Internal tree cycling and atmospheric archiving of mercury: examination with concentration and stable isotope analyses
<p>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 facilitat...
| Main Authors: | , , , , |
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| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2022-09-01
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| Series: | Biogeosciences |
| Online Access: | https://bg.copernicus.org/articles/19/4415/2022/bg-19-4415-2022.pdf |
| Summary: | <p>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<span class="inline-formula"><sub>2</sub></span>-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 <span class="inline-formula">µ</span>g kg<span class="inline-formula"><sup>−1</sup></span>) from the <i>background period</i> (BGP; facility closed; 1992–present) to <i>secondary industrial period</i> (2ndIP; no HgCl<span class="inline-formula"><sub>2</sub></span> wood treatment; 1962–1992) to <i>primary industrial period</i> (1stIP; active HgCl<span class="inline-formula"><sub>2</sub></span> wood
treatment; <span class="inline-formula">≈</span> 1900–1962). Mass-dependent fractionation (MDF) Hg
stable isotope data are shifted negative during industrial periods (<span class="inline-formula"><i>δ</i><sup>202</sup></span>Hg of 1stIP: <span class="inline-formula">−</span>4.32 <span class="inline-formula">±</span> 0.15 ‰,
2ndIP: <span class="inline-formula">−</span>4.04 <span class="inline-formula">±</span> 0.32 ‰, BGP: <span class="inline-formula">−</span>2.83 <span class="inline-formula">±</span> 0.74 ‰; 1 SD). Even accounting for a <span class="inline-formula">≈</span> <span class="inline-formula">−</span>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<span class="inline-formula"><sub>2</sub></span> reduction and
Hg<span class="inline-formula"><sup>0</sup></span> volatilisation. Similar MDF (<span class="inline-formula"><i>δ</i><sup>202</sup></span>Hg: <span class="inline-formula">−</span>3.90 <span class="inline-formula">±</span> 0.30 ‰; 1 SD) in bark Hg (137 <span class="inline-formula">±</span> 105 <span class="inline-formula">µ</span>g kg<span class="inline-formula"><sup>−1</sup></span>) 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 <span class="inline-formula">Δ<sup>199</sup></span>Hg between periods of different industrial activities were
significant (<span class="inline-formula">Δ<sup>199</sup></span>Hg of 1stIP: 0.00 <span class="inline-formula">±</span> 0.03 ‰, 2ndIP: <span class="inline-formula">−</span>0.06 <span class="inline-formula">±</span> 0.04 ‰, BGP: <span class="inline-formula">−</span>0.13 <span class="inline-formula">±</span> 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.</p> |
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| ISSN: | 1726-4170 1726-4189 |