Comparison of the Isotopic Composition of Hg and Pb in Two Atmospheric Bioaccumulators in a Pyrenean Beech Forest (Iraty Forest, Western Pyrenees, France/Spain)

Julien P G Barre,Emmanuel Tessier,Alberto De Diego,David Elustondo Valencia,Sylvain Bérail,David Amouroux,Gaëlle Deletraz,Cristina Sola-Larrañaga,Jesús Miguel Santamaría,Silvia Queipo-Abad

doi:10.3389/fenvc.2020.582001

Abstract

Mercury (Hg) and lead (Pb) isotopic compositions were investigated in mosses and lichens collected in a large mountainous beech forest (Iraty Forest) located on the French-Spanish Pyrenean border. Hg isotopic signature in topsoil samples were also analyzed in selected sampling sites. This is the first work that uses the complementary information of both isotopic systems in two distinct atmospheric bioaccumulators. Mosses and lichens present characteristic accumulation due to their integration times, displaying different information on metal pollution over the area. Hg and Pb concentrations in annual moss shoots represent recent atmospheric accumulation, while whole lichen thalli integrates a process of accumulation over a longer period. Lead isotope ratios in mosses are consistent with reported data corresponding to the actual European atmospheric background (206Pb/207Pb ∼ 1.158), while Hg isotopic composition reflects potential uptake of both dry and wet Hg depositions. For lichens, Pb isotopic composition exhibits the contribution of a longer integration period of both industrial Pb emissions and legacy of leaded gasoline pollution. Hg isotopes in lichens discriminate two main groups: a larger one representing the background atmospheric contribution and a second one corresponding to unexpected higher Hg content. The similarities in odd and even Mass-independent fractionation of Hg isotopes between topsoils and lichens from the larger group, support the idea that foliage uptake is the main input of Hg in soils. The second group of lichens exhibits more negative δ202Hg (down to –4.69‰) suggesting a new source of fractionation in this area, probably related to lichens aging and/or stubble and grass fires due to pastoral activities. This study demonstrates that using both Hg and Pb isotopic signature in lichens and mosses allows to trace atmospheric sources and environmental pathways of these metals in forested ecosystems. This original data set in a remote environment provides also new information on the fate of atmospheric Pb and Hg depositions.

Highlights

Lichens and mosses present excellent characteristics when monitoring air pollution by metals (Bargagli, 2016)
This study reports for the first time the combination of Hg and Pb isotopic systems in mosses, lichens and soils collected at different altitudes in the French (North) and Spanish (South) slopes of the Iraty Forest, one of the largest beech forests in the south of Europe
The plot of 206Pb/207Pb vs. 1/[Pb] for lichens and mosses (Figure 2) allows to define three possible reservoirs of deposited Pb based on the isotopic signature: a local geogenic source, the “European” atmospheric background and a more regional anthropogenic and industrial source (“French” industrial lead). 206Pb/207Pb ratio in lichens for the highest concentrations of Pb, is representative of the “French” industrial Pb defined in previous works (Monna et al, 1997; Carignan et al, 2005; Geagea et al, 2008)

Summary

Introduction

Lichens and mosses present excellent characteristics when monitoring air pollution by metals (Bargagli, 2016). Several recent studies have evaluated mosses and lichens as biomonitors of metal pollution in forest ecosystems and remote areas (Barre et al, 2013; Barre et al, 2015; Agnan et al, 2017; Fabri et al, 2018; Kłos et al, 2018; Ratier et al, 2018; Panichev et al, 2019; Shotyk and Cuss, 2019; Klapstein et al, 2020) These studies suggest that both bioaccumulators provide complementary information depending on their differences in trace elements uptake and integration times (Bargagli et al, 2002; Szczepaniak, 2003; Giordano et al, 2013). Both particulate bound mercury (PBM) and gaseous oxidized mercury (GOM) have more positive MIF compared to gaseous elemental mercury (GEM), which usually displays negative Δ199Hg (Gratz et al, 2010; Demers et al, 2013; Rolison et al, 2013; Fu et al, 2016a; Zheng et al, 2016)

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