Predicting Behavior\nand Fate of Atmospheric Mercury\nin Soils: Age-Dating METAALICUS Hg Isotope Spikes with Fallout Radionuclide\nChronometry

Soils accumulate anthropogenic mercury (Hg) from atmospheric\ndeposition\nto terrestrial ecosystems. However, possible reemission of gaseous\nelemental mercury (GEM) back to the atmosphere as well as downward\nmigration of Hg with soil leachate influence soil sequestration of\nHg in ways not sufficiently understood in global biogeochemical models.\nHere, we apply fallout radionuclide (FRN) chronometry to understand\nsoil Hg dynamics by revisiting the METAALICUS experiments 20 years\nafter enriched isotope tracers (<sup>198</sup>Hg, <sup>200</sup>Hg, <sup>201</sup>Hg, and <sup>202</sup>Hg) were applied to two boreal watersheds\nin northwestern Ontario, Canada. Hg spikes formed well-defined peaks\nin organic horizons of both watersheds at depths of 3–6 cm\nand were accurately dated to the year of spike application in 6 of\n7 cases (error = −0.8 ± 1.2 years). A seventh site was\ndepleted by ca. 90% of both the <sup>200</sup>Hg spike and background\nHg, and the spike was dated 16 years older than its application. Robust\nFRN age models and mass balances demonstrate that loss of Hg is attributable\nto its specific physicochemical behavior at this site, but more work\nis required to attribute this to reemission or leaching. This study\ndemonstrates the potential of FRN chronometry to provide insights\ninto Hg accumulation, mobilization, and fate in forest soils.