Seasonal Variations of the Rare Earth Elements in a Boreal River

Abstract

Weathering reactions on the continents lead to extensive fractionation between the dissolved rare earth element (REs composition of river waters and that of river suspended particles and continental rocks. Both the concentration and the extent of fractionation of dissolved trivalent REE in river waters appear to be dependent on pH and on the presence of colloidal matter. The critical phase is most likely river colloids. River water stripped of their suspended particles and colloids by settling and coagulation, may have a solution phase with trivalent REE composition as fractionated as in sea water. This implies that fractionation by oceanic geochemical cycles is not the sole process behind the evolved pattern of sea water (Sholkovitz, 1995). Large temporal variations in the dissolved REE concentration have been observed in river water over time scales of the order of a few days. Temporal changes in REE fractionation have also been observed. For example, both negative and positive Ce-anomalies have been recorded in samples from the same river (Elderfield et al., 1990). The REE data base in river water is based on several rivers but there are only few samples in each river, and the reasons for the temporal variations have been little studied. In this study weekly sampling of the dissolved and suspended particulate phase during 18 months in the Kalix River, Northern Sweden, is presented together with ultra filtration data (colloidal phase). The dissolved ( 0.45 lam) in the river was dominated by a Fe-rich phase, especially during winter. The amount of detrital particles in the suspended phase was relatively low except during high water discharge (Ingri and Widerlund, 1994). Approximately 10% of the particulate REE (>0.45 gm) where transported in detrital particles during winter. At maximum ashed suspended concentration at spring-flood, about 30% of the LREE and up to