Abstract
Arsenic (As) is commonly sequestered at the sediment–water interface (SWI) in mining-impacted lakes through adsorption and/or co-precipitation with authigenic iron (Fe)-(oxy)hydroxides or sulfides. The results of this study demonstrate that the accumulation of organic matter (OM) in near-surface sediments also influences the mobility and fate of As in sub-Arctic lakes. Sediment gravity cores, sediment grab samples, and porewaters were collected from three lakes downstream of the former Tundra gold mine, Northwest Territories, Canada. Analysis of sediment using combined micro-X-ray fluorescence/diffraction, K-edge X-ray Absorption Near-Edge Structure (XANES), and organic petrography shows that As is associated with both aquatic (benthic and planktonic alginate) and terrestrially derived OM (e.g., cutinite, funginite). Most As is hosted by fine-grained Fe-(oxy)hydroxides or sulfide minerals (e.g., goethite, orpiment, lepidocrocite, and mackinawite); however, grain-scale synchrotron-based analysis shows that As is also associated with amorphous OM. Mixed As oxidation states in porewater (median = 62% As (V), 18% As (III); n = 20) and sediment (median = 80% As (-I) and (III), 20% As (V); n = 9) indicate the presence of variable redox conditions in the near-surface sediment and suggest that post-depositional remobilization of As has occurred. Detailed characterization of As-bearing OM at and below the SWI suggests that OM plays an important role in stabilizing redox-sensitive authigenic minerals and associated As. Based on these findings, it is expected that increased concentrations of labile OM will drive post-depositional surface enrichment of As in mining-impacted lakes and may increase or decrease As flux from sediments to overlying surface waters.
Highlights
Twenty-first century climate warming has disproportionately affected high northern latitudes, causing greater and more rapid increases in temperature and duration of icefree seasons relative to lower latitudes (ACIA 2005)
This study aims to address the knowledge gaps presented in Galloway et al (2018) and Miller et al (2019, 2020) and focuses on determination of the origin of sedimentary organic matter (OM) in sub-Arctic lakes, and the variable influence of aquatic- and terrigenous-derived OM on the mobility of As in three mining-impacted lakes located in the central Northwest Territories (NT), Canada
This N 2-dried material was used for scanning electron microscope (SEM)-based automated mineralogy, electron microprobe analysis (EPMA), synchrotronbased bulk X-ray Absorption Near-Edge Structure (XANES), micro-X-ray fluorescence (μ-XRF), and micro-X-ray diffraction (μ-XRD) to determine the mineralogy of As-bearing hosts and As speciation in the sediments of lakes sampled near Tundra Mine
Summary
Twenty-first century climate warming has disproportionately affected high northern latitudes, causing greater and more rapid increases in temperature and duration of icefree seasons relative to lower latitudes (ACIA 2005). Changing redox conditions in the water column and near-surface sediments driven by seasonal variations and longer term changes associated with increased OM flux can cause the release of As from sediments to overlying surface waters (Martin and Pedersen 2002; Bauer and Blodau 2006; Couture and Van Cappellen 2011; Anawar et al 2013; Barrett et al 2019; Palmer et al 2019; Schuh et al 2019). As climate warming continues to impact sensitive sub-Arctic and Arctic ecosystems, an improved understanding of the effects of increasing OM on the long-term stability of As is needed to predict the impacts of future climate variations on As mobility in northern lake environments
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