areas on three different geomorphic surfaces and determined total elemental contents by X-ray fluorescence analysis. In comparing the contents of Cerrado soils to soils worldwide, the contents of Sc, V, Cr, Ga, Zr, Nb, Ce, and Th are ≥1.2 times the world average, Y, La, and U are about the same, and Mn, Co, Ni, Cu, Zn, Rb, Sr, Ba, and Pb are <0.4 times the world average. Trace elements that accumulated during weathering are either pentavalent (Nb 5+), tetravalent (Ti 4+, Zr 4+, U 4+, Th 4+), or trivalent (Sc 3+, V 3+, Cr 3+, Ga 3+, Y 3+, La 3+, Ce 3+) at the acid pH of these soils, while trace elements that were depleted are either divalent or monovalent (Rb +, Mn 2+, Co 2+, Ni 2+, Cu 2+, Zn 2+, Sr 2+, Ba 2+, and Pb 2+) at acid pH. Thus, trace element geochemistry over long periods of pedogenic weathering at acid pH mirrors major element geochemistry in that mono- and divalent cations are leached and depleted, while tri-, tetra-, and pentavalent elements accumulate.