Deep Water Masses and Sediments Are Main Compartments for

Article pubs.acs.org/est

Deep Water Masses and Sediments Are Main Compartments for Polychlorinated Biphenyls in the Arctic Ocean Anna Sobek*,† and Ö rjan Gustafsson†,‡ †

Department of Applied Environmental Science (ITM) and ‡Bolin Centre for Climate Research, Stockholm University, 10691, Stockholm Sweden S Supporting Information *

ABSTRACT: There is a wealth of studies of polychlorinated biphenyls (PCB) in surface water and biota of the Arctic Ocean. Still, there are no observation-based assessments of PCB distribution and inventories in and between the major Arctic Ocean compartments. Here, the first water column distribution of PCBs in the central Arctic Ocean basins (Nansen, Amundsen, and Makarov) is presented, demonstrating nutrient-like vertical profiles with 5−10 times higher concentrations in the intermediate and deep water masses than in surface waters. The consistent vertical profiles in all three Arctic Ocean basins likely reflect buildup of PCBs transported from the shelf seas and from dissolution and/or mineralization of settling particles. Combined with measurement data on PCBs in other Arctic Ocean compartments collected over the past decade, the total Arctic Ocean inventory of ∑7PCB was estimated to 182 ± 40 t (±1 standard error of the mean), with sediments (144 ± 40 t), intermediate (5 ± 1 t) and deep water masses (30 ± 2 t) storing 98% of the PCBs in the Arctic Ocean. Further, we used hydrographic and carbon cycle parametrizations to assess the main pathways of PCBs into and out of the Arctic Ocean during the 20th century. River discharge appeared to be the major pathway for PCBs into the Arctic Ocean with 115 ± 11 t, followed by ocean currents (52 ± 17 t) and net atmospheric deposition (30 ± 28 t). Ocean currents provided the only important pathway out of the Arctic Ocean, with an estimated cumulative flux of 22 ± 10 t. The observationbased inventory of ∑7PCB of 182 ± 40 t is consistent with the contemporary inventory based on cumulative fluxes for ∑7PCB of 173 ± 36 t. Information on the concentration and distribution of PCBs in the deeper compartments of the Arctic Ocean improves our understanding of the large-scale fate of POPs in the Arctic and may also provide a means to test and improve models used to assess the fate of organic pollutants in the Arctic.

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settling of particles and by transport off the shelves.16−18 Consequently, these removal processes are likely important also for hydrophobic POPs, such as PCBs, because of their strong affinity for particulate organic matter.19−24 Studies in the Arctic Ocean and other major ocean systems suggest that vertical export of PCBs sorbed to organic matter have a significant influence on their overall fate.19,20,24 Modeling studies predict that Arctic deep waters may contain higher concentrations of PCBs compared to surface waters, due to mineralization of settling organic matter and release of PCBs to the water phase.20,24 However, this has yet to be tested by observations of PCBs in subsurface water layers of the Arctic Ocean, observations which are lacking, in part due to the inaccessibility of the ice-covered Arctic Ocean and the technical challenges posed by collecting, handling, and cleanly analyzing these substances in intermediate and deep Arctic Ocean seawater. The aim of this study was to provide an observation-based assessment of the large-scale distribution and cycling of PCBs

INTRODUCTION Chemical pollution of natural systems is a major concern worldwide.1−3 Even the abyssal seafloor of the Arctic Ocean is anthropogenically affected.4 Polychlorinated biphenyls (PCBs) are legacy pollutants targeted by the Stockholm Convention on Persistent Organic Pollutants (POPs).5,6 PCBs bioaccumulate, affect environmental and human health, and are transported long distances to pristine environments.1,3,7−10 Although banned in many countries for more than 30 years, PCBs are ubiquitously spread throughout the global biogeospheres8,11 and still belong to one of the ecotoxicologically most significant pollutant groups in the Arctic and elsewhere.5−7,12 The global environmental distribution of these organochlorine POPs results from a combination of their physicochemical properties and the hydrological, geophysical, and biogeochemical characteristics of the environment. This combination originally led to the proposition that POPs, such as many of the PCBs, are subject to a northbound transfer in a large-scale global distillation with eventual cold trapping in regions such as the Arctic.13 Now it is evident that only a very small fraction of global emissions of PCBs is transported to the Arctic.9,14,15 Organic matter is transferred to sediment and deep and intermediate Arctic Ocean water masses via both vertical © 2014 American Chemical Society

