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Eddy-Pump: Pelagic carbon pump processes along the eddying Antarctic Polar Front in the Atlantic Sector of the Southern Ocean
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Eddy-Pump: Pelagic carbon pump processes along the eddying Antarctic Polar Front in the Atlantic Sector of the Southern Ocean 1. Introduction The Southern Ocean influences earth's climate in many ways. It hosts the largest upwelling region of the world oceans where ~80% of deep waters resurface (Morrison et al., 2015). A prominent feature is the broad ring of cold water, the Antarctic Circumpolar Current (ACC), which encircles the Antarctic continent and connects all other oceans. The ACC plays a major role in the global heat and freshwater transports and oceanwide cycles of chemical and biogenic elements, and harbours a series of unique and distinct ecosystems. Due to the upwelling of deep-water masses in the Antarctic Divergence, there is high supply of natural CO2 as well as macronutrients, leading to the worldwide highest surface nutrient concentrations. Despite the ample macronutrients supply, phytoplankton concentration is generally low, limited either by low micronutrient (iron) availability, insufficient light due to deep wind-mixed layers or grazing by zooplankton, or by the combination of all, varying temporally and regionally. In preindustrial times, the Southern Ocean might have been a source of atmospheric CO2 (Morrison et al., 2015). The contemporary Southern Ocean south of 30°S is currently considered a sink of CO2 mainly due to the increase in atmospheric concentrations from ~280 to over 400 ppm (parts per million). 40% of the anthropogenic CO2 taken up by the oceans entered via the region south of 40°S (Khatiwala et al., 2009). There are indications of a weakening of the CO2 sink in recent decades because of a stronger upwelling and associated outgassing of natural CO2 (Le Quéré et al., 2007), based on both atmospheric CO2 inverse and ocean biogeochemistry simulations. This trend was confirmed by an ocean circulation model with embedded biogeochemistry (e.g. Lovenduski et al., 2008) and by the trends in measured pCO2 (partial pressure of CO2) differences between the ocean and the atmosphere in the Indian sector of the Southern Ocean (Metzl, 2009). On the other hand, doubt arose with respect to the robustness of atmospheric inversion results and performance of the coarse ocean circulation models used, where eddy activity was not included (Law et al., 2008). Moreover, Böning et al. (2008) showed with observational data that the meridional overturning of the Southern Ocean is not very sensitive to the observed increased wind stress in the ACC, a mechanism which Le Quéré et al. (2007) and others held responsible for elevated upwelling and concomitant outgassing of natural CO2. Key to explain the difference in views is the eddy activity of the ACC which might increase along with a strengthened wind stress curl and so be able to counteract the wind-forced steepening of isopycnals (Böning et al., 2008; Meredith et al., 2012). The later finding of Landschützer et al. (2015) that the Southern Ocean carbon sink regained its strength between 2002 and 2012 supports the view of a rather dynamic ocean carbon cycle. Varying levels of eddy kinetic energy associated with changes in the wind forcing may not only affect the meridional overturning and thus the physical carbon pump but also the biological carbon pump. Mesoscale dynamics linked to eddies and front meanders shape the physical and chemical environment of primary production in several ways (Strass et al., 2002). It induces mesoscale upwelling that can supply micronutrients (iron) to the photic zone stimulating carbon fixation through photosynthesis (Hense et al., 2003). The cross-front circulation that connects mesoscale up- and downwelling cells also modifies the water column stratification, mixed layer depths and therefore the light environment in which phytoplankton can thrive. Moreover, the subduction that is linked to the cross-front circulation not only sequesters anthropogenic CO2 as dissolved inorganic carbon (Sallée et al., 2012) but also displaces phytoplankton to depth, possibly inducing its sinking as particulate organic carbon (Strass et al., 2002; Omand et al., 2015), and thus potentially enhances the biological carbon pump. But also the large-scale wind-driven upwelling can bring iron into the surface layer and stimulate phytoplankton growth and consequently the biological carbon pump south of the Antarctic Polar Front (APF), as indicated by a coupled ecosystem-general circulation model and regression analysis (Hauck et al., 2014). Productivity in the Southern Ocean varies not only across the ACC with its embedded fronts but also along the ACC axis. In the Atlantic sector of the Southern Ocean highest productivity is usually observed downstream of land masses, in particular the Patagonian shelf, the Antarctic Peninsula and South Georgia - as was also the case during Eddy-Pump (Fig. 1). The plume of high productivity observed downstream of South Georgia extends eastward along the meandering Polar Front where longitudinal bands with high chlorophyll content can also be observed. Benthic oxygen measurements have revealed that both areas show very high rates of carbon export reaching the sediment surface (Sachs et al., 2009). Significant differences between these two regions, however, have been observed both in the magnitude, inter-annual variability and composition of benthic fluxes as well as in chlorophyll a concentrations and sediment composition (Abelmann et al., 2006; Sachs et al., 2009; Venables and Meredith 2009). These differences have been attributed to differences in magnitude and mode of iron supply (Venables and Meredith, 2009). Apart from its physical and chemical drivers, the biologically mediated carbon export critically depends on phytoplankton species assemblages and food web structure (e.g. Longhurst and Harrison, 1988, Smetacek et al., 2004; Assmy et al., 2013; Wallace et al., 2013). Dominance of phytoplankton and zooplankton key species and food webs in the ACC change strongly with the meridional gradients across fronts but also zonally along the ACC (Abelmann et al., 2006, http://atlas.biodiversity.aq/). The relationship between plankton composition, productivity and the http://dx.doi.org/10.1016/j.dsr2.2017.02.009
0967-0645/ © 2017 Elsevier Ltd. All rights reserved.
