Article Cite This: Environ. Sci. Technol. 2018, 52, 14099−14109
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Unprecedented Increases in Total and Methyl Mercury Concentrations Downstream of Retrogressive Thaw Slumps in the Western Canadian Arctic Kyra A. St. Pierre,*,§,† Scott Zolkos,§,† Sarah Shakil,§,† Suzanne E. Tank,† Vincent L. St. Louis,† and Steven V. Kokelj‡ †
Department of Biological Sciences, University of Alberta, Edmonton, Alberta T6G 2E3, Canada Northwest Territories Geological Survey, Yellowknife, Northwest Territories X1A 2L9, Canada
Environ. Sci. Technol. 2018.52:14099-14109. Downloaded from pubs.acs.org by UNIVERSITE DE SHERBROOKE on 01/11/19. For personal use only.
‡
S Supporting Information *
ABSTRACT: Retrogressive thaw slumps (RTSs) are thermokarst features created by the rapid thaw of ice-rich permafrost, and can mobilize vast quantities of sediments and solutes downstream. However, the effect of slumping on downstream concentrations and yields of total mercury (THg) and methylmercury (MeHg) is unknown. Fluvial concentrations of THg and MeHg downstream of RTSs on the Peel Plateau (Northwest Territories, Canada) were up to 2 orders of magnitude higher than upstream, reaching concentrations of 1,270 ng L−1 and 7 ng L−1, respectively, the highest ever measured in uncontaminated sites in Canada. MeHg concentrations were particularly elevated at sites downstream of RTSs where debris tongues dammed streams to form reservoirs where microbial Hg methylation was likely enhanced. However, > 95% of the Hg downstream was typically particle-bound and potentially not readily bioavailable. Mean openwater season yields of THg (610 mg km−2 d−1) and MeHg (2.61 mg km−2 d−1) downstream of RTSs were up to an order of magnitude higher than those for the nearby large Yukon, Mackenzie and Peel rivers. We estimate that ∼5% of the Hg stored for centuries or millennia in northern permafrost soils (88 Gg) is susceptible to release into modern-day Hg biogeochemical cycling from further climate changes and thermokarst formation.
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INTRODUCTION Across permafrost landscapes, thaw-driven ground subsidence (thermokarst) is one of the most rapid mechanisms by which previously frozen material can be reactivated into contemporary biogeochemical cycles.1 Among the most striking examples of thermokarst features are retrogressive thaw slumps (RTSs), which can mobilize millions of cubic meters of terrestrial material to aquatic environments.2−4 Climate warming and intensifying precipitation are driving widespread RTS development in the ice-rich, glaciated terrain of the western Canadian Arctic.5 On the Peel Plateau in the Northwest Territories, Canada, RTSs significantly increase downstream sediment and solute concentrations;3,6 however, the effect of RTSs on the mobilization of mercury (Hg) into fluvial networks is yet unknown. Soils in northern permafrost regions store more Hg (1656 ± 962 Gg) than global vegetation, the ocean, atmosphere, and all other soils combined.7 A substantial proportion of this Hg (136−863 Gg)8 is found within the active layer and thus susceptible to exposure during seasonal thaw. Because RTSs rapidly erode both active layer and the underlying permafrost soils, RTSs can be expected to mobilize vast quantities of Hg into northern aquatic ecosystems.9 Across the Arctic region, © 2018 American Chemical Society
Hg is of particular concern because its neurotoxic form, methylmercury (MeHg), readily biomagnifies through food webs10 into upper trophic level organisms that are consumed as part of traditional diets.11 Indeed, rapid Arctic warming and intensifying precipitation12 are already increasing permafrost thaw and facilitating the release of archived Hg into modern biogeochemical cycles,13,14 with potentially critical implications for downstream aquatic ecosystems. Thermokarst wetlands and thaw ponds have been identified as Hg methylation hotspots due to the concurrent inputs of Hg and development of reducing conditions required for microbial methylation following permafrost thaw.15,16 Although streams and rivers draining thermokarst terrain are the most efficient ways to transport Hg to downstream ecosystems over great distances, research on both total Hg (THg, all forms of Hg in a sample) and MeHg release from RTSs is sparse and has focused on lacustrine environments,17,18 thus leaving an important gap in our Received: Revised: Accepted: Published: 14099
September 21, 2018 November 25, 2018 November 26, 2018 November 26, 2018 DOI: 10.1021/acs.est.8b05348 Environ. Sci. Technol. 2018, 52, 14099−14109
Article
Environmental Science & Technology
Figure 1. Map of retrogressive thaw slump (RTS) study sites in the Stony Creek and Vittrekwa River watersheds on the Peel Plateau (NWT, Canada). Note the locations of RTS features FM3 (inset photos (a) and (b)), SE and HD on the map. Photo of SE depicts the vertical headwall and scar zone beneath. Photo of HD shows a transient upstream reservoir, caused by rapid debris tongue growth following intense rainfall during July 2015. See Littlefair et al. (2017)25 for additional photos of RTSs on the Peel Plateau. Major Arctic river watersheds (http:arcg.is/9qf5K) and inset basemap from Esri ArcGIS Online; active RTS from Segal et al. (2016).19
