Eurasian river spring flood observations support net Arctic Ocean mercury export to the atmosphere and Atlantic Ocean

Significance Elevated levels of mercury in Arctic marine wildlife have been linked to midlatitude anthropogenic mercury emissions which are transported to the Arctic Ocean by air. Modeling studies, however, suggest that Arctic rivers contribute equal amounts of mercury to the Arctic Ocean. In this study, we provide comprehensive mercury data on large Eurasian rivers. We find that the spring flood mercury flux from Eurasian rivers is indeed large, which confirms a new Arctic mercury cycling paradigm: Mid-latitude anthropogenic emissions reach the terrestrial Arctic by air, whereby vegetation uptake transfers atmospheric mercury to tundra and boreal peat soils. Springtime snowmelt subsequently mobilizes peat soil mercury to the Arctic Ocean, where photochemistry drives net export of mercury back to the atmosphere. Midlatitude anthropogenic mercury (Hg) emissions and discharge reach the Arctic Ocean (AO) by atmospheric and oceanic transport. Recent studies suggest that Arctic river Hg inputs have been a potentially overlooked source of Hg to the AO. Observations on Hg in Eurasian rivers, which represent 80% of freshwater inputs to the AO, are quasi-inexistent, however, putting firm understanding of the Arctic Hg cycle on hold. Here, we present comprehensive seasonal observations on dissolved Hg (DHg) and particulate Hg (PHg) concentrations and fluxes for two large Eurasian rivers, the Yenisei and the Severnaya Dvina. We find large DHg and PHg fluxes during the spring flood, followed by a second pulse during the fall flood. We observe well-defined water vs. Hg runoff relationships for Eurasian and North American Hg fluxes to the AO and for Canadian Hg fluxes into the larger Hudson Bay area. Extrapolation to pan-Arctic rivers and watersheds gives a total Hg river flux to the AO of 44 ± 4 Mg per year (1σ), in agreement with the recent model-based estimates of 16 to 46 Mg per year and Hg/dissolved organic carbon (DOC) observation-based estimate of 50 Mg per year. The river Hg budget, together with recent observations on tundra Hg uptake and AO Hg dynamics, provide a consistent view of the Arctic Hg cycle in which continental ecosystems traffic anthropogenic Hg emissions to the AO via rivers, and the AO exports Hg to the atmosphere, to the Atlantic Ocean, and to AO marine sediments.

Midlatitude anthropogenic mercury (Hg) emissions and discharge reach the Arctic Ocean (AO) by atmospheric and oceanic transport. Recent studies suggest that Arctic river Hg inputs have been a potentially overlooked source of Hg to the AO. Observations on Hg in Eurasian rivers, which represent 80% of freshwater inputs to the AO, are quasi-inexistent, however, putting firm understanding of the Arctic Hg cycle on hold. Here, we present comprehensive seasonal observations on dissolved Hg (DHg) and particulate Hg (PHg) concentrations and fluxes for two large Eurasian rivers, the Yenisei and the Severnaya Dvina. We find large DHg and PHg fluxes during the spring flood, followed by a second pulse during the fall flood. We observe well-defined water vs. Hg runoff relationships for Eurasian and North American Hg fluxes to the AO and for Canadian Hg fluxes into the larger Hudson Bay area. Extrapolation to pan-Arctic rivers and watersheds gives a total Hg river flux to the AO of 44 ± 4 Mg per year (1σ), in agreement with the recent model-based estimates of 16 to 46 Mg per year and Hg/ dissolved organic carbon (DOC) observation-based estimate of 50 Mg per year. The river Hg budget, together with recent observations on tundra Hg uptake and AO Hg dynamics, provide a consistent view of the Arctic Hg cycle in which continental ecosystems traffic anthropogenic Hg emissions to the AO via rivers, and the AO exports Hg to the atmosphere, to the Atlantic Ocean, and to AO marine sediments. mercury | Arctic | rivers | ocean | modeling M ercury (Hg), in its methylated form (MeHg), is an environmental toxicant that has been associated with long-term neurocognitive deficits in children and impaired cardiovascular health in adults (1,2). Compared with other world regions, the Arctic is largely devoid of anthropogenic Hg emission point sources. Gaseous elemental Hg (GEM), the dominant form of Hg in emissions, has a long atmospheric lifetime (months) and can therefore be transported from emission-source regions to the Arctic (3). Thus, elevated Hg levels in Arctic biota have been linked to atmospheric transport of anthropogenic Hg emissions from the