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

Abstract

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.

Paleolimnological characterization of metal(loid) sources and transport processes in Canadian subarctic lakes

“…American emissions Our flux deconstruction approach indicates that atmospheric pollution (long-range and local) was the dominant driver of Hg accumulation in bogs from ~1880 onward and that atmospheric Hg deposition peaked during the 1970s. The post 1970s decline in atmospheric Hg deposition recorded by bogs corroborates findings from tree-ring reconstructions in the Mackenzie River Valley in the central Northwest Territories (Ghotra et al, 2020). Our bog records are also consistent with decreasing Hg concentrations in air (gaseous elemental Hg) and in precipitation measured at monitoring sites in southern Canada since the 1990s (Cole et al, 2014) as well as studies modelling Hg deposition based on anthropogenic emissions and atmospheric transport models .…”

“…This trend contrasts with an ice core record from the western Canadian Arctic suggesting increasing deposition of Pb over the same period (Marx et al, 2016;Osterberg et al, 2008). Similarly, our reconstructions of atmospheric Hg deposition align with atmospheric Hg reconstruction from Mackenzie River tree-rings (Ghotra et al, 2020), showing a peak in the 1970s, but contrast with a recent tree-rings reconstructions from the western Canadian Arctic (Yukon) showing stable or increasing atmospheric Hg after the 1970s (Clackett et al, 2018(Clackett et al, , 2021. The Yellowknife area follows the continental Arctic trend in Pb and Hg accumulation rather than the western Canadian Arctic trend, which are respectively influenced by the Continental Polar and Arctic air masses and the Maritime Polar air mass, the former being more impacted by long-range influences from Asia (Dastoor et al, 2015).…”

“…The Yellowknife area follows the continental Arctic trend in Pb and Hg accumulation rather than the western Canadian Arctic trend, which are respectively influenced by the Continental Polar and Arctic air masses and the Maritime Polar air mass, the former being more impacted by long-range influences from Asia (Dastoor et al, 2015). The Mackenzie River tree-rings reconstruction showed a new increase in atmospheric Hg concentration in the last 20 years (Ghotra et al, 2020), whereas there was no conclusive evidence of an increase in atmospheric Hg deposition in our bog records for the last 20 years. Only two of our ombrotrophic peat records (HW3 and WB-P1) had more than one sample dated post-2000, and Hg accumulation rate was slowly increasing in one (HW3) and slowly decreasing in the other (WB-P1) (Figure 5.20).…”

A Continental and Marine-Influenced Tree-Ring Mercury Record in the Old Crow Flats, Yukon, Canada

“…More data is needed to determine if a Hg rebound is also observed at the Old Crow Flats, which is observed in the 5 year resolution tree-ring THg record from white spruce trees in the Mackenzie Delta (250 km northeast of the Old Crow Flats), which ends in 2015. 28 In contrast, sediment records from Alaska show a continued increase in THg flux to present, rather than a peak after the 1970s immediately followed by decline. 42 An enrichment factor (EF) of 1.62 was calculated using the concentration in the first segment of the breakpoint analysis (2.51 ng g −1 THg) and the maximum average THg concentration (4.06 ng g −1 THg), which occurred in 1993.…”

“…The EF reported in this study is smaller than the EF calculated at the Scree Hill site in central Yukon, which was estimated at 1.9, 13 and is similar to the EF of 1.54 estimated in the Mackenzie Delta. 28 Amos et al 46 compiled peat bog and lake sediment cores, used 1760−1880 as the baseline, similar to the baseline used in this study, and calculated an EF of 3 in sediment cores and calculated an EF of 4.3 in peat bog. It is important to note that the EF metric will be affected by the period selected as the preindustrial baseline, which could be subject to the length of the temporal record and the medium of the record.…”