Geochemistry of Quaternary sediments in terraces of the Tinto River (SW Spain): Paleoenvironmental implications

The Tinto River shows very high concentrations of toxic elements and acidity from its source in the mining area to its mouth. In this paper, we studied the evolution of the levels of toxic elements during part of the Quaternary from the analysis of fluvial deposits that, forming terraces, were originated by the dynamics of the Tinto River. Using a detailed geomorphological cartography, up to nine terrace levels were differentiated in the lower sector of the basin, whose ages range from the Pleistocene to the Holocene. Sampling and geochemical analyses (major elements and trace) were carried out in six of these levels, in addition to sediments of the current flood plain. The results of these analyses do not show any bias from the mid Pleistocene (T8 — Terrace 8) to the early Holocene (T1). Moreover, these geochemical contents are similar to the average values obtained by other authors for the entire South-Portuguese Zone of the Iberian Massif. However, the current sediments of the Tinto River (T0) show a sharp increase in the levels of As, Cu, Pb and Zn compared to the rest, which indicates a serious increment of contamination by toxic elements. Although there are evidences on the existence of a natural process of oxidation of sulfides since the Miocene, the amounts of toxic elements released should be very small compared to those generated since the beginning of the mining activities in the Iberian Pyrite Belt.