Understanding the high mercury concentrations observed in Arctic ecosystems requires in-depth knowledge of mercury cycling. Here, we show that variations in mercury concentration in the sediments of the North American Arctic Margin and the Arctic Ocean basins can be explained by carbon sources and cycling. Sedimentary mercury concentrations are predicted (p < 0.001) considering three carbon sources and, in some areas, the recapture of soluble mercury released during burial. Terrigenous organic carbon dominates mercury delivery (50-90

The Mackenzie River is North America's largest contributor of freshwater and sediment to the Arctic Ocean. Here, we evaluate the potential of rare earth elements (REE) as tracers of its sediment sources and fate, from the river mouth to the deep Arctic Ocean. We collected sediment cores from 21 sites, from the delta to the marine shelves, slopes and basins and measured the spatial and down-core distribution of total, leached and residual REE concentrations. Our results show that the proportion of leached REE is highest in the delta. This proportion decreases with distance from the river, suggesting mixing with other sediment sources, REE loss to the residual phase, or REE scavenging via adsorption and complexation in coastal waters. Normalized REE concentrations plotted against their atomic number provide regional signatures. The leached REE signatures indicate medium REE enrichment in the Mackenzie Delta, an enrichment that diminishes with distance from the delta. We then used a similarity index (SI) to investigate the divergence amongst REE signatures, with riverine and deep marine basin values as endmembers for the calculation. Our results highlight the influence of the Mackenzie Region sediments on the Beaufort Sea margin. Overall, our findings demonstrate that REE are relevant tracers for identifying sediment sources and that tracking REE distribution from the delta to the deep Arctic Ocean offers additional insights into sediment transport mechanisms.

The global carbon and iron cycles are intimately linked as redox-sensitive iron oxides readily bind organic carbon in a variety of environmental settings, including marine and lacustrine sediments. While these iron-organic carbon complexes sequester vast quantities of organic carbon, the composition of the organic matter within them remains unknown for lacustrine environments. Here we present C K1s and Fe L3,2 edge Near Edge X-ray Absorption Fine Structure (NEXAFS) spectra of surface sediments and authigenic iron complexes from adjacent basins of a pristine boreal lake located in Quebec, Canada, with contrasting oxygen exposure regimes. We demonstrate differences in organic carbon speciation in sediments from both basins, as well as co-localization of organic carbon and iron on a sub-micron scale in 100 nm thick samples. Differences in redox cycling across these two basins allow for a direct comparison of the effect of oscillating redox conditions on the composition of organic carbon sequestered by iron. Our results suggest that reactive organic molecules, which may be polysaccharides, were found preferentially associated with iron in the perennially oxic sediments compared to more phenol rich organics in the seasonally anoxic sediments, highlighting the importance of iron oxides in the protection and preservation of labile organic compounds. Traces of aliphatic carbon were observed in sediments from the anoxic basin, alongside carboxyl and aromatic functionalities. This carboxyl-rich aliphatic material could possibly interact with the sediment mineral matrix either through a ligand exchange mechanism between the mineral phases and the carboxyl functionalities, or via non-specific hydrophobic interactions involving the aliphatic moieties. Finally, our work also shows that OC:Fe ratios should be used with caution when inferring a binding mechanism between OC and iron oxides.

The biogeochemical cycles of iron and organic carbon (OC) are closely interconnected in terrestrial and aquatic systems. In ocean waters, the concentration of reactive Fe is tightly controlled by soluble organic ligands. In soils, Fe stabilizes OC by forming aggregates that shield OC from degradation. In lake sediments however, the role of Fe in the preservation of OC has not been explored as extensively yet. We investigated Fe-OC interactions in sediment collected from Lake Tantare, in which two basins are characterized by contrasting redox conditions. These contrasting redox conditions provide an opportunity to assess their importance in the formation of stable Fe-OC complexes. On average, 30.1 +/- 6.4 % of total OC was liberated upon reductively dissolving reactive iron. The Fe-associated and the non-Fe-associated OC pools were characterized at the elemental (OC, TN), isotopic (delta 13C, delta 15N) and functional group (FTIR) levels. Large differences in OC:Fe and TN:Fe ratios between the two basins were found which were not linked to OM chemical composition but rather to differences in reactive iron concentrations stemming from the higher abundance of iron sulfides in the anoxic basin. Nevertheless, since the affinity of OM for iron sulfides is lower than that for iron hydroxides, using OC:Fe and TN:Fe ratios as a diag- nostic tool for the type of OM -Fe interactions should be done with care in anoxic environment. Same caution should be considered for oxic sediments due to the variation of the proportion of iron hydroxides associated with OM from sample to sample.