Received: Revised: Accepted: Published: 6719

February 13, 2014 May 4, 2014 May 20, 2014 May 20, 2014 dx.doi.org/10.1021/es500736q | Environ. Sci. Technol. 2014, 48, 6719−6725

Environmental Science & Technology

Article

CB52, CB70, CB90/101, CB105, CB110, CB118, CB138, CB149, CB153, CB180, CB194 and CB199) were determined in pooled GF/F and PUF samples following previously described methods (see Supporting Information for more details).10,29 Field blanks and laboratory blanks were analyzed in parallel with the samples to identify any contamination originating from the described sampling and analytical steps. Field blanks consist of GF/F and PUF which were brought on the expedition and treated the same way as the filter and absorbent used for the samples, but with 75 years for the ArDWL.31,38 Water from the shelves, convecting down the slope to deeper water layers in the central Arctic Ocean, brings also particles from biological production from the shelves,37 thus reflecting the same process of vertical transport of PCBs from surface water to deeper water masses. The atmosphere is a significant source of PCBs to the World’s oceans, including primarily exchange of gaseous PCBs but also wet and dry deposition of PCBs associated with aerosols and/or water.39 Estimating the two-way gaseous exchange with the atmosphere was not an aim of this work. Instead net atmospheric uptake of PCBs was assumed to equal the vertical flux of PCBs associated with settling marine POC to deeper water layers. Ocean currents transporting both dissolved and particulate-bound PCBs will also contribute to the final concentration of PCBs at deeper waters,23,38 which is accounted for in the mass balance by transport with ocean currents. Net atmospheric uptake was calculated separately for each shelf sea and interior basin, based on average annual vertical flux of POC for each of these regions (Table 1). PCBs associated with terrestrial POC from riverine discharge may be transported far into the Arctic Ocean by ocean currents and ice.31 To avoid including this pool of PCBs twice in the mass balance, only the vertical flux of marine POC was considered as a vector for net atmospheric deposition. A fraction of 36% of the total vertical transport of POC was assumed to correspond to marine POC on the shelves and 21% in the central Arctic Ocean basins.18 These numbers are based on the largest available assessment of the organic carbon mass balance of the Arctic Ocean.18 The lower fraction of marine POC in the shelf seas compared to the central basins can be understood by the very oligotrophic character of the central Arctic Ocean basins in

Figure 1. Vertical PCB concentration profiles illustrated for CB52 and CB 153 (pg L−1) in the different water masses and basins of the central Arctic Ocean. Concentrations of additional PCB congeners in the different water masses are presented in the Supporting Information Table S5. PML = polar mixed layer; AtWL = Atlantic water layer; ArDWL = Arctic deep water layer.

Table 1. Area, Mean Volume, Vertical Export of POC and Sea Ice Volume in the Arctic Ocean Arctic seas Chukchi Sea East Siberian Sea Laptev Sea Kara Sea Barents Sea Beaufort sea SNCAA Central Arctic Ocean basins PML and halocline AtWL ArDWL

areaa (103 km2)

mean deptha (m)

vertical exportb (g C m−2 yr−1)

sea ice volumec (km3)

