The impact of vegetation on REE fractionation in stream waters of a small forested catchment (the Strengbach case)

Abstract

Previous studies on waters of a streamlet in the Vosges Mountains (Eastern France) have shown that strontium and rare earth elements (REE) mainly originate from preferential dissolution of apatite during weathering. However, stream water REE patterns normalized to apatite are still depleted in the light REE (LREE, La–Sm) pointing to the presence of an additional LREE depleting process. Vegetation samples are strongly enriched in LREE compared to stream water and their Sr and Nd isotopic compositions are comparable with those of apatite and stream water. Thus, the preferential LREE uptake by vegetation might lead to an additional LREE depletion of surface runoff in the forested catchment. Mass balance calculations indicate, that the yearly LREE uptake by vegetation is comparable with the LREE export by the streamlet and, therefore, might be an important factor controlling LREE depletion in river water. This is underlined by the observation that rivers from arctic and boreal regions with sparse vegetation appear to be less depleted in LREE than rivers from tropical environments or boreal environments with a dense vegetation cover.

Introduction

Natural waters are the main pathway for the transport of elements and particles from the surface and subsurface of the continents to the oceans. Their chemical and isotopic composition is the result of interaction with the environment and especially controlled by erosional processes. Therefore, major river systems have been studied to estimate the fluxes of continent-derived material to the oceans and to shed light upon erosion processes on a global scale (Martin and Meybeck, 1979, Stallard and Edmond, 1983, Meybeck, 1987, Négrel et al., 1993, Blum et al., 1994, Gaillardet et al., 1995, Gaillardet et al., 1997, Dupré et al., 1996). Alteration leads to the disaggregation of rocks and minerals, the formation of soils and also allows the removal of chemical elements as well as of larger and smaller particles from altered rocks and soils by surface runoff. Previous studies have shown that REE are powerful geochemical tracers that provide information about the origin of the suspended and the dissolved river load and about elemental fractionation between particulate and solution phases (Goldstein et al., 1984, Stordal and Wasserburg, 1986, Goldstein and Jacobsen, 1987, Goldstein and Jacobsen, 1988a, Goldstein and Jacobsen, 1988b, Elderfield et al., 1990, Sholkovitz, 1992, Allègre et al., 1996, Tricca et al., 1999, Stille et al., 2003). The fractionation of the REE in river water between dissolved and particulate load as well as immobilization of the REE in the river sediment can be extensive and is strongly controlled by weathering reactions, surface adsorption and solution chemistry (Sholkovitz, 1995, Byrne and Sholkovitz, 1996, Byrne and Liu, 1998). With the exception of Ce (IV) the lanthanides have trivalent oxidation state in most natural waters. The elements of the lanthanide series are characterized by a gradual decrease in the ionic radii with increasing atomic number (“lanthanide contraction” from La³⁺ to Lu³⁺; e.g., Brookins, 1989) leading to a slightly different behavior for heavy REE (HREE, Dy–Lu) and light REE (LREE, La–Sm) during chemical processes such as coprecipitation, adsorption or complexation. The configuration of the valence electrons does not change throughout the series because the additional electrons are systematically filled into the f-electron shell. From the literature it is known that competition between free and complexed REE ions, surface adsorption as well as REE scavenging by colloidal particles may strongly fractionate the relative lanthanide concentrations (Byrne and Kim, 1990, Byrne and Li, 1995, Byrne and Sholkovitz, 1996). As a consequence, REE concentrations in natural waters and fractionation of their distribution patterns strongly depend on pH, availability of potential complex ligands, and the presence of particles and colloids (Byrne and Sholkovitz, 1996).

Besides weathering, solution and surface chemistry, other factors such as vegetation have rarely been considered to be of importance for the REE distributions in river water although plants are actually known to accumulate REE under natural conditions (Sun et al., 1999, Yang et al., 1999, Zhimang et al., 2000, Ozaki and Enomoto, 2001, Akagi et al., 2002, Krachler et al., 2003). Therefore, vegetation might, especially in tropical and temperate zones, be an important REE sink potentially capable of fractionating the REEs in surface runoff. The aim of the present study is to determine in how far vegetation may fractionate the relative lanthanide concentrations in natural surface waters. To do this, the Strengbach streamlet and its uppermost catchment in the Vosges mountains (France) has been chosen, because the isotopic and REE characteristics of this site have already previously been extensively studied (Amiotte-Suchet et al., 1999, Riotte and Chabaux, 1999, Tricca et al., 1999, Aubert et al., 2001, Aubert et al., 2002a, Aubert et al., 2002b, Aubert et al., 2004).