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Mercury stocks in discontinuous permafrost and their mobilization by river migration in the Yukon River Basin [1]

['M Isabel Smith', 'Department Of Earth Sciences', 'University Of Southern California', 'Los Angeles', 'Ca', 'United States Of America', 'Http', 'Yutian Ke', 'Division Of Geological', 'Planetary Sciences']

Date: 2024-08 

The YRB spans more than 330 000 km2 in regions of northwestern Canada and central Alaska and is underlain by areas of continuous and discontinuous permafrost [30]. The Yukon River has the highest flow-weighted annual THg concentration out of the six major Arctic rivers [29], and the YRB is one of the six major freshwater contributors to the Arctic Ocean, supplying 3–32 times more THg to the oceans than the 8 other major northern hemisphere river basins [19]. This makes the YRB an important focus of study in the context of riverine THg inputs in a changing Arctic.

Yukon river waters contain a range of sediment sizes, which are expected to influence organic carbon (OC) and Hg contents. To capture some of this variability, we focused on two regions in the YRB underlain by discontinuous permafrost with distinct riverbed sediment characteristics (figure 1). Our sites were chosen near Alaska Native communities that are at different risk levels for erosion, flooding, and permafrost thaw [31, 32] to coincide with overarching collaborative efforts to understand the effects of erosion in the YRB. At both sites, the river channel migrates laterally through cutbank erosion and point bar deposition at rates of meters per year [33].

Figure 1. Study sites located in interior Alaska in the YRB (watershed boundary-yellow shaded region [34], tributaries-blue line) (A). Samples were collected along an anastomosing, gravel-bedded reach of Yukon River (B) and a sand-bedded reach of the Koyukuk River (C), a single-threaded meandering river that is a major tributary of the Yukon River. Sampling locations were located near the villages of Huslia (purple square in A) and Beaver (orange square in A). Dots represent cutbanks (red; n = 56) and point bars (blue; n = 29) that have been characterized. Samples were collected in June 2022 (Huslia: 18 cutbanks, 6 point bars; Beaver: 13 cutbanks, 6 point bars) and September 2022 (Huslia: 15 cutbanks, 8 point bars; Beaver: 10 cutbanks, 6 point bars) (supplementary, dataset S1). To capture a holistic view of the floodplain, sites were selected to span a range in ages, terrain type, and permafrost presence determined from geomorphic maps [33, 35, 36]. Seasonal variation in water level affected sampling site accessibility, so sites from June and September are complimentary, but not identical.] (A) Microsoft® Bing™ Maps Platform screen shot(s) reprinted with permission from Microsoft Corporation [37]. Yukon River Watershed Boundary shapefile reproduced [34] with permission from Yukon River Inter-Tribal Watershed Council. (B) and (C) Map data: Google, Maxar Technologies, CNES/Airbus, Landsat/Copernicus [38]. Download figure: Standard image High-resolution image

2.2.1. Field sampling procedures

Sediment samples were collected from exposed riverbanks and pits dug (∼0.5–1 m deep) into point bar deposits (figures 1(B), (C) and supplementary, text S1). Stratigraphic columns were measured from the top of the bank to the waterline or from the top of the pit to the bottom of the pit, which was usually frozen ground. Descriptions of stratigraphic columns, distinct bed thickness, sample depth, and substrate class (gravel, sand, peat, mud) were recorded. The surficial 5–10 cm of exposed sediments were removed before sample collection. Paired samples were collected for analysis of geochemistry and bulk density (details in supplement, text S1). At selected permafrost cutbanks, we sampled both thawed material on the surface (effectively the 'active layer' of material exposed on the vertical bank) and frozen material recovered by drilling into the bank with a hole saw.