midlatitudes (3). The discovery of atmospheric Hg depletion events (AMDEs), depositing large amounts of atmospheric Hg to sea ice, reinforced a paradigm in which the atmosphere has a central role in shuttling Hg emissions to the Arctic marine environment (3,4). The first Hg mass budget for the Arctic Ocean (AO) suggested a dominant role of atmospheric Hg deposition under steady-state conditions (5). Subsequent research has documented that between 60 and 80% of AMDE-deposited Hg is rapidly reduced and reemitted to the atmosphere (3). Recent studies using 3D coupled atmosphereocean models suggest that Arctic river inputs have been a po-tentially underestimated source of Hg to the AO (6,7). The ratio of watershed to ocean basin surface is higher for the AO than for any other ocean basin (Fig. 1). Models, constrained by marine and atmospheric observations, suggest the AO to be a net source of Hg to the atmosphere (7)(8)(9) and have led to the suggestion that boreal soils and rivers are a key intermediate in trafficking midlatitude Hg emissions (via deposition to boreal soils) to the AO (10). Obrist et al. (11) documented how low Hg wet deposition to Arctic tundra is swamped by large annual atmospheric GEM uptake by tundra vegetation and soils, which potentially explains the large modeled river Hg flux. Model river Hg fluxes are severely underconstrained, however, with only a handful of low-water-stage Hg observations made on Eurasian rivers that represent 80% of Arctic river runoff (12,13). Recent studies indicate that permafrost stores more Hg compared to other soils and oceans combined (11,14). In the context of these findings and suggestions, understanding the Arctic Hg budget and AO Hg dynamics hinges on new observations of seasonal Eurasian river Hg fluxes.

Significance
Elevated levels of mercury in Arctic marine wildlife have been linked to midlatitude anthropogenic mercury emissions which are transported to the Arctic Ocean by air. Modeling studies, however, suggest that Arctic rivers contribute equal amounts of mercury to the Arctic Ocean. In this study, we provide comprehensive mercury data on large Eurasian rivers. We find that the spring flood mercury flux from Eurasian rivers is indeed large, which confirms a new Arctic mercury cycling paradigm: Mid-latitude anthropogenic emissions reach the terrestrial Arctic by air, whereby vegetation uptake transfers atmospheric mercury to tundra and boreal peat soils. Springtime snowmelt subsequently mobilizes peat soil mercury to the Arctic Ocean, where photochemistry drives net export of mercury back to the atmosphere.
Coquery et al. (13) made unique total dissolved Hg (DHg) and particulate Hg (PHg) observations on the Lena, Ob, and Yenisei estuaries and river end members during the low-water stages of September 1991 and 1993. The authors observed low DHg values of 5.0, 2.8, and 1.5 pmol L −1 and low PHg values of 4.3, 34, and 1.1 pmol L −1 for the Lena, Ob, and Yenisei river end members, respectively. Annual DHg and PHg fluxes were estimated by taking into account water and sediment discharge and assuming that spring flood DHg is 3× higher than the September observations. A total Hg (THg = DHg + PHg) river flux of 6.0 Mg y −1 was computed for the Lena, Ob, and Yenisei and extrapolated to 15 Mg y −1 for total Eurasian river runoff into the AO (13). On the North American continent, river Hg concentrations have been more extensively observed for the Yukon, Mackenzie, Nelson, and Churchill rivers (15)(16)(17)(18). Leitch  Numerical models of global and Arctic Hg cycles have provided additional constraints on Arctic Hg fluxes and dynamics. Fisher et al. (7) used atmospheric GEM observations to constrain the coupled atmosphere-ocean GEOS-Chem simulation of Hg dynamics in the Arctic. The authors were able to reproduce the summertime rebound in GEM concentrations by invoking a missing source of Hg to the Arctic surface ocean from riverine inputs (7). Based on atmospheric observations and an initial parameterization for air-sea exchange of Hg 0 evasion, a terrestrial THg flux of ∼95 Mg y −1 (80 Mg y −1 from rivers and 15 Mg y −1 from coastal erosion) was needed to reproduce the observed summer GEM peak. Later work using an ocean model with more realistic physics and ecology for the Arctic lowered this estimate to a THg flux of 62 Mg y −1 , portioned into 46 Mg y −1 from rivers and 16 Mg y −1 from coastal erosion (8). A different modeling study suggested a lower river THg flux of 16 to 30 Mg y −1 , assuming that river Hg derives solely from atmospheric Hg(II) deposition (19).