Abstract. The complexity of organic matter (OM) degradation mechanisms represents a significant challenge for developing biogeochemical models to quantify the role of aquatic sediments in the climate system. The common representation of OM by carbohydrates formulated as CH2O in models comes with the assumption that its degradation by fermentation produces equimolar amounts of methane (CH4) and dissolved inorganic carbon (DIC). To test the validity of this assumption, we modeled using reaction-transport equations vertical profiles of the concentration and isotopic composition (δ13C) of CH4 and DIC in the top 25 cm of the sediment column from two lake basins, one whose hypolimnion is perennially oxygenated and one with seasonal anoxia. Our results reveal that methanogenesis only occurs via hydrogenotrophy in both basins. Furthermore, we calculate, from CH4 and DIC production rates associated with methanogenesis, that the fermenting OM has an average carbon oxidation state (COS) below −0.9. Modeling solute porewater profiles reported in the literature for four other seasonally anoxic lake basins also yields negative COS values. Collectively, the mean (±SD) COS value of −1.4 ± 0.3 for all the seasonally anoxic sites is much lower than the value of zero expected from carbohydrates fermentation. We conclude that carbohydrates do not adequately represent the fermenting OM and that the COS should be included in the formulation of OM fermentation in models applied to lake sediments. This study highlights the need to better characterize the labile OM undergoing mineralization to interpret present-day greenhouse gases cycling and predict its alteration under environmental changes.

Vertical profiles of the concentration and isotopic composition (delta C-13) of methane (CH4) and dissolved inorganic carbon (DIC), as well as of ancillary parameters, were obtained in the top 25 cm of a sediment column in a seasonally anoxic basin from an oligotrophic boreal lake. Modeling the profiles of CH4 and DIC concentrations and those of their delta C-13 signatures with reaction-transport equations allowed us to determine the organic matter (OM) degradation rates according to various reactions and to constrain the in situ isotopic fractionation factors and diffusivity coefficients of CH4 and DIC. This exercise reveals inter alia that (i) CH4 production occurs below a depth of 5 cm, with the highest production rate between 5 and 7.5 cm depth, (ii) all CH4 is produced through hydrogenotrophy, and (iii) methanogenesis yields a production rate of CH4 about three times greater than that of DIC. This latter observation indicates either that fermentation of OM is not the exclusive source of H-2 sustaining hydrogenotrophy, or that the commonly assumed model molecule CH2O does not adequately represent the fermenting OM, since its fermentation yields identical rates of CH4 and DIC production. The porewater profiles of Fe and SO42- suggest that some H-2 may be produced during the reoxidation of reduced sulfur by Fe(III), but the rate of H-2 production via this process, if active, would be insignificant in comparison to that required to sustain the estimated rate of hydrogenotrophy. We deduce that the imbalance between CH4 and DIC production rates is rather due to the fermentation of organic substrates that are more reduced than CH2O, i.e., having a negative average carbon oxidation state (COS). From the constraints on reaction rates and on fermentation pathways imposed by the delta C-13 data, we infer that the organic substrate fermenting between depths of 5 and 7.5 cm should have a COS of -1.87. We thus submit that CH4 is produced in the sediments of the seasonally anoxic basin of our boreal lake through hydrogenotrophy coupled to the fermentation of reduced organic substrates that can be represented by a mixture of fatty acids (e.g. C16H32O2; COS of -1.75) and fatty alcohols (e.g., C(16)H(3)4O; COS of -2.00). This study emphasizes the importance of characterizing the sedimentary OM undergoing mineralization in order to improve diagenetic model predictions of CH4 cycling in boreal lakes and of its significance in climate change. (C) 2018 Elsevier Ltd. All rights reserved.

