Mercury (Hg) in some Arctic marine mammals has increased to levels that may be toxic
to Northern peoples consuming them as traditional food. It has been suggested that
sunlight-induced atmospheric reactions called springtime atmospheric Hg depletion
events (AMDEs) result in the loading of -150-300 tons of Hg to the Canadian Arctic
archipelago each spring and that AMDEs are the ultimate source of Hg to Arctic foodwebs.
AMDEs result from the oxidation of gaseous elemental Hg0 (GEM) in Arctic atmospheres
to reactive gaseous Hg (RGM) and particulate Hg (pHg), both of which fall out of the
atmosphere to snowpacks. We studied the springtime cycling of Hg between air and snowpacks
near Churchill, Manitoba, for 2 years to determine the net input of Hg to Hudson Bay
from AMDEs. In 2004, we monitored atmospheric concentrations of GEM, pHg, and RGM
while simultaneously measuring concentrations of total Hg (THg) in surface snow collected
over the sea ice on Hudson Bay. During numerous springtime AMDEs, concentrations of
THg in surface snow increased, often to over 60 ng/L, demonstrating that AMDEs resulted
in deposition of oxidized Hg (Hg(II)) to snowpacks. However, immediatelyfollowing
AMDEs, average concentrations of THg in snow declined drastically from between 67.8+/-7.7
ng/L during AMDEs to only 4.25+/-1.85 ng/L four or more days following them. In 2003,
we measured THg in surface snow collected daily over the sea ice and total gaseous
Hg (TGM) concentrations in the interstitial airspaces of snowpacks. When concentrations
of THg in the surface snow decreased, concentrations of TGM in interstitial airspaces
of the snowpack increased sharply from between approximately 1.4-3.4 ng/m(3) to between
approximately 20-150 ng/m(3), suggesting thatthere was a reduction of deposited Hg(II)
to GEM, which then diffused out of snowpacks. At snowmelt in both 2003 and 2004, average
concentrations of THg in meltwater collected over Hudson Bay were only 4.04+/-2.01
ng/L. Using concentrations of THg in meltwater and snow water equivalent, we estimated
a net springtime loading of only 2.1+/-1.7 mg/ha of Hg to Hudson Bay from AMDEs, indicating
that only a small portion of the Hg(II) deposited during AMDEs enters Hudosn Bay each
spring.