Mercury (Hg) is a pollutant of global concern for human and ecosystem health. This is particularly true in the Arctic where indigenous populations are excessively exposed to dietary Hg from fish and marine mammal consumption. The deposition of Hg from the atmosphere to Earth surfaces and its re-emission via biogeochemical reduction processes determine Hg concentrations in Earth surface reservoirs, such as soils, snow, and runoff into Arctic lakes and surface Ocean. Stable Hg isotopes are a promising new tool to identify the dominant pathways of atmospheric Hg deposition and potential re-emission processes. We investigated the Hg isotope signatures of different plant species, organic and mineral soil horizons and bedrock of the Arctic tundra at Toolik Field Station, Alaska. We measured stable Hg isotope signatures of atmospheric Hg0, Hg0 in interstitial snow and soil air and Hg in snow, representing atmospheric HgII scavenged during snowfall or deposited during halogen-driven atmospheric mercury depletion events (AMDE). The results suggest that vegetation uptake of gaseous elemental mercury from the atmosphere is the dominant deposition pathway to the terrestrial ecosystem, and contributes about 70% to Hg in organic soils. We have observed large mass-independent Hg isotope anomalies in snow during spring, with minimal D199Hg values of -1.4‰, which are in agreement with previously observed Hg isotope signatures during the AMDE season at the Arctic coast. However, the transfer of HgII from AMDE's to the terrestrial ecosystem does not represent a significant source (0-5 %), suggesting that most HgII deposited during AMDE's was re-emitted prior to snow-melt. The Hg isotope signatures in the soils and the atmosphere showed no indication for substantial re-emission of gaseous elemental mercury from the soils. The Hg stable isotope results agreed well with micro-metrological flux measurements conducted at the same location. Both independent approaches suggest that vegetation uptake of gaseous elemental Hg represents the dominant Hg flux between the atmosphere and Arctic tundra soils and they thus represent a net sink for atmospheric Hg. Changes in environmental conditions, such as warming and thawing of permafrost due to climate change or fires could however lead to a remobilization of the large Hg pool stored in Arctic tundra soils.
