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 Hg0) 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 Hg0 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 Hg0 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 Hg0 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 Hg0 cycling across the subarctic and northern boreal forest.