The isotopic composition of Nd in a boreal river: a reflection of selective weathering and colloidal transport

Abstract

In this study the Nd concentrations (C Nd) from 18 months of weekly sampling of filtered water (<0.45 μm) in the Kalix River, northern Sweden, are reported with ε Nd(0) and 147Sm/ 144Nd ratios determined in samples representing major flow events as well as maxima and minima in C Nd. The C Nd varies by a factor of ten, between 200 pmol/L to 2100 pmol/L, and there is a strong relation between high discharge and high C Nd. The Nd in the Kalix River is mainly transported on particles (>90%), dominated by a colloidal phase primarily composed of organic C and Fe. The ε Nd(0) and 147Sm/ 144Nd only vary within a narrow range, −27.1 to −24.8 and 0.103 to 0.110 respectively, with no obvious relationship to C Nd and discharge. The ε Nd(0) and 147Sm/ 144Nd in the river water is significantly lower than in the unweathered till and average bedrock in the catchment and show a closer resemblance with the isotopic characteristics found in humic substances and plant material. These data show that the isotopic composition of Nd exported from a large boreal drainage basin does not directly reflect that of the bulk bedrock in the catchment. The isotopic composition is controlled by selective weathering and the Nd transport is dominated by organic colloidal particles.