Spatial distribution and magnification processes of mercury in snow from high-elevation glaciers over the Tibetan Plateau
Thursday, 18 December 2014
The Tibetan Plateau is one of the most extreme cold regions in the world and a source of fresh water to 1.4 billion people. In this study, between 2008 and 2010, surface snow samples were retrieved in replicate from four high-elevation glaciers with an average elevation of 5200 m above sea level from the Tibetan Plateau and were analyzed for total Hg (HgT). The spatial distribution of Hg for glaciers and magnification processes of Hg in snow with increasing altitude over the Tibetan Plateau are investigated. The total Hg concentrations in snow samples ranged from <1 to 15 ng/L, with the highest concentrations at Mt. Muztagata, and the lowest concentrations at Mt. Nyainqêntanglha. This may be explained by the high concentration and annual flux of particulate matter deposition at Mt. Muztagata, the maximum from which particulate matter loads decrease from northwest to southeast over the Tibetan Plateau. Because Hg deposited on the plateau is primarily associated with particulate matter, only 31% of Hg was lost (e.g., reemitted back to the atmosphere) during the transition of fresh snow to coarse-grained snow. This demonstrates that Hg in snow over the Tibetan Plateau may be less influenced by the effect of post-depositional processes such as reemission and photoreduction than that in the Arctic. Moreover, the increases of HgT with increasing altitude ("altitude effect") found on four high-elevation glaciers (e.g., 3-fold in Mt. Muztagata) suggests that atmospheric Hg is cold-trapped and magnified toward low temperature and/or high-elevation regions, implying that the glaciers over the Tibetan Plateau play an important sink role for global Hg cycling. In the context of a warming climate, the Tibetan Plateau may shift from a current sink to a source in the future, and the accumulated Hg released by accelerated glacier thinning and retreat may endanger ecosystems and human health in the glacier-fed downstream regions.