Large annual pan-Arctic river THg fluxes have also been estimated by extrapolating Hg/dissolved organic carbon (DOC) observations and DOC river budgets. Kirk  In this study, we present seasonal observations on DHg, PHg, DOC, and select DMeHg concentrations and fluxes for two large Eurasian rivers: the Yenisei and the Severnaya Dvina (S. Dvina). We provide additional observations on a smaller river, the Great Whale (G. Whale) River in Canada, to better constrain THg runoff into the Hudson Bay. We interpolate and extrapolate our comprehensive observations to the larger Arctic watershed and use a 3D model (8) to assess net sea-air exchange of Hg over the AO. We then use our revised Arctic river Hg flux together with observations on tundra Hg dynamics and AO Hg observations to revisit the Arctic Hg budget and cycling.

Materials and Methods
Sampling and Analysis. The AO watersheds and the Yenisei, S. Dvina, and G. Whale rivers are represented in Fig. 1. Hg samples were filtered in the field using preburnt quartz filters and a Teflon filter holder into acid-cleaned 500 mL fluorinated ethylene propylene Teflon bottles, acidified to 0.36 M HCl, and stored cold and in the dark until transport to France for analysis. DHg is defined here as the sum of inorganic and MeHg in the filtrate. PHg is defined as the inorganic and MeHg fraction retained on the quartz filter. DHg was measured by atomic fluorescence spectrometry following the US Environmental Protection Agency Method 1631 (21). We measured DMeHg as the sum of monomethyl (MM)Hg and dimethylmercury in the filtrate. DMeHg was analyzed via isotope dilution using GC-inductively coupled plasma-MS (21). Whole quartz filters were analyzed for PHg by combustion cold vapor atomic absorption spectrometry (Milestone DMA-80). DOC was measured on a Shimadzu TOC-VCSN Analyzer. Full details on all methods, including quality assurance/control and blanks, can be found in SI Appendix, Supplementary Text. Flux Calculations. Our 2012-2016 observations for DHg, PHg, and THg concentrations were multiplied by daily water discharge (from Roshydromet) to obtain daily Hg fluxes, which were then interpolated and integrated to estimate annual Hg fluxes. Data interpolation was performed using discharge vs. DHg, PHg, or THg concentration or yield trends, following published guidelines (17). DMeHg was measured only during the 2012 spring flood for the S. Dvina (April to August) and the G. Whale (May) rivers. Correlations between DHg and DMeHg are used to estimate S. Dvina DMeHg concentration and fluxes from January to March and from September to December.
Air-Sea Exchange Modeling. We simulated Hg 0 evasion from the AO configuration of the Massachusetts Institute of Technology general circulation model (MITgcm). The MITgcm includes carbon cycling and plankton dynamics and has a horizontal resolution of 36 km and 50 vertical ocean layers, with a coupled sea-ice model, and boundary conditions from a 1°× 1°global simulation (15). The oceanic Hg simulation was forced using atmospheric Hg 0 concentrations and Hg deposition from the GEOS-Chem atmospheric chemical transport model (8). We use the model to assess the impact of Arctic riverine Hg discharges on AO Hg 0 evasion to the atmosphere.
2012-2013 S. Dvina Hg and DOC. Seasonal discharge for the S. Dvina River peaks during the spring and fall floods, and total annual discharge varied substantially between 2012 (128 km 3 y −1 ) and 2013 (85 km 3 y −1 ) (Fig. 2). DOC levels in the S. Dvina during base flow (5.6 mg L −1 ) and spring flood (18 mg L −1 ) were typical of Russian boreal rivers (22). Base flow DHg, PHg, and THg levels were 3.8, 19.9, and 23.7 pmol L −1 ; spring flood DHg, PHg, and THg levels were up to 52, 55, and 107 pmol L −1 ; and fall flood DHg, PHg, and THg levels were up to 68, 39 and 107 pmol L −1 , respectively (Fig. 2). DHg, PHg, and THg are all correlated with discharge (r 2 values of 0.31, 0.50, and 0.54, respectively) and with DOC (r 2 values of 0.69, 0.23, and 0.48, respectively), similar to previous observations on North American rivers (17) (SI Appendix, Fig. S1 A and B). DMeHg was measured in 2012 and ranged from 0.25 to 0.87 pmol L −1 , representing 1 to 4% of DHg (Fig. 2). DMeHg was anticorrelated with DHg (r 2 = 0.40) and discharge (r 2 = 0.75), but not with DOC (r 2 = 0.11). Low DHg and elevated Within the first two snow melt and discharge pulses, DHg, PHg, DMeHg, and THg (but not DOC) are nonlinearly anticorrelated with discharge, suggesting a flushing effect of the Hg carrying phases. Consequently, THg is not correlated with discharge (r 2 = 0.03) over the entire month of observations. Similarly weak THg vs. discharge correlations were made for the Nelson and Churchill rivers (r 2 values of 0.05 and 0.34, respectively) that also run off into Hudson Bay (23). Further identification of the underlying hydrogeochemical dynamics for the G. Whale is beyond the scope of this study. Due the progressive increase in discharge during the spring flood, THg yield does correlate well with runoff and is similar among the Nelson, Churchill, and G. Whale rivers (see below).
