Mercury export from glacierized Alaskan watersheds as influenced by bedrock geology, watershed processes, and atmospheric deposition

Abstract

Mercury (Hg) export from glacierized watersheds is poorly understood, with very few studies worldwide on Hg concentration and speciation in glacier snow, ice, and meltwater, and on Hg fluxes to downstream freshwater and coastal ecosystems. In addition to bedrock-derived geogenic Hg, glaciers may be releasing legacy accumulations of natural and anthropogenically-sourced Hg trapped in glacier ice melt each summer season. Our prior work showed that a glacierized stream in southeast Alaska had the highest reported flux of inorganic mercury of all known non-mining impacted streams, highlighting the strong ability of glaciers to mobilize the trace metal. Here we present data on Hg concentrations, speciation, partitioning, and fluxes from two more glacierized watersheds (Herbert and Mendenhall Rivers), and we compare them to an adjacent non-glacierized, forested-wetland stream (Peterson Creek). Results show that the glacierized streams carried Hg largely in the particulate form, whereas the forested-wetland stream carried it largely in the filtered fraction, at 20 fold higher concentration than in the glacierized streams, and with a higher percent of Hg in its methylated form. Yet, considering the higher water and sediment yields (as mass per watershed area per year) of the glacierized streams during the summer melt season, the yield of total Hg (unfiltered) from the Mendenhall glacier was approximately 80 times higher than from the Herbert glacier and 50 times higher than in Peterson Creek and presents the highest watershed yields of total Hg and methyl-Hg reported in the literature to date. Incongruous yields out of the two glacierized streams can likely be explained by differences in underlying bedrock geology. Based on the sediments entrained in Mendenhall meltwater, the late Paleozoic to Paleocene metasedimentary and volcanic rocks being eroded in the terminal 3 km of the Mendenhall are elevated above mean crustal concentrations by at least 4–17 fold. Differences in speciation between the glacierized and non-glacierized streams are likely accounted for by glacial and watershed geochemical conditions that variably promote Hg methylation.