Previous studies have demonstrated that the analysis of biogeochemical tracers along baleen can provide seasonal, annual, and longer term insights into whale movements, habitat use, diet, and ecosystem processes. We measured the mercury (Hg) concentration and stable carbon (δ 13 C) and nitrogen (δ 15 N) isotope compositions along baleen plates of bowhead whales ( Balaena mysticetus ) harvested between 1988 and 1996 from the Bering–Chukchi–Beaufort (BCB) and the Eastern Canada–West Greenland (EC–WG) populations. These measurements were compared among individuals and between populations to assess seasonal cycling and to determine if bowhead whales forage year-round rather than fasting in winter. Individuals from the BCB population had synchronous cycles with higher Hg concentrations and lower δ 13 C values in summer and lower Hg concentrations and higher δ 13 C values in winter. EC–WG individuals also had synchronized periodic variations with peak values in isotopic ratios during summer. These results reflect an annual cycle of seasonal migration between distinct food webs in both populations. Spring and fall feeding activity was indicated in the intra-annual Hg and δ 13 C cycles along the baleen plates of all whales in our study. These shorter periods, spanning about half the annual growth, have not been previously described in the baleen of these populations, and are consistent with separate foraging areas along the migration routes. The results of this study provide further support for year-round foraging in a species previously thought to fast during winter. Future monitoring of seasonal foraging patterns in baleen would help to determine whether bowhead whales alter their foraging patterns in response to shifts in zooplankton community availability, composition, and phenology.

Concentrations of redox-sensitive elements (S, Mn, Mo, U, Cd, Re) were analyzed in a set of 27 sediment cores collected along the North American Arctic margin (NAAM) from the North Bering Sea to Davis Strait via the Canadian Archipelago. Sedimentary distributions and accumulation rates of the elements were used to evaluate early diagenesis in sediments along this section and to estimate the importance of this margin as a sink for key elements in the polar and global oceans. Distributions of Mn, total S and reduced inorganic S demonstrated that diagenetic conditions and thus sedimentary carbon turnover in the NAAM is organized regionally: undetectable or very thin layers (<0.5 cm) of surface Mn enrichment occurred in the Bering–Chukchi shelves; thin layers (1–5 cm) of surface Mn enrichment occurred in Barrow Canyon and Lancaster Sound; and thick layers (5–20 cm) of surface Mn enrichment occurred in the Beaufort Shelf, Canadian Archipelago, and Davis Strait. Inventories of authigenic S below the Mn-rich layer decreased about fivefold from Bering–Chukchi shelf and Barrow Canyon to Lancaster Sound and more than ten-fold from Bering–Chukchi shelf to Beaufort Shelf, Canadian Archipelago and Davis Strait. The Mn, total S and reduced inorganic S distributions imply strong organic carbon (OC) flux and metabolism in the Bering–Chukchi shelves, lower aerobic OC metabolism in Barrow Canyon and Lancaster Sound, and deep O2 penetration and much lower OC metabolism in the Beaufort Shelf, Canadian Archipelago, and Davis Strait. Accumulation rates of authigenic S, Mo, Cd, Re, and U displayed marked spatial variability along the NAAM reflecting the range in sedimentary redox conditions. Strong relationships between the accumulation rates and vertical carbon flux, estimated from regional primary production values and water depth at the coring sites, indicate that the primary driver in the regional patterns is the supply of labile carbon to the seabed. Thus, high primary production combined with a shallow water column (average 64 m) leads to high rates of authigenic trace element accumulation in sediments from the Bering–Chukchi shelves. High to moderate primary production combined with deep water (average 610 m) leads to moderate rates of authigenic trace element accumulation in sediments from Lancaster Sound. Low to very low primary production combined with moderate water depths (average 380 m) leads to low rates of authigenic trace element accumulation in sediments in the Beaufort Shelf, Davis Strait and Canadian Archipelago. Authigenic Mo accumulation rates show a significant relationship with vascular plant input to the sediments, implying that terrestrial organic matter contributes significantly to metabolism in Arctic margin sediments. Our results suggest that the broad and shallow shelf of the Chukchi Sea, which has high productivity sustained by imported nutrients, contributes disproportionately to global biogeochemical cycles.

