400-Year Record of Atmospheric Mercury from Tree-Rings in Northwestern Canada.

Tree-rings are a promising high-resolution archive for gaseous atmospheric mercury (composed primarily of Hg) reconstruction, but the influence of cambial age (ring number from pith) and tree-specific differences are uncertainties with potential implications for interpreting tree-ring Hg signals. We address these uncertainties and reconstruct the last 400 years of Hg change using a tree-ring Hg data set from 20 white spruce ( Picea glauca) trees from a pristine site in central Yukon. Cambial age has no significant influence on tree-ring Hg concentration, but tree-specific differences in mean concentration are prevalent and must be normalized to a common mean to accurately constrain long-term trends in the mean tree-ring Hg record. Our record shows stable, low Hg concentrations prior to ∼1750 CE, a persistent rise from ∼1750-1950 (increasing more rapidly post-1850), a pause from ∼1951-1975, and then a resumed increase to record-high levels at present. This general pattern is reflected in other proxy-based Hg reconstructions worldwide. This study provides a novel long-term Hg reconstruction in the Western subarctic from one of the most widely distributed boreal tree species in North America and, therefore this proxy may also hold potential for investigating broader spatial patterns in Hg cycling across the subarctic and northern boreal forest.