Mercury distribution and variation on a high-elevation mountain glacier on the northern boundary of the Tibetan Plateau

- Average Hg concentrations in snow were at high levels in the LHG glacier.
- Most of the Hg in the snow and ice is associated with particulate matter.
- An increase of Hg concentrations with depth was found in snowpits.
- Increases of Hg with increasing altitude were not found in surface snow.
- The highest Hg concentrations were found in surface ice.

In July 2013, snow/ice and streamwater samples were retrieved from a high-elevation mountain glacier (Laohugou Glacier No. 12) on the northern boundary of the Tibetan Plateau and were analysed for total Hg (HgT). Concentration levels of HgT in snow samples were higher than those from the central and southern part of the Tibetan Plateau. Most of the Hg in the snow/ice was found to be associated with particulate matter. A small daily variation of HgT in surface snow further suggested that snow Hg on the Tibetan Plateau was less influenced by the effect of post-depositional processes such as photoreduction. The variations in HgT peak concentration on the vertical profiles of the snowpits indicated that settling of particulate matter and percolation of snowmelt had caused an increase in Hg concentrations with increasing depth. No positive relationship was found between HgT and elevation in our study region, indicating that the “altitude effect” on Hg in surface snow may only exist on extremely high-elevation glaciers. Moreover, HgT concentrations in the surface ice were found to be the highest observed in this glacier. With ongoing climate warming, this is of particular concern in western China as the accumulated Hg released by accelerated glacier thinning and retreat may endanger ecosystems and human health in the glacier-fed downstream regions.