Adjacent glacierized watersheds with contrasting bedrock geology exhibit strong differences in annual export of mercury

Abstract:

As glaciers melt each summer, they release both bedrock-derived geogenic mercury (Hg) as well as legacy accumulations of Hg trapped in glacier ice. We present Hg concentrations, speciation, partitioning, and fluxes from 2 glacierized watersheds (Herbert and Mendenhall) and an adjacent wetland/forested (non-glacial) stream (Peterson Creek), located in southeast Alaska. The glacier streams carried Hg largely in the particulate form, whereas Peterson carried it dominantly in the filtered fraction. Peterson streamwater had the highest filtered total and methyl-Hg concentrations. However, the turbid Mendenhall glacier outflow contained the highest particulate total and methyl-Hg concentrations, and the yield (as mass per watershed area per year) of total unfiltered Hg from the Mendenhall glacier was approximately 80 times higher than from the Herbert glacier and 50 times higher than in Peterson Creek. Area-weighted flux of unfiltered methyl-Hg was 4-fold higher in glacial Mendenhall than in Peterson. These area- normalized fluxes of mercury out of Mendenhall glacier are the highest watershed yields of total Hg (780 g km^-2yr^-1) and methyl-Hg (0.82 g km^-2yr^-1) reported to date. Differences in the yields out of the two glaciers can be explained by contrasts in underlying bedrock geology near their termini. These findings underscore that glaciers may be oversized conduits for mercury export to downstream freshwater and marine areas, and that subglacial bedrock geology is likely a key factor in determining the magnitude of the flux of Hg.