620 987 498 926 1512 178 146 4489

80 58 48 131 200 124 338 2748

39 1 1 10 33 13 27 2

1482 2360 279 519 847 426 1250 10 732

4489

300

4489 4489

570 1848

a From refs 30 and 31. bFrom ref 17. cCalculated from data on ice thickness and area.46,47 SNCAA = Shelf of Northern Canadian Arctic Archipelago. AtWL = Atlantic water layer; ArDWL = Arctic deep water layer.

the agreement between concentrations in dated Greenland sediment cores34 and annual emissions32 (Supporting Information, Figure S1). The flux of PCBs to Greenland lakes is dominated by atmospheric deposition. Atmospheric transport and deposition will also be a major process for the Arctic Ocean, since the load of PCBs in the Arctic Ocean reflects what is deposited on the oceans and in the Arctic river catchments. PCBs traveling from their emission source toward either Greenland lakes or the Arctic Ocean may be temporally retained in various compartments along their way.34 However, as PCBs deposited in the Arctic river catchments mainly will be in contact with snow, ice, and frozen soil, during drainage to the rivers; about 90% of PCB river export to the Arctic Ocean 6721

dx.doi.org/10.1021/es500736q | Environ. Sci. Technol. 2014, 48, 6719−6725


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Figure 2. Extrapolated ∑7 PCB concentrations (pg L−1) in the Arctic Ocean: (A) surface water concentrations; (B) depth profiles along a section in the Central Arctic Ocean (indicated with a black arrow in panel a) from the Nansen Basin, through the Amundsen Basin to the Makarov Basin (data and references displayed in Table S5). Gray symbols indicate sampling locations. The figure was made with the Software Ocean Data View.48

flux of organic carbon compared to ocean currents or river discharge.

combination with large amounts of terrestrial organic carbon being transported with ice and surface water of the Polar mixed layer from the shelves to the central Arctic Ocean.31 Uncertainty Estimations. Estimates of vertical export of POC, ocean currents, and freshwater discharge via rivers display a significant interannual variability and are accompanied by uncertainties.17,40,41 To account for the variability and uncertainty of estimations of the system, the range of cumulative fluxes was calculated using the range or reported uncertainty of vertical export, ocean currents, and river discharge.17,40,41 The resulting relative uncertainty is presented in Supporting Information, Table S6. As the mass balance covers fluxes approximated over 70 years, the high-amplitude variability between single years will most likely not be significant for the multiyear average,40,42 and thus the estimates presented in Table S6 most likely reflect worst-case scenarios. Both the inventory and estimated cumulative fluxes of PCBs are based on the assumption that measured concentrations are representative of the respective compartments in which they were measured, given the sluggish years-to-decades turnover times of subsurface Arctic Ocean compartments. This is a common assumption in marine science. To estimate uncertainties associated with the inventories and fluxes due to the variability in the measured PCB concentrations, we used the standard deviation calculated for the available data points for each compartment. For the compartments for which there were less than three data points, we applied a worst-case scenario by assuming the relative standard deviation (RSD) from the compartment with the highest measured variability. The propagated standard deviation for the cumulative fluxes or sum of inventories was then calculated. The uncertainties for the cumulative fluxes associated with the environmental system dominate over the uncertainties due to PCB measurements or concentration variability (see Table S6), and are therefore used in the presentation of the fluxes. Inventories are presented with ±1 standard error of the mean based on measured PCB concentration variability. The propagated uncertainty of the inventory calculated based on the cumulative fluxes into and out of the Arctic Ocean assumes that the variability in the environmental system (Table S6) represents the standard error of the mean. A sensitivity analysis of the mass balance for the Arctic Ocean was done, by analyzing what effect a 10%, 50%, and 100% difference in vertical carbon export, ocean currents, and river discharge has on the total mass balance. The results are presented in Supporting Information, Table S7 and shows that the mass balance is less sensitive to variability in vertical

