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

Highlights

• Disparate mercury yields from two glacierized watersheds point to bedrock differences.

• Total Hg yield from the Mendenhall glacier is the highest in the literature.

• Snow and glacial outflow data suggest subglacial Hg methylation.

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.

Introduction

Glaciers in Alaska currently account for the largest mass budget loss of glacier ice and corresponding contribution to sea level rise of any region in the world (Gardner et al., 2011, Zemp et al., 2019). Regular summertime glacial melt pulses release solutes and particulates that were stored in glacier ice, providing a delayed return of materials that were integrated into the glacier ice as precipitation decades or centuries prior (Beal et al., 2015, Sharma et al., 2015, Ferrario et al., 2017). As glaciers retreat, they liberate legacy atmospheric contaminants such as mercury (Hg) faster than they accumulated. Although remote, glaciers in southeast Alaska are likely repositories of decades to centuries of atmospherically deposited Hg originating from regional and distant global pollution sources as well as natural sources such as wildfire, volcanic eruptions, and dust (Wiedinmyer and Friedli, 2007, Pearson et al., 2019). In particular, Alaska is downwind of major air currents that carry Hg from southeast Asia, where coal-fired power plants have been driving up Hg emissions in the past few decades (Streets et al., 2019). Regional lake sediment cores show a tripling of Hg deposition in the 1–2 centuries compared to pre-industrial centuries (Lepak et al., 2020)

Fast moving alpine glaciers are highly effective agents of landscape erosion, mobilizing naturally occurring elements from bedrock sources and transporting them to downstream watersheds and coastal ecosystems (Hallet et al., 1996, Herman et al., 2015). As glaciers move down valley, they excavate their underlying bedrock via quarrying and abrasion, incorporating bedrock-derived particulates into glacier ice and solutes into meltwater. Subglacial meltwater flow may flush out entrained and subglacial material, particularly during episodes of intense summer rainfall or enhanced melt (Gimbert et al., 2016). Characterizing and quantifying the flux of water, solutes, and particulates from glaciers is important in the context of understanding erosion rates and feedbacks in a warming climate as well as the delivery of nutrients and contaminants to downstream ecosystems.

The concentration, partitioning, and speciation of the Hg in stream water have implications for water quality standards and aquatic health. The limited data on glacier meltwater streams, spanning areas from Alaska (Schuster et al., 2011, Nagorski et al., 2014, Vermilyea et al., 2017), Canada (St. Pierre et al., 2019), Greenland (Søndergaard et al., 2015), and China (Sun et al., 2016, Sun et al., 2017, Paudyal et al., 2017), show that such waters carry the Hg predominantly as inorganic Hg and in the particulate fraction, unlike wetland and forested streams, where several percent of the Hg is in the more toxic monomethyl-form and the Hg is predominantly dissolved and/or colloidal (Brigham et al., 2009, Dittman et al., 2010, Nagorski et al., 2014).

While much work has been done on flux rates of major elements from glacierized watersheds, only a few studies exist on the export of geogenic and anthropogenic-sourced Hg directly from glacial meltwater. Several studies have estimated fluxes from Tibetan glaciers (Sun et al., 2016, Sun et al., 2017, Paudyal et al., 2017); another quantified Hg contributions from small ice caps and glaciers into a lake in the Yukon, Canada (Zdanowicz et al., 2018); and another measured Hg fluxes from a glacier in northeastern Greenland into the receiving Zackenberg River (Søndergaard et al., 2015). A recent study in Arctic Canada estimated both total Hg and methylmercury (MeHg) flux out of seven glacial streams feeding into Lake Hazen (St. Pierre et al., 2019). Schuster et al. (2011) quantified Hg yields (mass per watershed area per year) from the partially glacierized Yukon River watershed, and found them to be 3 to 32-fold higher than other measured major Arctic rivers, and they attributed the increase to contributions from thawing permafrost. A seasonal study on the glacial Lemon Creek in Juneau, Alaska, showed that it had the highest total Hg yields of any unmined watershed reported in the literature, with 20 g km⁻²y^-1 (Vermilyea et al., 2017). These studies show that glacier-influenced streams have some of the highest yields of total Hg relative to many other stream types.

Here we evaluate a melt season of Hg export from two glacierized watersheds and an adjacent non-glacial (forest and wetland-dominated) watershed. We examine the concentration, hydrologic timing, speciation, and partitioning of aqueous and particulate Hg in the watersheds and calculate their fluxes and yields by watershed and relate them to underlying bedrock geology, landscape factors, and atmospheric deposition.