Internal tree cycling and atmospheric archiving of mercury: examination with concentration and stable isotope analyses

摘要: Trees predominantly take up mercury (Hg) from the atmosphere via stomatal assimilation of gaseous elemental Hg (GEM). Hg is oxidised in leaves/needles and transported 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 HgCl-contaminated site using total Hg (THg) and Hg stable isotope analyses. Tree ring samples are indicative of significant increases in THg concentrations (up to 521 µg kg) from the background period (BGP; facility closed; 1992–present) to secondary industrial period (2ndIP; no HgCl wood treatment; 1962–1992) to primary industrial period (1stIP; active HgCl wood treatment;  1900–1962). Mass-dependent fractionation (MDF) Hg stable isotope data are shifted negative during industrial periods (Hg 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 stomatal uptake, these data are indicative of emissions derived from industrial activity being enriched in lighter isotopes associated with HgCl reduction and Hg volatilisation. Similar MDF (Hg: 3.90  0.30 ‰; 1 SD) in bark Hg (137  105 µg kg) suggests that stomatal assimilation and downward transport is also the dominant uptake mechanism for bark Hg (reflective of negative stomatal-uptake …