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RESULTS AND DISCUSSION Vertical Profiles of PCB Concentrations. Concentrations of individual PCB congeners in the Arctic Ocean central basins increased with depth with the highest concentrations observed in the ArDWL and the lowest concentrations in the PML (illustrated for PCB 52 and 153 in Figure 1). This is the first empirical data set demonstrating such nutrient-like vertical profiles of PCBs in the central Arctic Ocean. An increase in PCB concentration with depth was observed also for the Barents Sea, the deepest of the seven Arctic Ocean shelf seas (Supporting Information, Table S5). Concentrations of the sum of the seven PCB congeners (∑7PCB) in the Nansen Basin increased from 0.7 pg L−1 in the surface PML to 3.6 pg L−1 and 4.5 pg L−1 in the AtWL and ArDWL, respectively (Figure 2). POC concentrations decreased accordingly by a factor of 10−30 from the PML to the ArDWL, supporting a strong particle dissolution/mineralization-influence on the vertical distribution of PCBs. The observed tendency of a concentration decrease from the Nansen Basin via the Amundsen Basin extending to the Makarov Basin (Figure 1 and Table S5) in intermediate and deep water masses is consistent with previously reported surface water concentrations in these basins, following a trend of decreasing concentrations along a northbound transect.10 Concentrations may also be expected to decrease with distance from the shelves, as has been observed for Al, indicating partly different origins of the water masses in the shelf seas and the central Arctic Ocean.36 Early reports of vertical profiles of PCBs in the northern North Atlantic Ocean, from observations in the mid1980s and early 1990s, displayed decreasing concentrations with depth,26,43 as did also observations in the Mediterranean Sea in the early 1990s.44 The discrepancy in the vertical profiles of PCBs probably reflects differences between the oceanic basins. A large fraction of the Arctic Ocean is ice covered, which limits atmospheric deposition. Therefore, losses of PCBs in surface waters due to sinking organic carbon are not counterbalanced by atmospheric deposition/exchange. In addition, the advective flow off the continental shelves is another characteristic unique for the Arctic Ocean which will further contribute to the nutrient-like vertical profiles observed in this study. Another aspect which may have contributed to the different vertical profiles of the Arctic Ocean and the Atlantic Ocean is the shorter time (10−20 years) for PCBs to build up 6722



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corresponding to 0.5% of the total amount of this congener emitted to the global environment (1930−2000).32 The cumulative net uptake from the atmosphere of ∑7PCB to the ocean during 1930−2000 was 2 ± 2 t in the central Arctic Ocean basins and 28 ± 25 t on the shelves (Figure 4).

in the subsurface water of the Atlantic Ocean at the time of sampling (1980s) compared to the Arctic Ocean in the early 21st century. Our observed PCB concentrations in Arctic Ocean subsurface water masses are qualitatively consistent with predicted vertical trends from modeling scenarios.20,24 Arctic Ocean Mass Balance of PCBs. The inventory of ∑7PCB in the entire Arctic Ocean (water, ice, sediment) is dominated by sediment, intermediate and deep water masses. Shelf sediment is the major compartment with an inventory of 136 ± 40 t,9 whereas the abyssal sediments are estimated to store 8 ± 2 t. The inventory of ∑7PCB in the intermediate AtWL and in the ArDWL is 5 ± 1 t and 30 ± 2 t, respectively. Surface water in the central Arctic Ocean (PML and halocline) stores 2 ± 0.3 t and the seawater of the seven shelf seas holds 1 ± 0.1 t,15 whereas the inventory of sea ice is 0.2 ± 0.02 t.29 This sums up to a total inventory of ∑7PCB in the entire Arctic Ocean of 182 ± 40 t, with sediment, intermediate and deep water masses currently comprising 98% (Figure 3 and Table

Figure 4. Historically accumulated inventories of ∑7PCB in the Arctic Ocean compartments during the 20th century. Arrows indicate cumulative (1930−2000) fluxes into and out of the Arctic Ocean through rivers, ocean currents, net atmospheric uptake, sediment, and ice. Uncertainties in inventories represent 1 standard error of the mean. InvMass balance and InvObserved represent the inventories calculated for the Arctic Ocean based on calculated cumulative fluxes into and out of the Arctic Ocean, and current PCB concentrations in the compartments of the Arctic Ocean, respectively.

