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-T) concentrations were significantly higher in the southern slopes (72 +/- 54 ng g(-1), 2SD, n = 21) than those in the northern slopes (43 +/- 26 ng g(-1), 2SD, n = 10) of Himalaya-Tibet. No significant relationship was observed between Hg T concentrations and soil organic carbon (SOC), indicating that the Hg-T variation was not governed by SOC. Soil from the southern slopes showed significantly negative mean delta Hg-202 (-0.53 +/- 0.50 parts per thousand, 2SD, n = 21) relative to those from the northern slopes (-0.12 +/- 0.40 parts per thousand, 2SD, n = 10). The delta Hg-202 values of the southern slopes are more similar to South Asian anthropogenic Hg emissions. A significant correlation between 1/Hg-T and delta Hg-202 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 Delta Hg-199 (-0.45 and 0.24 parts per thousand) were observed in frozen soils. Most of soil samples displayed negative Delta Hg-199 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-T concentrations and Hg isotopes indicated that Himalaya-Tibet, even its northern part, may have been influenced by transboundary atmospheric Hg pollution from South Asia. (C) 2019 Elsevier Ltd. All rights reserved.