A 400-year tree-ring reconstruction of gaseous elemental mercury in northwestern Canada

Atmospheric mercury (consisting primarily of Hg⁰) from anthropogenic and natural emission sources is a globally dispersed pollutant owing to its long atmospheric residence time ( 1 year). Eventual Hg deposition in terrestrial and aquatic environments, and conversion to toxic forms such as methylmercury, is a major environmental and health concern. International efforts to monitor and mitigate Hg⁰ emissions are underway, but the monitoring network is sparse and many stations have only a few years of observations. Recent studies have found tree-rings are a promising high-resolution natural archive for reconstructing atmospheric Hg⁰ and can potentially be used to place recent Hg⁰ changes in a long-term context and evaluate the efficacy of global mitigation efforts. However, as it is a relatively new proxy, several uncertainties about the influence of internal factors on long-term variability in tree-ring Hg have not been assessed, including cambial age (ring number from pith) and tree-specific differences. In this study, we address these uncertainties using a tree-ring Hg dataset developed from 20 white spruce (Picea glauca) trees from a boreal woodland site in central Yukon, and use these data to reconstruct the last 400 years of atmospheric Hg⁰ change. We find that cambial age has no significant influence on tree-ring Hg concentration, but tree-specific differences in mean concentration are common and must be normalized to most accurately constrain the Hg signal that is common to a sample population of trees. Our site average tree-ring Hg record documents relatively stable and low pre-industrial atmospheric Hg⁰ concentrations, followed by rise from 1750-1950 CE, a relaxation in trend from 1950-1975, and then a continued increase to the highest mean values of the record. Contrary to some monitoring sites in temperate North America and western Europe, our record suggests that atmospheric Hg⁰ concentrations in central Yukon have not subsided in recent years, which may reflect continued cycling of legacy Hg from terrestrial reservoirs and rising Hg⁰ emission in upwind industrial regions (i.e., Eurasia). Additional tree-ring Hg records from this region are needed to substantiate the reconstructed trends and determine if they are related to regional atmospheric change.