Eu anomalies in soils and soil water from a boreal hillslope transect – A tracer for Holocene lanthanide transport?

Concentrations of lanthanides (La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm and Lu) were measured in soils and soil water from a hillslope transect in the Krycklan catchment in northern Sweden. Soil water was collected at different depths and at different distances from the draining stream – from the uphill Podzol to the peaty riparian zone – throughout the course of a year. Combined with soil profiles from the same sites this provided a comprehensive dataset to study the fractionation and transport of lanthanides from boreal forest soils to streams. The soil water was characterised by high concentrations of lanthanides in the riparian zone, more than an order of magnitude higher than in the uphill mineral soil, and when normalised against local unweathered till the soil water exhibited distinctly negative Eu anomalies, particularly in deeper soil layers. The soil samples displayed both positive and negative Eu anomalies, but positive Eu anomalies were primarily associated with strongly weathered soils, whereas negative soil Eu anomalies were observed in organic soils at depths were large amounts of lanthanides are transported towards the stream. Since reduction of Eu seems unlikely to occur in this transect, we propose that the Eu anomalies in soils and soil water are caused by the partial occurrence of Eu in more weathering-resistant minerals than the other lanthanides. Based on the collected data and the hydrological and biogeochemical understanding of the studied transect we therefore propose a conceptual model for how Eu anomalies arise and how they can be used to interpret the Holocene transport and accumulation of lanthanides in soils. The results suggest that Eu anomalies have the potential to become a powerful tracer for studying weathering and soil development.