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.