Geochemistry of rare-earth elements in Holocene sediments of an acidic estuary: Environmental markers (Tinto River Estuary, South-Western Spain)

Concentration and fractionation patterns of rare earth elements (REE) have been studied in sediments from three cores collected in the Tinto River estuary. REE mean concentration (104.4 ppm) and mean values for LREE / HREE ratios (6.3) are lower than observed in sediments from other rivers and estuaries. NASC (North American Shale Composite) normalized patterns are typically convex and subparallel, and show a clear depletion of LREE relative to MREE and HREE. These characteristics indicate that sediments were deposited in an environment affected by acid mixing processes. Vertical evolution of the fractionation patterns and (La / Gd)NASC and (La / Yb)NASC ratios in the three cores show an increasing of acid drainage influence in the estuary, caused by the displacement of acid mixing processes from the inner zones of the estuary to the outwards. Under these conditions (with pH ranging between 2.5 and 5), LREE remained in the dissolved phases, whereas HREE and, in a lesser extent, MREE are adsorbed onto particle surfaces.