Distribution of rare earth elements (REEs) in sediment cores of fjords in the Norwegian coast.
Supervisor: Tomasz Ciesielski (Arctic Tech, Svalbard)
Rare Earth Elements (REEs) are a group of 16 trace metals, that includes the lanthanide series (La – Lu), plus yttrium (Y), with a cohesive geochemical behavior which makes them useful tools for assessing geochemical processes, provenance and depositional history of sediments in aquatic systems. In this study, the content and distribution of REEs in sediment cores from three Norwegian fjords was investigated, Balsfjord in the northern coast, Trondheimsfjord in the central, and Oslofjord in the southeastern coast. Extraction of 13 REEs from the sediment was carried out with a modified BCR sequential extraction, providing REEs in three leached fractions (I – exchangeable, II – reducible/bound to Fe-Mn oxyhydroxides, III – oxidizable/bound to organic matter) and a residual one (IV). Total REE content in Balsfjord averaged 249.27 ± 12.48 mg kg-1, in Trondheimsfjord, 186.24 ± 17.75 mg kg-1, and in Oslofjord, 307.66 ± 26.25 mg kg-1. The percentage of the contribution of each fraction shows the bulk of REEs is in the residual fraction and that Fe-Mn oxyhydroxides are major carriers of labile REEs to the sediment. Fractionation patterns in all cores show preferential scavenging of certain elements for each fraction, but the resulting signature displays LREE enrichment over HREE (LaN/YbN = 1.29 – 1.88), comparable to other coastal environments. Ce/Ce* (1.04 – 1.10) and Gd/Gd* (0.99 – 1.03) suggest it was inherited from the source material. Vertical distribution is quite stable throughout the cores, and a diagenetic signal could not be distinguished. Marked decreases in REEs content in Trondheimsfjord are likely due to reduced precipitation and river discharge. REEs concentrations are close to European Shale (EUS) values, and significant correlations with Al suggest a predominant terrigenous origin, although further mineralogical and petrological studies are needed to establish provenance. No anthropogenic REE input was identified in any of the sites, REEs concentrations found in this study can be used as background levels for future comparisons.