Sulfur controls the fate of many geochemical elements in lake sediments, including iron, phosphorus and environmentally important trace elements. We measured the speciation of pore-water and sediment-bound sulfur (aqueous sulfate and sulfides, elemental sulfur, iron monosulfide, pyrite, organic sulfur) and supporting geochemical variables (carbon, oxygen, iron) in the sediments of a perennially oxygenated and a seasonally anoxic basin of an oligotrophic lake in Québec, using a combination of pore-water analyses, sequential extractions and X-ray absorption near edge structure. A non-steady state early diagenetic model was developed and calibrated against this extensive dataset to help unravel the pathways and quantify the rates of S transformations. Results suggest that the main source of S to the sediments is the settling of organic ester-sulfate (R-O-SO3-H). Hydrolysis of these compounds provides an additional source of sulfate for anaerobic microbial oxidation of sedimentary organic matter, releasing sulfide to the pore-water. Reduced solid-bound S species accumulate as thiols (R-SH) and iron sulfides in the perennially oxygenated and seasonally anoxic basin, respectively. The model-estimated rate constant for R-SH formation is lower than previously estimated for this particular lacustrine site, but similar to that proposed for marine shelf sediments. The solid sediment S profiles, however, carry the imprint of the time-dependent sulfate input to the lake. Iron sulfide enrichments formed during past decades of elevated atmospheric SO4 deposition are presently dissolving. In the sediments of the perennially oxygenated basin this reaction hampers the build-up of Fe(III) (oxy)hydroxide near the sediment–water interface.

Sediment porewater was analyzed at several sampling dates in two adjacent basins of an oligotrophic boreal lake, one basin perennially oxygenated (Basin A) and the other occasionally anoxic (Basin B). Depth concentration profiles of methane (CH4), dissolved inorganic carbon (DIC), and electron acceptors were modeled with a one-dimensional transport-reaction equation to constrain the depth intervals (zones) where solutes are produced/consumed in the top 10 cm of the sediment column, and to obtain the net reaction rates in each zone. This multicomponent geochemical modeling reveals that CH4 was produced below 4-7 cm depth at lower rates in Basin A (250-800 fmol cm(-2) s(-1)) than in Basin B (1900-6500 fmol cm(-2) s(-1)) and that methanogenesis accounted for 30-64% and 84-100% of the sediment organic matter (OM) mineralization in Basins A and B, respectively. We show that methanogenesis did not always yield equimolar amount of CH4 and DIC, as would be expected from the fermentation of the model molecule CH2O. While similar to 50% of the CH4 produced in Basin A is oxidized in the sediment column, this proportion decreases to similar to 20% in Basin B. Dioxygen is by far the main electron acceptor for CH4 and OM oxidations in both basins. Methanotrophy in the sediment, however, is not limited to the similar to 4-mm thick surface layer in which O-2 diffuses from bottom water but occurs down to 4-7 cm depth where O-2 is transported through bioirrigation. Thermodynamic calculations suggest that, in addition to O-2, Fe oxyhydroxides, and sulfate may serve as oxidants for methanotrophy in that zone. We predict that Basin B sediments release more CH4 than DIC whereas Basin A sediments mainly export DIC. This study highlights that small changes in hypolimnetic O-2 levels may significantly alter the magnitude of OM mineralization pathways and the fate of CH4 in boreal lake sediments.

