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

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.

Fractionation of mercury stable isotopes in lichens

“…The δ 202 Hg in lichens ranged from -0.5 to -3.0 ‰, and the Δ 199 Hg from -0.0 to -0.5 ‰. This was in line with the previously published data (Barre et al, 2020;Blum et al, 2012;Carignan et al, 2009). The in-situ δ 202 Hg was the lowest in Pokljuka, with -3.1 ± 0.6 ‰ among the five samples measured there.…”

“…Hg isotopes in lichens have already been utilised in studies attempting to discriminate pollution sources (Barre et al, 2018(Barre et al, , 2020Olson et al, 2019;Wang et al, 2020). In general, Hg isotopes in lichens have negative MDF (-2.5 to -0.5 ‰ for δ 202 Hg) and MIF (-0.8 to 0.0 ‰ for Δ 199 Hg) (Blum et al, 2014), while atmospheric Hg exhibits mostly positive MDF (0.0 to 1.5 ‰ for δ 202 Hg) and negative MIF (-0.4 to 0.0 ‰ for Δ 199 Hg) (Demers et al, 2013;Gratz et al, 2010;Sonke, 2011;Szponar et al, 2020;Wang et al, 2021).…”