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Fig. 1. Upper panel: Track of Polarstern cruise ANT-XXVIII/3 ‘Eddy-Pump’, superimposed on a map of the chlorophyll a (Chla) concentration monthly composite of February, 2012, obtained from the Ocean Colour Climate Change Initiative dataset, Version 3, European Space Agency (http://www.esa-oceancolour-cci.org/). Lower panels: Chlorophyll a concentrations measured underway during ANT-XXVIII/3.
magnitude of POC export is variable, and the processes responsible for this variability are still poorly understood and quantified. The Eddy-Pump program was designed to identify and better describe the interaction of physical, biological and chemical processes and their combined effects on the carbon flux to the ocean interior and deep sea. With the overarching common goal of improving the current understanding of the Southern Ocean's role in the global climate system, Eddy-Pump integrated the specific disciplines and relevant fields of work. To achieve its goal, Eddy-Pump followed a dedicated survey strategy, concentrating on areas of contrasting environmental conditions along the ACC focussing on evidently most relevant but least understood processes, and applying state-of-the-art methods. 2. Cruise itinerary Following its research strategy, Eddy-Pump concentrated on the band of latitudes between 50°S and 60°S where the upwelled deep-water masses interact with the atmosphere before they are subsequently subducted at the Sub-Antarctic Front. To reveal the effect of carbon pump processes during a significant part of the growth season, process studies were conducted for days to weeks within eddies or meanders, which were tracked before and during the cruise using satellite remote sensing of sea surface height anomalies and ocean colour. The study sites were selected to represent different productivity regimes presumed to be dominated by different plankton assemblages and with contrasting export fluxes. The data for Eddy-Pump were collected 2012 during Polarstern cruise ANT-XXVIII/3, which started January 7 in Cape Town (Fig. 1). The route from Cape Town led southwest until 44°S and 10°E. A transect with hydrographic stations was conducted along 10°E until 53°S, thereby crossing the Subantarctic Front, the Antarctic Polar Front and the Southern Polar Front. Passing stations at 52°S, 8°W in chlorophyll-poor water (Fig. 1) and 52°S, 12°W, the route led to the region around 51°12'S, 12°40'W above the topographic structure of the West Mid-Atlantic Ridge, where we sampled a grid of stations within a phytoplankton bloom for more than 2 weeks. After a short visit of the British Antarctic Survey station on South Georgia we steamed to the Georgia Basin in the northwest of that island. In the region around 50°S, 38°W a grid of 30 stations at intervals of 24 nautical miles was carried out. The station work was finished at March 7 and Polarstern sailed to Punta Arenas, Chile, where the cruise ended on March 11, 2012 (Fig. 1). 2
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3. Major findings The physical environment encountered during Eddy-Pump is described by Strass et al. (2016, this issue), who used measurements of pressure, temperature and conductivity and the derived variables salinity and density, of mixed layer turbulence parameters, horizontal current vectors as well as dynamic heights and flow trajectories obtained from surface drifters and submerged floats. This information provides the background for the analyses of biological and chemical processes and of biogeochemical fluxes addressed by other papers in this issue. The section along 10°E between 44°S and 53°S showed a classical ACC structure with well-known hydrographic fronts: the Subantarctic Front (SAF) at 46.5°S, the Antarctic Polar Front (APF) split in two, at 49.25°S and 50.5°S, and the Southern Polar Front (SPF) at 52.5°S. Each front was associated with strong eastward flows, with weaker or even reversed flows interspersed between. The West Mid-Atlantic Ridge Survey, centered around 51°12'S, 12°40'W, showed a weak and poorly resolved meander structure between the APF and the SPF. During the first eight days of the survey the oceanographic conditions at the Central Station at 12°40’W remained reasonably constant. However after that, conditions became more variable in the thermocline with conspicuous temperature inversions and inter-leavings accompanied by increased currents, and a decrease in temperature in the surface layer. At the very end of the period of observation the conditions in the thermocline returned to being similar to those observed during the early part of the period with however the higher mixed layer temperature. The Georgia Basin Survey showed a very strong zonal jet at its northern edge that connected to a large cyclonic meander