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New publication in Biogeosciences examines internal tree cycling and atmospheric archiving of mercury with concentration and stable isotope analyses
Trees predominantly take up mercury (Hg) from the atmosphere via stomatal assimilation of gaseous elemen-tal Hg (GEM). Hg is oxidised in leaves/needles and trans-ported to other tree anatomy including bole wood, where it can be stored long-term. Using Hg associated with growth rings facilitates archiving of historical GEM concentrations. Nonetheless, there are significant knowledge gaps on the cycling of Hg within trees.
We investigate Hg archived in tree rings, internal tree Hg cycling, and differences in Hg uptake mechanisms in Norway spruce and European larch sampled within 1 km of a HgCl2-contaminated site using total Hg (THg) and Hg stable isotope analyses. Tree ring samples are indicative of significant increases in THg con-centrations (up to 521 µg kg−1) from the background pe-riod (BGP; facility closed; 1992–present) to secondary in-dustrial period (2ndIP; no HgCl2 wood treatment; 1962–1992) to primary industrial period (1stIP; active HgCl2 wood treatment; ≈ 1900–1962). Mass-dependent fractiona-tion (MDF) Hg stable isotope data are shifted negative dur-ing industrial periods (δ202Hg of 1stIP: −4.32 ± 0.15 ‰, 2ndIP: −4.04 ± 0.32 ‰, BGP: −2.83 ± 0.74 ‰; 1 SD). Even accounting for a ≈ −2.6 ‰ MDF shift associated with stom-atal uptake, these data are indicative of emissions derived from industrial activity being enriched in lighter isotopes associated with HgCl2 reduction and Hg0 volatilisation.