About 60% of the net atmospheric uptake took place in the Barents Sea, due to high primary production and vertical export of organic carbon in this area (export normalized to area in the Barents Sea is a factor 30−190 higher than in other shelf seas17). Galbán-Malagón et al. (2012)20 estimated current removal of PCBs from the atmosphere due to vertical export of POC to be 0.44 ng per m2 per day for individual PCB congeners close to Spitsbergen in summer. In this study we calculated a mean vertical export of PCBs in the more productive Barents Sea during Arctic spring and summer of 0.17 ± 0.06 ng per m2 per day (based on PCB data from 2001). We expect the importance of net atmospheric uptake of POPs in the Arctic Ocean to increase in the future as a consequence of global warming, since climate change will expand the Arctic Ocean areas with high primary production and vertical export of organic carbon.16,17 The huge freshwater discharge by the Arctic rivers (11% of global freshwater input) into the Arctic Ocean is reflected in the river discharge of ∑7PCB of approximately 0.6 t per year.35 We estimated the cumulative import to the Arctic Ocean by the great Arctic rivers of ∑7PCB for 1930−2000 to 115 ± 11 t (Figure 4). Ocean currents entering the Arctic Ocean from the Pacific Ocean (the Bering Strait) and the Barents Sea (St Anna Trough and the West Spitsbergen Current) imported approximately 52 ± 17 t of ∑PCB7 to the Arctic Ocean during 1930 to 2000. Export through ocean currents was the only significant pathway out of the Arctic Ocean with approximately 22 ± 10 t of ∑PCB7, while sediment and ice leaving the Arctic Ocean exported 2 ± 2 t and 0.6 ± 0.5 t, respectively. The inventory of ∑7PCB in the Arctic Ocean during the 20th century illustrates a significant increase during the 1970s and 1980s and a stabilization of the Arctic Ocean inventory in the early 1990s (Figure 4). The current Arctic Ocean inventory (Figure 3; Table 2, and Supporting Information, Table S7) of

Figure 3. Inventory of ∑7PCB in the Central Arctic Ocean. PML = polar mixed layer; AtWL = Atlantic water layer; ArDWL = Arctic deep water layer. Chlorination degree of PCBs is indicated in the circle diagrams. The sizes of the circles are proportional to inventories in the compartments (Supporting Information, Table S7). The circles for ICE and PML (also including the inventory of the halocline) had to be enlarged to be visible in the Figure. Uncertainties represent 1 standard error of the mean.

S7) and as much as 75% stored in shelf sediments. Consequently, both ice and PML are insignificant compared to other compartments for storing PCBs. On the basis of these results we conclude that climate change is unlikely to cause release of a major fraction of hydrophobic POPs currently held in the Arctic Ocean. A previous compilation of observations of PCBs in the Arctic Ocean shelf sediments contributed with a detailed accounting of a substantial inventory and burial flux of PCBs in the Arctic Ocean (and global) shelf sediments.9 Recently the first multimedia model report appeared that included shelf sediments, and found this compartment to contribute substantially to the modeled PCB dynamics in the Arctic.45 Water masses are dominated by tri-, tetra-, and pentachlorinated PCBs (65−80%),10,14 while sediment is dominated by penta- and hexachlorinated PCBs (>80%; Figure 3).9 The dominance of lighter more volatile PCB congeners in the water suggests atmospheric deposition to be important, while the dominance of the more hydrophobic PCB congeners in sediment may reflect sorption to particulate organic carbon and more efficient transport to sediments. The predominant congener in water was PCB 52 with a total Arctic Ocean inventory of 22 ± 3 t (Supporting Information, Table S7), 6723