which joined an anticyclonic eddy in the southeastern quadrant. The water mass contrasts in this survey were stronger than in the West Mid-Atlantic Ridge Survey, but similar to those met along 10°E with the exception that the warm and saline surface water typical of the northern side of the SAF was not covered by the Georgia Basin Survey. The trajectories of surface buoys and subsurface drifters following the large-scale flow through the Atlantic sector revealed a northward deflection of the ACC when crossing the Mid-Atlantic ridge, superimposed by mesoscale eddies. Mixed layers found during Eddy-Pump were typically deep, but varied between the three survey areas; the mean depths and standard variations of the mixed layer along the 10°E section were 77.2 ± 24.7 m, at the West Mid-Atlantic Ridge 66.7 ± 17.7 m, and in the Georgia Basin 36.8 ± 10.7 m. Using transient tracer and carbonate system data collected along the 10°E transect compared with historic cruises in the same area, Tanhua et al. (2016, this issue) analyzed temporal changes in ventilation and in inorganic carbon storage in the southeast Atlantic sector of the Southern Ocean. The transient tracer data suggest a significant speed-up of ventilation in the summer-warmed upper part of Antarctic Intermediate Water (AAIW) between 1998 and 2012, which is consistent with the high storage rate of anthropogenic carbon Cant. A shift of the more prevalent northern Cant storage to more southern storage in near surface waters was detected in the early 2000s. Beneath the AAIW the extended multiple linear regression (eMLR) method did not reveal significant storage of Cant. However, the presence of the transient tracer CFC-12 through the whole water column suggests that some Cant should be present, but at concentrations not reliably quantifiable. The observed temporal variability in the ocean interior seems at a first glance to be out of phase with observed surface ocean Cant fluxes, but this can be explained by the time delay for the surface ocean signal to reach the interior of the ocean. Jones et al. (2015, this issue) investigated the influence of eddy structures on the seasonal depletion of dissolved inorganic carbon (DIC) and on carbon dioxide (CO2) disequilibrium. This study focused on the highly dynamic northern edge of the Georgia Basin, which is impacted by both the Subantarctic Front (SAF) and Antarctic Polar Front (APF). Cyclonic cold core as well as anticyclonic warm core eddy structures were found to be hotspots of carbon uptake relative to the rest of the ACC surveyed during Eddy-Pump. Carbon uptake was highest in the cold core where greatest CO2 undersaturation (−78 μatm) and substantial surface ocean DIC deficit (5.1 mol m−2) occurred. In the presence of high wind speeds, the cold core eddy acted as a strong sink (25.5 mmol m−2 day−1) for atmospheric CO2. Waters of the warm core displayed characteristics of the Polar Frontal Zone (PFZ), with enhanced CO2 undersaturation (−59 μatm) and depletion of DIC (4.9 mol m−2). A mechanism proposed for the enhanced carbon uptake across both eddy structures is based on the Ekman eddy pumping theory. First, the cold core is seeded with productive (high chlorophyll-a) waters from the Antarctic Zone and biological productivity is sustained through upwelled nutrients that counteracts DIC inputs from deep waters. Second, horizontal entrainment of due to biological uptake low DIC surface waters from the PFZ, which downwell within the warm core, cause relative DIC depletion in the upper water column. The observations suggest that the formation and northward propagation of cold core eddies in the region of the APF could transfer low DIC waters towards the site of Antarctic Intermediate Water formation and enhance CO2 drawdown into the deep ocean. Sampling during Eddy-Pump also provided first insights into the concentrations of mercury (Hg) species in the Atlantic sector of the Southern Ocean (Canário et al., 2016, this issue). Results showed high spatial variability in the concentrations of total mercury (HgT) and methylmercury (MeHg). HgT (0.93 ± 0.69 ng L−1) and MeHg (0.26 ± 0.12 ng L−1) levels were similar or higher than those reported in previous works in high latitude studies. The highest values were found at a location (53°S, 10°E) south of the South Polar Front, an area with strong currents and horizontal gradients between different water masses. Vertical profiles showed a great variability even for stations sampled at the same location or area dominated by the same oceanographic features. A decrease of HgT accompanied by an increase in MeHg with depth was observed in some sites, suggesting the occurrence of an Hg methylation process, while at other stations a concurrent decrease or increase of both mercury species was observed. In spite of these