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

Abstract

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 HgCl2-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-1) from the background period (BGP; facility closed; 1992-present) to secondary industrial period (2ndIP; no HgCl2 wood treatment; 1962-1992) to primary industrial period (1stIP; active HgCl2 wood treatment; ≈ 1900-1962). Mass-dependent fractionation (MDF) Hg stable isotope data are shifted negative during 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 stomatal uptake, these data are indicative of emissions derived from industrial activity being enriched in lighter isotopes associated with HgCl2 reduction and Hg0 volatilisation. Similar MDF (δ202Hg: -3.90 ± 0.30 ‰; 1 SD) in bark Hg (137 ± 105 μg kg-1) suggests that stomatal assimilation and downward transport is also the dominant uptake mechanism for bark Hg (reflective of negative stomatal-uptake MDF shift) rather than deposition to bark. THg was enriched in sapwood of all sampled trees across both tree species. This may indicate long-term storage of a fraction of Hg in sapwood or xylem solution. We also observed a small range of odd-isotope mass-independent fractionation (MIF). Differences in Δ199Hg between periods of different industrial activities were significant (Δ199Hg of 1stIP: 0.00 ± 0.03 ‰, 2ndIP: -0.06 ± 0.04 ‰, BGP: -0.13 ± 0.03 ‰; 1 SD), and we suggest MIF signatures are conserved during stomatal assimilation (reflect source MIF signatures). These data advance our understanding of the physiological processing of Hg within trees and provide critical direction to future research into the use of trees as archives for historical atmospheric Hg.