Origin and fluxes of atmospheric REE entering an ombrotrophic peat bog in Black Forest (SW Germany): Evidence from snow, lichens and mosses

Abstract

The fate of the Rare Earth Elements (REE) were investigated in different types of archives of atmospheric deposition in the Black Forest, Southern Germany: (1) a 70 cm snow pack collected on the domed part of a raised bog and representing 2 months of snow accumulation, (2) a snow sample collected close to the road about 500 m from the peat bog, (3) two species of lichens and (4) a peat profile representing 400 years of peat accumulation as well as a “preanthropogenic” sample and the living moss layer from the top of the core. REE concentrations in peat are significantly correlated to Ti which is a lithogenic conservative element suggesting that REE are immobile in peat bog environments. Snow, lichens and peat samples show similar PAAS (Post Archean Australian Shale) normalized REE distributions suggesting that the complete atmospheric REE signal is preserved in the peat profile. However, the annual flux of REE accumulated by the peat is ca. 10 times greater than that of the bulk winter flux of REE. This difference probably indicates that the REE concentrations in the snowpack are not representative of the average REE flux over the whole year. Despite the pronounced geological differences between this site (granite host-rock) and a previously studied peat bog in Switzerland (limestone host-rock) similar REE distribution patterns and accumulation rates were found at both sites. Given that both sites confirm an Upper Continental Crust signature, the data suggests both sites are influenced by regional and not local, soil-derived lithogenic aerosols.

Introduction

In the last decades, there has been an increasing interest in trace metals in natural environments (e.g., Nriagu and Pacyna, 1988, Vernet, 1992, de Vries and Bakker, 1995, Hernandez et al., 2003) and particularly in atmospheric precipitation (Struempler, 1976, Galloway et al., 1982, Barrie et al., 1987, Noll et al., 1990, Atteia, 1994, Barbaris and Betterton, 1996, Takeda et al., 2000, Roy and Négrel, 2001, Ferrari et al., 2002, Nieminen et al., 2002, Walker et al., 2003, Zhang and Liu, 2004). This is of importance because atmospheric deposition constitutes a major contribution of numerous heavy metals, which are of potential toxicity for ecosystems. In remote areas (at high latitudes for instance), records of elevated concentrations of these trace elements provide evidence of long range transport of anthropogenic pollutants from urbanized regions (e.g., Boutron and Patterson, 1987, Berg et al., 1994, Halstead et al., 2000).

Among these trace metals, data for Rare Earth Elements (REE) in atmospheric precipitation are still very scarce (Freydier et al., 1998, Ikegawa et al., 1999, Aubert et al., 2002, Zhang and Liu, 2004) since these elements are often below the ng L⁻¹ level, even in areas affected by human activities and thus mostly require a pre-concentration before analysis. Nevertheless, the knowledge of the REE behaviour at the atmosphere–soil interface is of importance because REE have similar and conservative behaviour and a relatively lack of human sources. Consequently, REE have been recently used as tracers and reference elements in broad fields of environmental studies (Greaves et al., 1999, Chiarenzelli et al., 2001, Sahoo et al., 2001, Shotyk et al., 2001, Bayon et al., 2002, Krachler et al., 2003, Stille et al., 2003, Gaiero et al., 2004).

Peat bogs are fed exclusively by atmospheric deposition and are readily age dated (Givelet et al., 2004). Thus, they have been widely used as archives of past atmospheric deposition (Shotyk et al., 1996, Shotyk et al., 2001, Shotyk et al., 2002, Görres and Frenzel, 1997, Kempter et al., 1997, Weiss et al., 1997, Benoit et al., 1998, Martinez-Cortizas et al., 1999, Roos-Barraclough et al., 2002).

Nevertheless, to our knowledge, it is only the fourth time including the present study that complete REE patterns are investigated in peat profiles (Yliruokanen and Lehto, 1995, Akagi et al., 2002, Krachler et al., 2003). Yliruokanen and Lehto show that REE patterns in Finnish peat bogs are similar to those of the surrounding rocks and that REE concentrations follow the status of the bog (minerotrophic vs. ombrotrophic), that is to say, the mineral content of the peat. The study by Krachler et al. (2003) on a peat core in the Swiss Jura (Étang de la Gruère: EGR), confirms that REE concentrations more or less follow concentrations of other conservative elements like Sc, that is to say, the mineral content. The peat profile from EGR also shows an increase of REE concentrations that corresponds to the beginning of the industrial period and increasing deposition of dust. Unlike soil-grown plants (Fu et al., 2001), peat mosses (Sphagnum) and grass (Carex) do not show a Ce anomaly in their REE patterns (Akagi et al., 2002, Fu et al., 2004). This is in agreement with a simple thermodynamic model (Akagi and Masuda, 1998) showing no possible formation of Ce anomaly at the low pH and E_H found in peat bogs. Therefore, the REE patterns of peat are not biologically fractionated and allow potentially the identification of an atmospheric signal.

Thus, the first objective of this study is to establish the fate of REE and particularly their potential mobility in a “chemically aggressive” environment because anoxic conditions and acidic pore waters should be mechanisms changing the atmospheric REE preservation in the peat column. This can be investigated by comparing REE behaviour to those of Zn and Ti. Both elements have contrasted and relatively well-known behaviours in peat. The second aim of this work is to compare the REE distributions in peat to those of the present deposition in order to evaluate the variations of the REE atmospheric signal over the years. For that purpose REE distribution patterns of snow (particles and wet deposition), as well as lichens, have been determined. Epiphytic lichens, which are very sensitive to environmental changes, are often used to monitor the present atmospheric inputs of elements and especially trace metals (e.g., Puckett, 1988, Loppi et al., 1994, Lippo et al., 1995, Zhang et al., 2002). Lichens with a slow growth rate (few millimetres per year) depend on nutrients from air constituents (wet, dry and gases) (Nash and Egan, 1988, Nash, 1996, Rossbach et al., 1999) and can accumulate trace elements (in particulate form or bounded to cation exchange sites) in the intercellular spaces of the thallus (Rodrigo et al., 1999). Lichens are powerful biomonitors because they are evergreen allowing bioindication at any time and for many years. Finally, REE distribution patterns and accumulation rates in the Black Forest peat bog are compared with those previously measured by Krachler et al. (2003) in EGR 100 km away in Northern Switzerland in order to determine in how far the atmospheric REE characteristics in peat bogs are of regional or supra-regional significance.