Mercury isotopes in frozen soils reveal transboundary atmospheric mercury deposition over the Himalayas and Tibetan Plateau.

The concentration and isotopic composition of mercury (Hg) were studied in frozen soils along a southwest-northeast transect over the Himalaya-Tibet. Soil total Hg (Hg) concentrations were significantly higher in the southern slopes (72 ± 54 ng g, 2SD, n = 21) than those in the northern slopes (43 ± 26 ng g, 2SD, n = 10) of Himalaya-Tibet. No significant relationship was observed between Hg concentrations and soil organic carbon (SOC), indicating that the Hg variation was not governed by SOC. Soil from the southern slopes showed significantly negative mean δHg (-0.53 ± 0.50‰, 2SD, n = 21) relative to those from the northern slopes (-0.12 ± 0.40‰, 2SD, n = 10). The δHg values of the southern slopes are more similar to South Asian anthropogenic Hg emissions. A significant correlation between 1/Hg and δHg was observed in all the soil samples, further suggesting a mixing of Hg from South Asian anthropogenic emissions and natural geochemical background. Large ranges of ΔHg (-0.45 and 0.24‰) were observed in frozen soils. Most of soil samples displayed negative ΔHg values, implying they mainly received Hg from gaseous Hg(0) deposition. A few samples had slightly positive odd-MIF, indicating precipitation-sourced Hg was more prevalent than gaseous Hg(0) in certain areas. The spatial distribution patterns of Hg concentrations and Hg isotopes indicated that Himalaya-Tibet, even its northern part, may have been influenced by transboundary atmospheric Hg pollution from South Asia.