Elemental signatures of an Amazonian Dark Earth as result of its formation process

Abstract

Amazonian Dark Earths (ADEs) are soils studied for archaeological purposes and as a potentially agricultural model for the humid tropic. We investigated the chemical composition of an ADE in comparison to an adjacent Ultisol without anthropic influence for better understanding the origin of ADE fertility and formation process. Three hundred ADE and Ultisol samples were collected at 10 cm depths from 0 to 100 cm soil profiles on the Caldeirão site at the Solimões River, Brazil and analyzed for elemental composition in an inductively coupled plasma mass spectrometer (ICP-MS). Results showed significantly increased concentrations of P, Ca, K, Mg, Mn, Ba, Zn, Sr, Cu, Rb, Ni, Li, Cd, Cs, Co, Tl, and Be and reduced concentrations of Se along the ADE profile relative to the control Ultisol. Additionally, it was also detected reduced concentrations of Al, U, and V in ADE superficial layers. Elemental enrichment factors in ADE varied from 1.6 to 15.9 times and the enriched elements have mostly been associated with organic residues. Besides the common elements used to characterize ADE sites, our results have shown altered concentrations of other uncommon elements (Al, Be, Cd, Cs, Li, Se, Tl, U, and V) that can be also used to unveil ADE sites and differentiate them from Amazonian soils without anthropic influence.