Tree-Ring Inferred Atmospheric Mercury Concentrations in the Mackenzie Delta (NWT, Canada) Peaked in the 1970s but Are Increasing Once More

Mercury (Hg) is a global pollutant
emitted to the atmosphere from
anthropogenic sources, which have varied over time in response to
increased industrialization and pollution control measures. Tree rings
have shown promise for reconstructing past atmospheric Hg(0) concentrations
to expand on the temporally and spatially limited data provided by
the instrumental record, producing high-resolution data with absolutely
dated chronologies. Here, we present a ∼400 year reconstruction
of atmospheric Hg(0) based on white spruce (Picea glauca) tree rings from a pristine coastal area in Northwestern Canada
(Mackenzie Delta, Northwest Territories). The tree-ring record shows
that Hg concentrations did not begin to increase significantly above
baseline until ∼1875, coinciding with increased industrialization
in Europe and North America. Hg concentrations reached a maximum in
the 1970s and subsequently declined until the end of the 20th century,
which is consistent with the trends in modeled global tropospheric
Hg(0) concentrations based on the latest Hg-emission inventories.
However, unlike many other locations in the northern hemisphere, the
Mackenzie Delta tree-ring record exhibits a rebound in Hg(0) concentrations
since the turn of the century, likely reflecting the increase in upwind
Asian industrial emissions. The relative enrichment in inferred modern
atmospheric Hg(0) relative to preindustrial concentrations [enrichment
factors (EF) = 1.54] is similar to other EF values obtained from tree
rings, but lower than those based on models and sedimentary archives.
Tree-ring Hg concentrations in the Mackenzie Delta were higher than
at a continental site also located in northwestern Canada, suggesting
that ocean Hg(0) evasion and permafrost slumping contribute to higher
local atmospheric concentrations.