Arctic River Hg Fluxes by Hg/DOC Extrapolation. Three different extrapolation methods can be used to estimate circum-Arctic river Hg fluxes. All critically depend on accurate estimates of freshwater discharge into the seas that make up the AO. The simplest method uses water discharge-weighted average Hg concentrations for select Arctic rivers and extrapolates these to the entire Arctic watershed using total discharge budgets (5,12). The second method uses THg/ DOC observations on select rivers multiplied by pan-Arctic DOC fluxes (which combine DOC observations and discharge) (6,20). The third method, well-illustrated for a recent estimate of the Arctic river DOC flux (22), combines seasonal observations of Hg on multiple rivers to derive relationships between drainage area (km 2 )-normalized discharge (water yield, or runoff in cm y −1 ) and Hg yield (i.e., Hg runoff in μg·m −2 ·y −1 ). In this study, we use the second and third methods to derive estimates for annual and daily riverine DHg, PHg, THg, and DMeHg inputs into the AO and Hudson Bay. An important aspect of these methods is the correction to be made for the 33.1% of Arctic watersheds that are not monitored for water discharge, DOC, and Hg (24). We provide separate estimates of THg flux to the AO from Eurasian and North American rivers and to the larger Hudson Bay area (i.e., Hudson Bay, Hudson Strait, James Bay, and Ungava Bay). We do not include Hg runoff to the Bering Sea, as Hg transport from the Pacific Ocean to the AO is taken into account separately in previous AO Hg budgets (5,9). DOC fluxes from North American and Russian rivers are historically better documented than Hg fluxes, in particular by the international Pan-Arctic River Transport of Nutrients, Organic Matter and Suspended Sediments (PARTNERS) program (22). Previous studies have used observations on THg/DOC ratios from the North American Yukon and Mackenzie rivers, together with pan-Arctic DOC fluxes, to estimate river THg fluxes to the AO of 108 Mg y −1 (20) and 50 Mg y −1 (6). Uncertainties in these estimates arise mainly from extrapolating THg/DOC ratios for North American rivers to Eurasian rivers and from uncertainty in DOC fluxes. In addition, the Yukon River has been suggested to have unnaturally high PHg/DOC and THg/DOC ratios due to the impact of legacy gold mining (6). In the absence of representative THg/DOC observations on Eurasian rivers, a key assumption in these studies has been that North American THg/DOC ratios apply to Eurasian rivers. SI Appendix,  Appendix, Table S2) that are between our Yenisei and S. Dvina river and North American river observations, based on continental lithology, geomorphology, and sediment discharge. We use our revised THg/DOC (and DHg/THg) ratios in SI Appendix, Table S1 to estimate a DOC-based pan-Arctic THg flux of 43 Mg y −1 (SI Appendix, Arctic River Hg Fluxes by Extrapolation of Watershed Hg Yields. Since DHg, PHg, and THg correlate with river discharge, we can use an alternative and more direct method to scale up the Yenisei, S. Dvina, G. Whale, and published Hg observations to all Arctic watersheds. Fig.  4 A-C shows the relationships between annual THg, DHg, and PHg flux and discharge, both normalized to watershed area, to give annual THg, DHg, and PHg yield (μg·m −2 ·y −1 ) and water yield (runoff, cm y −1 ). Published Hg fluxes for the Mackenzie River in 2003-2005 (16), the Nelson and Churchill rivers in 2003-2007 (15), and the Saint Lawrence River (26) are included. Yukon River observations (17) are not included because of the anthropogenic impact on its river basin (see ref. 6 and below). Annual Hg yields correlate well with water yields for all North American and Eurasian rivers that drain into the AO (NA/EA→AO) (Fig. 4 A-C). An exception to the trends are the low Hg yields of the G. Whale and Saint Lawrence watersheds on the eastern Hudson Bay (Fig. 4 A-C). This observation is not trivial because North Quebec, which drains into the eastern Hudson Bay and Atlantic Ocean, has among the highest wet precipitation and runoff across the entire (sub)Arctic (27). Extrapolating the linear NA/EA→AO relationship to the eastern Hudson