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Table 2. Contemporary Inventory of ∑PCB7 (t) in Arctic Ocean Water Masses, Ice and Sediment. Uncertainties Represent 1 Standard Error of the Mean. The Inventories for Additional PCB Congeners Are Presented in Supporting Information, Table S8 compartment

∑PCB7 (t)

sea ice Seven Arctic Ocean Shelf Seas seawater sediment Central Arctic Ocean Basinsa PML+ halocline AtWL ArDWL abyssal sediment total

0.2 ± 0.01

No. 621-2007-4631) and EU 7 FP project ArcRisk (Contract No. 1346810). Ö .G. acknowledges an Academy Research Fellow grant from the Swedish Royal Academy of Sciences.

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(1) Schwarzenbach, R. P.; Escher, B. I.; Fenner, K.; Hofstetter, T. B.; Johnson, C. A.; von Gunten, U.; Wehrli, B. The challenge of micropollutants in aquatic systems. Science 2006, 313, 1072−1077. (2) Rockström, J.; Steffen, W.; Noone, K.; Persson, A.; Chapin, F. S.; Lambin, E. F.; Lenton, T. M.; Scheffer, M.; Folke, C.; Schellnhuber, H. J.; Nykvist, B.; de Wit, C. A.; Hughes, T.; van der Leeuw, S.; Rodhe, H.; Sorlin, S.; Snyder, P. K.; Costanza, R.; Svedin, U.; Falkenmark, M.; Karlberg, L.; Corell, R. W.; Fabry, V. J.; Hansen, J.; Walker, B.; Liverman, D.; Richardson, K.; Crutzen, P.; Foley, J. A. A safe operating space for humanity. Nature 2009, 461, 472−475. (3) Simonich, S. L.; Hites, R. A. Global distribution of persistent organochlorine compounds. Science 1995, 269, 1851−1854. (4) Bergmann, M.; Klages, M. Increase of litter at the Arctic deep-sea observatory HAUSGARTEN. Mar. Pollut. Bull. 2012, 64, 2734−2741. (5) Arctic Pollution; AMAP Assessment Report; Arctic Monitoring and Assessment Programme (AMAP); Oslo, 2009. (6) United Nations Environment Programme. Stockholm Convention on Persistent Organic Pollutants (POPs); 2001; amended in 2009; http://chm.pops.int/. (7) Letcher, R. J.; Bustnes, J. O.; Dietz, R.; Jenssen, B. M.; Jorgensen, E. H.; Sonne, C.; Verreault, J.; Vijayan, M. M.; Gabrielsen, G. W. Exposure and effects assessment of persistent organohalogen contaminants in arctic wildlife and fish. Sci. Total Environ. 2010, 408, 2995−3043. (8) Nizzetto, L.; MacLeod, M.; Borga, K.; Cabrerizo, A.; Dachs, J.; Di Guardo, A.; Ghirardello, D.; Hansen, K. M.; Jarvis, A.; Lindroth, A.; Ludwig, B.; Monteith, D.; Perlinger, J. A.; Scheringer, M.; Schwendenmann, L.; Semple, K. T.; Wick, L. Y.; Zhang, G.; Jones, K. C. Past, present, and future controls on levels of persistent organic pollutants in the global environment. Environ. Sci. Technol. 2010, 44, 6526−6531. (9) Jonsson, A.; Gustafsson, Ö .; Axelman, J.; Sundberg, H. Global accounting of PCBs in the continental shelf sediments. Environ. Sci. Technol. 2003, 37, 245−255. (10) Sobek, A.; Gustafsson, Ö . Latitudinal fractionation of polychlorinated biphenyls in surface seawater along a 62 degrees N−89 degrees N transect from the southern Norwegian Sea to the North Pole area. Environ. Sci. Technol. 2004, 38, 2746−2751. (11) Meijer, S. N.; Ockenden, W. A.; Sweetman, A.; Breivik, K.; Grimalt, J. O.; Jones, K. C. Global distribution and budget of PCBs and HCB in background surface soils: Implications or sources and environmental processes. Environ. Sci. Technol. 2003, 37, 667−672. (12) Macdonald, R. W.; Barrie, L. A.; Bidleman, T. F.; Diamond, M. L.; Gregor, D. J.; Semkin, R. G.; Strachan, W. M. J.; Li, Y. F.; Wania, F.; Alaee, M.; Alexeeva, L. B.; Backus, S. M.; Bailey, R.; Bewers, J. M.; Gobeil, C.; Halsall, C. J.; Harner, T.; Hoff, J. T.; Jantunen, L. M. M.; Lockhart, W. L.; Mackay, D.; Muir, D. C. G.; Pudykiewicz, J.; Reimer, K. J.; Smith, J. N.; Stern, G. A.; Schroeder, W. H.; Wagemann, R.; Yunker, M. B. Contaminants in the Canadian Arctic: 5 years of progress in understanding sources, occurrence and pathways. Sci. Total Environ. 2000, 254, 93−234. (13) Wania, F.; Mackay, D. A global distribution model for persistent organic-chemicals. Sci. Total Environ. 1995, 160−61, 211−232. (14) Carrizo, D.; Gustafsson, Ö . Distribution and inventories of polychlorinated biphenyls in the polar mixed layer of seven Pan-Arctic shelf seas and the interior basins. Environ. Sci. Technol. 2011, 45, 1420−1427. (15) Wania, F. Assessing the potential of persistent organic chemicals for long-range transport and accumulation in polar regions. Environ. Sci. Technol. 2003, 37, 1344−1351. (16) Gustafsson, Ö .; Andersson, P. S. Th-234-derived surface export fluxes of POC from the Northern Barents Sea and the Eurasian sector of the Central Arctic Ocean. Deep-Sea Res. Pt. 1 2012, 68, 1−11.

