Lanthanides and yttrium in the sediments of the lower Minho River (NW Iberian Peninsula): imprint of tributaries

Abstract

Rare earth elements have been used as sediment tracers in river, estuarine and coastal environments but rarely applied as fluvial tributary tracers. Lanthanides (Ln) and yttrium (Y) were quantified in fluvial sediments of the Minho River lower course (NW Iberian Peninsula), where the catchment contains heterogeneous lithologies, to define the characteristic imprints of tributaries and their relevance in the riverine system. Surface sediments were sampled at 36 points in the lower Minho riverbed and its nine main tributaries.The < 2-mm fraction was sieved and ground, and ≈ 100 mg was completely acid-digested with HF and aqua regia in closed Teflon bombs at 100 °C for 1 h. The residue was evaporated, re-dissolved with HNO3 and Milli-Q water, heated 20 min at 75 °C and diluted to 50 cm3 with Milli-Q water. Lanthanides and yttrium were determined using a quadrupole ICP-MS equipped with a Peltier impact bead spray chamber and a concentric Meinhard nebuliser. Blanks accounted for less than 1% of the element concentrations in the samples. The precision and accuracy of the analytical procedures were controlled through reference materials AGV-1 and MAG-1. Contents ranged from 22 to 153 mg Ln kg−1 and 1.5–22.9 mg Y kg−1 and ES-normalised light-heavy Ln fractionation, (LN/HN), varied between 0.6 and 2.6. These wide ranges, together with Eu and Ce anomalies and element-by-element Ln, varied with changes in parental rocks of the lower Minho basin. Minho sediments showed negative Ce anomalies (0.81 ± 0.29) and positive Eu anomalies (1.23 ± 0.18). Sediments received traces of granitic pegmatites and gneissic peralkaline rocks from two tributaries: the Gadanha (22.9 mg Y kg−1; 83 mg Ln kg−1; 0.60 LN/HN; 0.51 Eu/Eu*; 0.88 Ce/Ce*) and the Louro (15.9 mg Y kg−1; 110 mg Lnkg−1; 0.97 LN/HN; 0.69 Eu/Eu*; 1.49 Ce/Ce*). The Tamuxe tributary, flowing through a slate and quartzite fault, provided the lowest source (1.6 mg Y kg−1; 28 mg Ln kg−1; 2.48 LN/HN; 1.01 Eu/Eu*; 0.55 Ce/Ce*). Lanthanide and Y signatures in sediments may be used to trace land-tributary-river influences. The imprints are observed downstream of fluvial confluences but not in all cases, responding to basin lithological changes, particularly for pegmatites and peralkaline rocks. The general REE trend is described using Y contents only. Tributaries, which are responsible for one-fifth of the Minho water load, provide one-half of their sediments. Non-homogeneous sediment patterns may be magnified in dammed rivers such as the Minho.