Bay, where 20 rivers have water yields >40 cm y −1 (37% of Canadian runoff), would result in a large, overestimated THg flux to the Hudson Bay and Strait. We therefore separated Hg-and water-yield relationships into (i) a North American/Eurasian component based primarily on the S. Dvina, Yenisei, and Mackenzie observations, but also including the Nelson and Churchill rivers, which drain watersheds on the drier, western Hudson Bay, and (ii) a Hudson Bay (and Strait) component that is based on a linear regression of the same Nelson and Churchill river data, as well as the G. Whale and Saint Lawrence river data. Although the Saint Lawrence River does not run off into the Hudson Bay (but into the Atlantic Ocean), it has similar watershed climate, land use, and low Hg yield as compared to the G. Whale, justifying our separation of Hudson Bay Hg runoff from the rest of North America. Based on Fig. 4 A-C, Hg yields can be formulated as follows: where Y is the annual yield of DHg, PHg, and THg (μg·m −2 ·y −1 ); R is the annual runoff for a given watershed (cm y −1 ); and HB is the Hudson Bay. Hg yield and runoff are defined as where F is the annual flux of THg, DHg, and PHg (kg y −1 ); A is the watershed area (km 2 ); and D is the annual discharge (km 3 y −1 ). Using Eqs. 1-8 together with published discharge and watershed area data for 116 Arctic rivers (24,28), we estimate annual pan-Arctic DHg, PHg, and THg fluxes of 22, 22, and 44 Mg y −1 , respectively (Dataset S2). The 10 largest Hg contributing rivers to the AO are summarized in Table 1. In calculating these fluxes, we correct discharge for the "river mouth to station" flow-rate ratios (24), and use corresponding watershed surface area at the river mouth. Dai and Trenberth (24) estimated that, on average, 33.1% of Arctic watersheds are unmonitored for discharge, which we also correct for ref. 24. DOC fluxes from the relatively smaller unmonitored high-latitude watersheds are thought to be enhanced (25). Hg fluxes from unmonitored watersheds are therefore also likely enhanced. This is confirmed in SI Appendix, Fig. S2, which shows more elevated THg yields for smaller watersheds throughout Eurasia. We therefore use mean THg yield for watersheds <10,000 km 2 to estimate THg fluxes from unmonitored watersheds ( Table 2) (Table 1). On average, multiannual model DHg, PHg, and THg fluxes are unbiased against observations within 10%. The multiannual 1σ uncertainties associated with model DHg, PHg, THg flux estimates are 35%, 48%, and 10%, respectively. Our revised river THg flux of 44 ± 4 Mg y −1 (1σ) and the recently revised erosion Hg flux of 30 Mg y −1 (9) agree (within uncertainty) with the model-based estimates of Zhang et al. (8). The revised annual budget will likely allow further refinement of other uncertainties in Arctic Hg models, such as evasion to the atmosphere and Hg export flux to the shelf and deep AO.
Daily River Hg Flux Regression Equations. Our observations will also allow a much finer temporal (i.e., daily) assessment of Hg fluxes to the AO, which are addressed below. Fig. 5 explores the relationships between daily DHg, PHg and THg flux and daily discharge, again normalized to watershed area, to give daily DHg, PHg, and THg yield (ng·m −2 ·d −1 ) and water yield (runoff, cm d −1 ). As before, we separated Hg inputs to the AO from inputs to the larger Hudson Bay. The observed correlations are good (r 2 values of 0.81 to 0.91) and again provide the possibility to couple Hg fluxes directly to daily gridded water discharge and watershed area in Arctic Hg models:   À Y PHg Á HB = 0.015 × ðRÞ-0.033, [14] where Y is the daily yield of THg, DHg, and PHg (ng·m  Similar to DHg, PHg, and THg, we can explore annual DMeHg yield vs. runoff relationships (Fig. 4D). We observe no trend in annual DMeHg yield vs. runoff, apart from the Eurasian S. Dvina having a higher yield of 0.043 μg·m −2 ·y −1 than North American rivers (0.008 ± 0.003 μg·m −2 ·y −1 ), presumably due to much higher DOC in the former. In the absence of additional Eurasian river data, we constrain North American/Eurasian and Hudson Bay yields for DMeHg to the AO to be independent of runoff:  (9).