The biogeochemical cycles of iron (Fe) and organic carbon (OC) are closely interconnected. The concentration of reactive Fe is tightly controlled by soluble organic ligands in oceanic waters. In soils, Fe stabilizes OC by forming aggregates that shield OC from degradation. In sediments however; the role of Fe in the preservation of OC is only starting to be explored. To quantify the amount of OC retained in the solid phase through its interaction with Fe, an approach based on the reductive dissolution of reactive solid phase Fe, was applied. In this work, we investigated Fe-OC interactions in lake sediments, using sediments collected from Lake Tantare, a system with two basins characterized by contrasting redox conditions in the summer. These contrasting redox conditions provided an opportunity to assess the importance of oxic/anoxic interfaces in the formation of stable OC-Fe complexes. We found 30.1 ± 6.4% of OC directly associated with Fe minerals. We characterized the Fe-associated and the non-Fe-associated OC pools at the elemental (OC, TN), isotopic (δ13C, δ15N) and functional group (FTIR) levels. We found large differences in OC:Fe and TN:Fe ratios among the two basins which were not related to differences in OM chemical composition but rather to differences in reactive iron concentrations stemming from the higher abundance of mackinawite (FeS). Since the affinity of OM for mackinawite is much lower than for iron hydroxides, using OC:Fe and TN:Fe ratios as a diagnostic tool for the type of OM-Fe interactions should be done with care in anoxic environment.

The continued growth of mining and upgrading activities in Canada's Athabasca oil sands (AOS) region has led to concerns about emissions of contaminants such as polycyclic aromatic hydrocarbons (PAHs). Whereas a recent increase in PAH emissions has been demonstrated within around 50 km of the main center of surface mining and upgrading operations, the exact nature of the predominant source(s) and the geographical extent of the deposition are still under debate. Here, we report a century-long source apportionment of PAHs using dual (δ(2)H, δ(13)C) compound-specific isotope analysis on phenanthrene deposited in a lake from the Athabasca sector of the Peace-Athabasca Delta situated ∼150 km downstream (north) of the main center of mining operations. The isotopic signatures in the core were compared to those of the main potential sources in this region (i.e., unprocessed AOS bitumen, upgrader residual coke, forest fires, coal, gasoline and diesel soot). A significant concurrent increase (∼55.0‰) in δ(2)H and decrease (∼1.5‰) in δ(13)C of phenanthrene over the last three decades pointed to an increasingly greater component of petcoke-derived PAHs. This study is the first to quantify long-range (i.e., >100 km) transport of a previously under-considered anthropogenic PAH source in the AOS region.

The downstream accumulation of polycyclic aromatic hydrocarbons (PAHs) in the Peace-Athabasca Delta (PAD), an ecologically important landscape, is a key issue of concern given the rapid development of the oil sands industry in Northern Alberta, Canada. In addition to PAHs derived from industrial activity (i.e., oil sands mining) within the Athabasca watershed, however, forest fires and erosion of fossil fuel deposits within both the Athabasca and Peace watersheds are two potentially important natural sources of PAHs delivered to the PAD. Consequently, evaluating the environmental impact of mining activities requires a quantitative understanding of natural, background PAHs. Here, we utilize molecular-level natural-abundance radiocarbon measurements on an amalgamated sediment record from a Peace River flood-susceptible oxbow lake in the northern Peace sector of the PAD to quantitatively discriminate sources of naturally occurring alkylated PAHs (fossil and modern biomass). A radiocarbon mass balance quantified a predominantly natural petrogenic source (93% petrogenic, 7% forest fire) for alkylated PAHs during the past ∼50 years. Additionally, a significant petrogenic component determined for retene, a compound usually considered a biomarker for softwood combustion, suggests that its use as a unique forest fire indicator may not be suitable in PAD sediments receiving Peace watershed-derived fluvial inputs.

In their paper, Parajulee and Wania (1) use a multimedia fate model to argue that emissions of polycyclic aromatic hydrocarbons (PAHs) in environmental impact assessments conducted to approve developments in the Athabasca oil sands region (AOSR) are likely underestimated. The discrepancy between their model and reported emissions was mainly attributed to indirect evaporative releases of PAHs from tailings ponds (TPs).