Abstract
Soils are a principal global reservoir of mercury (Hg), a neurotoxic pollutant accumulated through a history of anthropogenic emissions to the atmosphere and subsequent deposition to terrestrial ecosystems. The fate of Hg deposition in soils remains fundamentally uncertain, however, particularly to what degree Hg is quantitatively retained versus re-emitted back to the atmosphere as gaseous elemental mercury (GEM). Here we introduce a new bottom-up soil mass balance based on fallout radionuclide (FRN) chronometry that allows direct quantification of historical Hg soil accumulation rates and comparison with measured contemporary atmospheric deposition. We show that soils spanning Arctic, boreal, temperate, and tropical ecosystems are strong and long-term sinks for atmospheric Hg, and that the soil sink strength decreases with latitude. Peak deposition reconstructed for years 1950-2000 strongly exceeds contemporary deposition fluxes by factors of approximately two. In the northeastern USA, trends in soil-derived Hg accumulation rates agree in timing and magnitude with records derived from regional lake sediments and atmospheric measurements. We show that typical soils are quantitatively efficient at retaining atmospheric Hg deposition, with exception of a subset of soils (about 20%, all temperate and boreal coniferous), where approximately 10% of Hg deposition is unaccounted for, suggesting that up to 2% of soil Hg may be lost by legacy emission of GEM back to the atmosphere when scaled across the landscape. The observation that most soil Hg is effectively sequestered long-term calls into question global model and mass balance studies that assume strong and continued re-cycling of legacy Hg pollution in the environment that prolongs the impacts of past Hg emissions. Availability of FRN chronometry to reconstruct soil Hg accumulation rates poses a powerful new tool to quantify Hg deposition and trends across much larger spatial scales than previously possible, and should advance the understanding of Hg deposition, accumulation, and fate in the context of changing global environment.