Study of rare earth elements in overbank sediments under the influence of different geochemical-lithological environments

Rare earth elements (REE) have been studied in overbank floodplain sediments in two Spanish areas with very distinct geochemical and lithological features (the areas of Madrid and Iberian Pyrite Belt-IPB). Considering the high representativity of these sediments, the objective of the study is to estimate the influence of these environments over REE composition and patterns. Total REE contents were determined in overbank sediment samples from seven vertical profiles, and some samples were subjected to a Tessier sequential extraction. The contents of sigma REE vary widely in these two areas from 109 mg/kg in Tinto profile (IPB Area) to almost 1000 mg/kg in Guadarrama profile (Madrid Area). In the Madrid Area, Eu negative anomalies have been detected in all samples after a NASC normalization of the results, as well as an enrichment in light rare earth elements (LREE). Samples analyzed by sequential extraction have shown that REE are distributed in the fractions bound to organic matter, carbonates and residual. In the IPB area, the overbank sediment samples are also enriched in LREE, but they do not show any Eu anomaly. However, positive Ce anomaly is detected along the Odiel-2 profile. In the Tinto and Odiel-1 profiles, considered to be under the influence of mining activities, REE are almost entirely concentrated in the residual fraction (> 95%), while in Odiel-2 profile they present a wider distribution in all the Tessier fractions. These different patterns and contents of the REE seem to be related to the impact of acid waters. Different behaviors and characteristics were detected in these studied areas. REE in overbank sediments are heavily influenced by the parent material, as well as by the geochemical environment.