(LIS), which reached its maximum extent ∼18 500 years ago on the Peel Plateau.20 Early Holocene warming from ∼12 000 to 8500 years ago caused an increase in active layer thickness and the redistribution of solutes and organic matter within the top few meters of soils.6 Subsequent cooling and upward permafrost aggradation preserved this geochemically distinct Holocene permafrost layer above deeper ice-rich Pleistocene tills. Today, cryostratigraphic and geochemical thaw unconformities in RTS headwalls demarcate the boundary between Holocene and Pleistocene permafrost (Figure 1b).6,21 RTS growth is driven by the ablation of an exposed permafrost headwall. RTS development is enhanced by rainfall, which promotes downslope flow of the saturated slurry, maintaining exposure of the headwall and perpetuating RTS growth (Figure 1).4 In fluvial environments, these evacuated permafrost and active layer materials accumulate as debris tongues that can be tens of meters thick and extend for hundreds of meters downstream.3,22 These materials can also rapidly enter stream networks as sediment- and solute-rich runoff through rill channels that form within the debris tongues.3,23
understanding of the biogeochemical cycling of this contaminant in northern ecosystems as the climate warms. In this study, we measured unfiltered (bulk) and filtered (dissolved) THg and MeHg upstream and downstream of eight RTSs on the Peel Plateau (Figure 1, Supporting Information (SI) Table S1) with the objectives to (1) quantify the effect of RTSs on downstream concentrations and fluxes of THg and MeHg; (2) characterize the partitioning of THg and MeHg between dissolved and particulate phases; (3) quantify the degree to which high concentrations of THg and MeHg persist within fluvial networks downstream of RTSs; and, (4) estimate the total mass of THg stored in permafrost regions susceptible to hillslope thermokarst.
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MATERIALS AND METHODS Study Area. The Stony Creek and Vittrekwa River watersheds of the Peel Plateau span ∼3000 km2 and contain more than 100 active RTSs, with several exceeding 20 ha in area (Figure 1).19 RTSs in this region expose ice-rich permafrost associated with the former Laurentide Ice Sheet 14100
DOI: 10.1021/acs.est.8b05348 Environ. Sci. Technol. 2018, 52, 14099−14109
Article
Environmental Science & Technology Sample Collection. Samples were collected at five RTSs accessed by foot and three RTSs accessed by helicopter from June to August in 2015 and 2016 (SI Table S1). Samples were collected from (1) the unaffected reach upstream of RTSs; (2) rills within the RTS debris tongue; and (3) the affected stream reach immediately downstream of RTSs (Figure 1a). In 2016, water samples were additionally collected along a transect extending 2.8 km downstream of the RTS SE (Figure 1), with targeted collection points upstream and downstream of major tributary inflows along this transect. Water samples for Hg analysis were collected into 250 mL certified precleaned glass amber bottles (Environmental Sampling Supply, Sam Leandro, CA) using the clean handsdirty hands sampling protocol.24 Four bottles were collected at each site: one each for the analyses of unfiltered and filtered THg and unfiltered and filtered MeHg. All samples were subsequently processed and preserved within 24 h to minimize biogeochemical transformation. Two bottles were filtered through HCl-washed disposable Nalgene 0.45 μm cellulose nitrate filter towers, and the filtrate was poured into new glass amber bottles. All samples were then acidified to 0.2% with trace-metal grade HCl and kept cool until analysis. RTS rill runoff was collected in clean Whirlpak or Ziploc bags, and frozen until analysis. These heavily sediment-laden samples were subsequently freeze-dried and treated as solid samples. Chemical Analyses. All samples were analyzed using standard protocols in the Canadian Association for Laboratory Accreditation-certified Biogeochemical Analytical Service Laboratory (BASL; University of Alberta. All quality control and assurance (QA/QC) measures are described in the SI. THg Analysis. THg concentrations in unfiltered (U-THg; bulk) and filtered (F-THg; dissolved) water samples were quantified by cold vapor atomic fluorescence spectrometry on a Tekran 2600 Mercury Analyzer following overnight oxidation with bromine chloride (BrCl) at 4 °C (EPA Method 1631). Excess BrCl in solution was confirmed prior to analysis by pipetting ∼15 μL of solution onto potassium iodide starch paper. Procedural recoveries were assessed by adding a known quantity of HgCl2 spike (Spex-CertiPrep, Metuchen, NJ), approximately equivalent to the sample concentration, to all unfiltered samples and 10% of dissolved samples. Unfiltered sample spike recoveries were typically low (