Implications for Arctic Hg Cycling. Fisher et al. (7) hypothesized that the summer rebound in Arctic atmospheric GEM could be explained by a large riverine flux of Hg to the AO and associated peak in marine Hg evasion to the atmosphere. Subsequent work by Zhang et al. (8), which coupled the atmospheric simulation from Fisher et al. (7) with a 3D ocean simulation for the Arctic, suggested that a lower input from Arctic rivers of 46 Mg y −1 would be sufficient to explain the summertime peak in atmospheric GEM concentrations. Riverine Hg flux estimates into the AO were relatively unconstrained before the observations presented in this work due to lack of data from the Eurasian Arctic rivers. Our observationally based estimate of AO river inputs (44 ± 4 Mg y −1 ) agrees with prior modeling estimates, presenting a consistent picture of Hg dynamics in the Arctic. The magnitude of the riverine Hg flux shown in this study leads to enhanced evasion in the AO in summer (Fig. 6A, June to July), which is sufficient to explain the summertime rebound in coastal AO atmospheric GEM concentrations. Net air-sea exchange of Hg is strongest over the continental shelf areas due to heat transfer from continental regions and turbulence from seaice rafting (8). Evasion is inhibited in the central AO due to the presence of sea ice. The seasonal dynamics of air-sea exchange of Hg are shown in Fig. 6B and illustrate how marine evasion of river-discharged Hg exceeds Hg deposition from May to August, after the spring flood.
Based on the ensemble of these observations and model studies, Fig. 7 summarizes the essential features of the modern Arctic Hg cycle. Midlatitude atmospheric Hg emissions are transported to the Arctic where terrestrial vegetation and soil Hg 0 uptake drives a large net Hg deposition of 210 Mg y −1 to  permafrost soils (11,29). The Hg net deposition number includes the active plant Hg 0 uptake of 6.5 μg·m −2 ·y −1 , the low Hg II wet deposition of 0.5 μg·m −2 ·y −1 , and the Hg II dry deposition of 2.5 μg·m −2 ·y −1 over the 22.79 million square kilometer permafrost soil area (11). The large active-layer soil Hg pool [408 to 863 Gg Hg, mean of 635 Gg (14,30)] supplies Hg to Arctic rivers and sustains a large spring flood Hg pulse to the coastal AO. Here, river THg (44 Mg y −1 ), together with coastal erosion Hg inputs (30 Mg y −1 ), is partially photoreduced in surface waters and emitted to the atmosphere during summertime (8). Midlatitude atmospheric Hg 0 also reaches the AO where wet and dry deposition, especially during springtime AMDE events, is of similar magnitude as terrestrial inputs (76 Mg y −1 ). However, the large Hg reemissions from ice and AO water (99 Mg y −1 ) exceed Hg deposition over the AO (Fig. 6B). On an annual basis, the AO is therefore a net Hg emission source (23 Mg y −1 ) to the atmosphere. The AO not only exports Hg to the atmosphere but also to the North Atlantic basin (30 Mg y −1 ), because marine THg levels in outgoing AO currents, at all depths, are more elevated than in incoming Atlantic waters (21,31,32). Lastly, AO Hg is exported to shelf (25 Mg y −1 ) and deep AO sediments (3 Mg y −1 ) (9). The above-outlined AO Hg export scenario for the modern AO has likely changed over the past 50 y. Since 1970, global anthropogenic Hg emissions to air have decreased by a factor of 2 (33). Northern Hemisphere atmospheric Hg deposition to soils has concomitantly decreased by a factor of 2 (34). In the 1980s, Atlantic Ocean surface and intermediate waters had higher Hg concentrations due to enhanced emissions from North America and Europe (31,35), and AO summer sea-ice extent was larger, possibly inhibiting the strong Hg 0 evasion observed today. Consequently, the AO Hg budget is in constant flux and likely not at steady state, except for the surface ocean, which adapts more rapidly to decadal Hg emission and climate drivers. Future work should investigate (i) if and by how much the Arctic river Hg flux will increase as permafrost further thaws, (ii) the transport and dynamics of terrestrial Hg in the transpolar drift current, and (iii) the Hg export flux from AO surface waters to shelf sediments and deep AO.