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

[摘要] Trees predominantly take up mercury (Hg) from theatmosphere via stomatal assimilation of gaseous elemental Hg (GEM). Hg isoxidised in leaves/needles and transported to other tree anatomy includingbole wood, where it can be stored long-term. Using Hg associated with growthrings facilitates archiving of historical GEM concentrations. Nonetheless,there are significant knowledge gaps on the cycling of Hg within trees. Weinvestigate Hg archived in tree rings, internal tree Hg cycling, anddifferences in Hg uptake mechanisms in Norway spruce and European larchsampled within 1 km of a HgCl 2 -contaminated site using total Hg (THg)and Hg stable isotope analyses. Tree ring samples are indicative ofsignificant increases in THg concentrations (up to 521  µ g kg −1 ) from the background period (BGP; facility closed; 1992–present) to secondary industrial period (2ndIP; no HgCl 2 wood treatment; 1962–1992) to primary industrial period (1stIP; active HgCl 2 woodtreatment; ≈  1900–1962). Mass-dependent fractionation (MDF) Hgstable isotope data are shifted negative during industrial periods ( δ 202 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 activitybeing enriched in lighter isotopes associated with HgCl 2 reduction andHg 0 volatilisation. Similar MDF ( δ 202 Hg: − 3.90  ±  0.30 ‰; 1 SD) in bark Hg (137  ±  105  µ g kg −1 ) suggests that stomatal assimilation and downwardtransport is also the dominant uptake mechanism for bark Hg (reflective ofnegative stomatal-uptake MDF shift) rather than deposition to bark. THg wasenriched in sapwood of all sampled trees across both tree species. This mayindicate 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 Δ 199 Hg between periods of different industrial activities weresignificant ( Δ 199 Hg of 1stIP: 0.00  ±  0.03 ‰, 2ndIP:  − 0.06  ±  0.04 ‰, BGP:  − 0.13  ±  0.03 ‰; 1 SD), and we suggest MIFsignatures are conserved during stomatal assimilation (reflect source MIFsignatures). These data advance our understanding of the physiologicalprocessing of Hg within trees and provide critical direction to futureresearch into the use of trees as archives for historical atmospheric Hg.

[学科分类] 大气科学