Tree rings recording historical atmospheric mercury: A review of progresses and challenges

Foliage uptake of atmospheric elemental mercury (Hg0) and subsequent translocation by the phloem is the main pathway for Hg accumulation in tree rings. Tree rings have been used as the emerging natural archive to directly reconstruct centennial trends of atmospheric Hg0 level. The tree-ring Hg records in remote regions have successfully reconstructed the peak of anthropogenic Hg emissions in Europe and North America in 1960s - 1970s and the distinct increase of Hg emissions in Asia since 1980s. Combining the Hg concentrations and isotopic signatures would provide historical atmospheric Hg trends and Hg emission source shifts. The mechanisms for Hg translocation, specifically the radial translocation and impacts of environmental and tree physiological factors, are yet to be clarified to explain the nonlinear relation between atmospheric Hg0 concentration and Hg signals in tree rings. Thus, we recommend to trace Hg accumulation and translocation processes and their Hg isotopic fractionation in tree rings, and examine the relationship between tree-ring Hg profile and atmospheric pollution level in specific tree species. Finally, we suggest to develop more statistical models to quantify environmental and tree physiological impacts on Hg accumulation and translocation in tree rings.