1 ± 0.2 136 ± 40 2 ± 0.3 5±1 30 ± 2 8±2 182 ± 40

a

PML = polar mixed layer; AtWL = Atlantic water layer; ArDWL = Arctic deep water layer.

∑PCB7 (based on observations of PCB concentrations in Arctic Ocean compartments) of 182 ± 40 t is consistent with the inventory based on mass balance (173 ± 36 t) cumulative fluxes into (197 ± 34 t), within, and out of (24 ± 2 t) the Arctic Ocean (Figure 4; Table 2 and Supporting Information, Table S8). Hence, although there are uncertainties associated with both the inventory and the mass balance of PCBs, the overall agreement between the observation-based inventory and estimated cumulative fluxes presented here constitute strong support for the mass balance and estimations of cumulative fluxes. This study contributes the first empirical data on PCBs in intermediate and deep water masses of the Arctic Ocean. Our results show that the main pathway of PCBs into the Arctic Ocean is river discharge with the only significant pathway out of the Arctic Ocean being via ocean currents. This information improves our understanding of the large-scale fate of POPs in the Arctic.

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ASSOCIATED CONTENT

S Supporting Information *

Details on inventory and mass balance calculations, inventories of PCBs in the Arctic Ocean, data used in mass balance calculations, measured PCB concentrations, and a water balance of the Central Arctic Ocean. This material is available free of charge via the Internet at http://pubs.acs.org.

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*Phone: + 46-8-6747230; e-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank crew, captain and colleageus onboard icebreaker Oden for the Arctic Ocean expeditions Swedarctic-2001, Swedarctic-2005, and onboard h/v Yacob Smirnitskyi for the International Siberian Shelf Study 2008 (ISSS-08). The Swedish Polar Research Secretariat provided efficient logistical support for the expeditions. Kerstin Grunder is thanked for performing the PCB analyses, Emma Karlsson and Amelie Kierkegaard for support with Figures 2 and 3. This work is in part funded by the Swedish Research Council (VR Contract 6724



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