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

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 Norwegian spruce and European larch sampled within 1 km of a Hg contaminated site (HgCl2 wood treatment facility) using total Hg (THg) and Hg stable isotope analyses. Tree rings from all trees are indicative of significant increases in THg concentrations (up to 521 µg/kg) from the background period (after facility closure; 1992 present) to the secondary industrial period (site active, but no HgCl2 treatment; 1962 1992) to the primary industrial period (active HgCl2 wood treatment; ≈1900-1962). Mass dependent fractionation (MDF) Hg stable isotope data that are shifted negative during the primary (δ202Hg: -4.32±0.15 ‰; secondary (-4.04±0.32 ‰; 1SD) compared to the background period (δ202Hg: -2.83±0.74 ‰; 1SD). This we relate to atmospheric releases resulting from reduction of HgCl2 and subsequent evaporation of GEM, enriching emitted GEM in lighter isotopes. Bark THg (137±105 µg/kg) displays similar MDF (δ202Hg: -3.90±0.30 ‰; 1SD) to bole wood from the industrial periods, which suggest that stomatal assimilation and downward transport is the dominant uptake mechanism for bark Hg rather than direct atmospheric deposition to bark (bark Hg reflects ≈-2.6 ‰ MDF shift associated with stomatal uptake). We also discern an enrichment in THg concentrations in sapwood of all sampled trees across both tree species including a background spruce tree. This may indicate long-term storage of a fraction of Hg in sapwood or xylem solution.

"

Co authors: Harald Biester, Tomas Navrátil, Stephan Kraemer, Lorenz Schwab.