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

Abstract

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 (HgT) concentrations were significantly higher in the southern slopes (72\u202f±\u202f54\u202fng\u202fg-1, 2SD, n\u202f=\u202f21) than those in the northern slopes (43\u202f±\u202f26\u202fng\u202fg-1, 2SD, n\u202f=\u202f10) of Himalaya-Tibet. No significant relationship was observed between HgT concentrations and soil organic carbon (SOC), indicating that the HgT variation was not governed by SOC. Soil from the southern slopes showed significantly negative mean δ202Hg (-0.53\u202f±\u202f0.50‰, 2SD, n\u202f=\u202f21) relative to those from the northern slopes (-0.12\u202f±\u202f0.40‰, 2SD, n\u202f=\u202f10). The δ202Hg values of the southern slopes are more similar to South Asian anthropogenic Hg emissions. A significant correlation between 1/HgT and δ202Hg 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 Δ199Hg (-0.45 and 0.24‰) were observed in frozen soils. Most of soil samples displayed negative Δ199Hg 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 HgT concentrations and Hg isotopes indicated that Himalaya-Tibet, even its northern part, may have been influenced by transboundary atmospheric Hg pollution from South Asia.