Mercury deposition in the snow pack of the arctic Alaskan coastal system

Extensive measurements of mercury (Hg), bromine, and ions in the snow pack on both the sea ice and tundra near Barrow, Alaska, show complex spatial and temporal patterns. Based on a comprehensive series of measurements made during the three winters between 2004 and 2006, elevated levels of Hg (50 to 500 ppt) were most often found when sampling diamond dust, surface hoar, frost flowers, or rime, all crystals grown from the vapor phase of water. Due to both the polar light cycle and near-surface re-emission processes, snow pack loading of Hg typically diminished rapidly with depth. Basal snow layers typically had levels less than 5 ppt, with most of the Hg concentrated in the top 10 cm. Similar snow pack loading was observed for snow on land and sea ice though with increasing distance inland (>100 km) the levels fell off. Observed levels were found to loosely correlate with mercury depletion events (MDEs). Hg loading remained elevated into the snow melt period. At the peak of snow melt run-off, Hg concentrations remained in excess of 10 ppt. Soil cores indicate increasing Hg loading with time, indicating that the run-off loading is due to winter deposition of Hg in the snow. We hypothesize that the bulk of the Hg loading in the coastal region occurs during winter. The deposition rate may be highly sensitive to sea ice conditions which are known to be changing as the climate warms.