Predicting Behavior and Fate of Atmospheric Mercury in Soils: Age-Dating METAALICUS Hg Isotope Spikes with Fallout Radionuclide Chronometry

Soils accumulate anthropogenic mercury (Hg) from atmospheric
deposition
to terrestrial ecosystems. However, possible reemission of gaseous
elemental mercury (GEM) back to the atmosphere as well as downward
migration of Hg with soil leachate influence soil sequestration of
Hg in ways not sufficiently understood in global biogeochemical models.
Here, we apply fallout radionuclide (FRN) chronometry to understand
soil Hg dynamics by revisiting the METAALICUS experiments 20 years
after enriched isotope tracers (198Hg, 200Hg, 201Hg, and 202Hg) were applied to two boreal watersheds
in northwestern Ontario, Canada. Hg spikes formed well-defined peaks
in organic horizons of both watersheds at depths of 3–6 cm
and were accurately dated to the year of spike application in 6 of
7 cases (error = −0.8 ± 1.2 years). A seventh site was
depleted by ca. 90% of both the 200Hg spike and background
Hg, and the spike was dated 16 years older than its application. Robust
FRN age models and mass balances demonstrate that loss of Hg is attributable
to its specific physicochemical behavior at this site, but more work
is required to attribute this to reemission or leaching. This study
demonstrates the potential of FRN chronometry to provide insights
into Hg accumulation, mobilization, and fate in forest soils.