Mercury Stable Isotope Composition of Lichens and Mosses from Northern Eurasia Reveals Hg Deposition Pathways and Sources

Mercury (Hg) concentrations in lichens and mosses can
be used as
surrogates for atmospheric Hg deposition to continental surfaces.
In this study, we collected and analyzed Hg concentrations (n = 334) and isotopic composition (n =
67) of epiphytic tree lichens and terricolous lichens and mosses from
remote locations across the Eurasian Arctic and sub-Arctic (50 to
72° N, 30 to 180° E). The total Hg (THg) concentrations
ranged from 13 to 7700 ng g–1. Epiphytic tree lichens
had significantly higher median THg levels (243 ng g–1) than terricolous lichens (35 ng g–1) and mosses
(74 ng g–1). THg is substantially higher in both
tree lichens and terricolous lichens near the Arctic Ocean shore and
up to 300 km inland. The combined δ202Hg, Δ199Hg, and Δ200Hg signatures suggest that
the elevated coastal Hg levels are delivered by marine air masses
rich in gaseous and particulate-oxidized HgII forms, such
as HgBr2. Similar to other vegetation Hg isotope studies,
inland terricolous lichen and moss Δ200Hg are near
zero, indicating a dominant (63%) atmospheric Hg0 origin
followed by HgII wet and dry deposition. Inland tree lichens
carry a more positive Δ200Hg of 0.15‰, similar
to the atmospheric HgII end-member, suggesting that they
preferentially accumulate HgII wet and dry deposition compared
to colocated terricolous lichens. Mosses from the European sub-Arctic
show a low δ202Hg of −3.1‰, which we
speculate to result from regional soil Hg0 emissions that
are recaptured by mosses. Overall, the Hg isotope variability of mosses
and lichens reveals latitudinal gradients in Hg deposition pathways
and identifies preferential Hg0 or HgII uptake.