2.2.2. Lab analysis

Sediment samples for geochemical analysis were freeze dried and split. Geochemical subsamples were ground into a homogeneous fine powder in an agate mortar and pestle (supplementary, text S2). THg contents were determined using a NIC direct mercury analyzer (MA-3000) at the University of Southern California using the United States Geological Survey Mercury Research Laboratory protocol [39, 40]. Analysis of reference material MESS-4 (90 ± 40 ng g−1, National Research Council Canada) showed a median value of 64.9 ± 2.6 ng Hg g−1 (supplementary, figure S1, with uncertainty reported as relative standard deviation, or RSD) and blanks were below detection. All sediment samples were run in multiples (100% duplicate, 18% triplicate; median RSD of 2.03%) (supplementary, figures S1 and S2). Total organic carbon (TOC) content was analyzed using an Elementar elemental analyzer at Woods Hole Oceanographic Institute's National Ocean Sciences Accelerator Mass Spectrometry Facility (NOSAMS, [41]). 38% of samples were analyzed in duplicate, yielding a median RSD of 5%. The analytical precision was assumed to be less than 1%. TOC (wt%) and THg content were used to calculate RHgC, reported as µg Hg g C−1 [40, 41]. Bulk density samples were weighed pre- and post-oven drying (80 °C) to determine water mass fraction and dry density. Samples were categorized visually using a grain-size card into substrate classes of sand, mud, peat, and gravel. For each field site, THg content and RHgC values were sorted by substrate composition and a one-way ANOVA test (ɑ < 0.05) was conducted to determine if substrate compositions were statistically different from each other.

2.2.3. Stock calculations

We calculated THg stocks for the most complete stratigraphic sections sampled (Huslia: 15 cutbanks and 11 bars; Beaver: 13 cutbanks and 16 bars, supplementary, table S1). Near-surface stocks were determined by integrating over 1 meter and 3 m depth to compare with previously published datasets [15–17]. Total stocks that can potentially be reworked by river lateral migration were determined for the full column depth (∼10–15 m), defined as the distance from the top of the cutbank (CB) or point bar (PB) to the bottom of the thalweg (the deepest part of the river). However, incomplete bank exposure and inability to dig below the thawed active layer meant we could not sample below the top ∼20%–50% of this sedimentary column. Thus, we estimated full column stocks for PB and CB by the sum of the sampled and inferred stocks for each stratigraphic layer in the column (equation (1), supplementary, table S1) For exposed sections of bank and bars, sampled stocks were directly calculated using measured layer thicknesses ( , km) from each identifiably stratigraphic layer, dry density of bank material ( , kg dry sediment km−3) and THg mass fraction ( , kg Hg kg dry sediment−1) from collected paired samples. Any missing stratigraphic information was supplemented with an average value from sediments of similar substrate composition from the same field location (supplementary, tables S2 and S3). To calculate THg stocks for the unsampled sections (the 'inferred' portion in figure 2), we determined unsampled column heights and inferred associated sediment properties. Total column heights, independent of river stage height, were determined based on bathymetric and elevation data (supplementary, table S1). Bathymetry was mapped via SONAR surveys at the time of sample collection, referenced by RTK-GPS (real-time kinematic geographic positioning system). Topography data were from National Center for Airborne Laser Mapping Light Detection and Ranging datasets from flights over Huslia on 21–23 August 2022, and over Beaver on 2–5 August 2021 (figure 2). The sampled sections (h 2 , h 3 ) were subtracted from total column height (H CB , H PB ) to determine the unmeasured section (h 1 , h 4 ) heights. Figure 2. Schematic showing different components of the THg stock (S) calculation and where the data for each variable was obtained. Sampled sections were directly measured in the field, while inferred sections were determined using average values based on substrate composition (supplementary, tables S1 and S4). S 2 represents the sampled cutbank stock, with h 2 the corresponding exposed height. S 3 represents the sampled point bar stock, accessed by digging a pit, with h 3 representing associated depth. Download figure: Standard image High-resolution image To infer sediment properties, we used our most complete stratigraphic sections (∼5–10 m thick), measured in late fall when the Koyukuk (Huslia) and Yukon (Beaver) Rivers were at low stage. We determined that 3 m was a characteristic maximum thickness for fine-grained overbank sediments at both sites (supplementary, figure S3). We then bootstrap resampled all measured beds below 3 m depth from the modern floodplain surface to estimate sediment properties of all unmeasured beds. We found that lower beds (>3 m) were predominantly sand in Huslia and a mixture of gravel and sand in Beaver (supplementary, figure S4). Our findings of grain size fining upward is typical of river lateral accretion deposits [42]. We calculated inferred section stocks using an average dry density and THg content (supplementary, table S4) based on substrate composition for each location.

2.2.4. Flux calculations

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[1] Url: https://iopscience.iop.org/article/10.1088/1748-9326/ad536e

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