differences, an overall positive correlation between HgT and MeHg was found. Differences between vertical profiles of Hg species were attributed to favourable environmental conditions for Hg methylation. The highest proportion of MeHg (% of HgT) was observed at sites with low dissolved oxygen or higher estimated remineralization rates. However, the concentrations of MeHg in these areas are more dependent on the environmental conditions than on the total concentration of Hg present in the water. During Eddy-Pump several zones with high phytoplankton biomass and productivity were investigated (Fig. 1). From North to South along 10°E two phytoplankton blooms, defined by chlorophyll a (Chla) concentrations above 1 mg m−3, were crossed at 49.7°S and between 50.3 and 51.3°S in the Antarctic zone (Cheah et al., 2016, this issue, Puigcorbé et al., 2016, this issue). More detailed process studies were carried out further west in another large-scale bloom around 12°W and north of the Georgia Basin around 39°W (Fig. 1; Hoppe et al., 2015, this issue; Roca-Martí et al., 2015, this issue; Iversen et al., 2016, this issue). Despite differences in hydrography and iron supply (Hoppe et al., 2015, this issue) as well as phytoplankton assemblage (Cheah et al., 2016, this issue; Klaas unpublished) for these different locations, results from primary production measurements (Hoppe et al., 2015, this issue) and bio-optical parameters measurements (Cheah et al., 2016, this issue) provided similar results. An important finding was the positive relationship between depth-integrated phytoplankton biomass, productivity and the mixed layer depth (Hoppe et al., 2015, this issue; Cheah et al., 2016, this issue). These results imply that biomass accumulation during diatom-dominated blooms in the Antarctic zone (Cheah et al., 2016, this issue; Klaas et al., unpublished) can occur despite deep mixed layer depths (70 to 100 m depth) as previously observed during an artificial iron fertilization experiment in the region (Smetacek et al., 2012). This was further confirmed by bio-optical and photoprotective pigment measurements (Cheah et al., 2016, this issue) showing that the phytoplankton communities found were shade adapted. 3
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Differences in light regime might, however, influence bloom temporal dynamics (Hoppe et al., 2015, this issue). Satellite imagery indicated that our study took place in the late stage of the bloom both at 12°W and at 39°W (Hoppe et al., 2015, this issue, Puigcorbé et al., 2016, this issue; RocaMartí et al., 2015, this issue). Hoppe et al. (2015, this issue) however measured high levels of primary production in both areas despite low (subnanomolar) iron concentrations. These results point to the importance of iron recycling (and potentially silicic acid recycling) for productivity, most probably through the action of zooplankton grazers. The low export efficiencies found in all locations from 234Th and sediment trap measurements (Puigcorbé, 2016, this issue; Roca-Martí, 2015, this issue), as well as individual measurements on faecal pellet production and sinking of one of the dominant zooplankton groups in the area (Salpa thompsoni, Iversen et al., 2016, this issue) supported this conclusion. Despite low export efficiencies, flux measurements obtained during Eddy-Pump are among the highest reported for open waters in the Southern Ocean (including results obtained from natural iron fertilization experiments, as discussed in detail by Puigcorbé et al. (2016) and Roca-Martí et al. (2015) both this issue) and found to also influence deep-sea microbial communities (Ruff et al., 2014), indicating the important role of the biological pump for the carbon cycle in the Atlantic sector of the Southern Ocean. Generally, mean abundance and biomass of macro-zooplankton increased from east to west (Pakhomov and Hunt, 2017, this issue; Pakhomov and Hunt, unpublished) from 293 to 788 ind.(1000 m)−3 and from 6.7 to 9.3 mg DW m−3, respectively. The highest densities were recorded in the chlorophyll-poor water at 52°S, 8°W, reaching 3261 ind.(1000 m)−3 and 21.9 mg DW m−3 mainly due to pelagic tunicates (Pakhomov and Hunt, 2017, this issue). While euphausiids, chaetognaths, amphipods and myctophids generally accounted on average for 50 to 80% by both numbers and mass, pelagic tunicates Salpa thompsoni were prominent components of the macroplankton community (Pakhomov and Hunt, 2017, this issue). Novel observations contributing to S. thompsoni ecology were carried out during the Eulerian study at the 12°W location. Rapid growth rates of salps were measured suggesting potentially fast turnover of their populations during the austral summer (Pakhomov and Hunt, 2017, this issue), further pointing to a stronger than expected biological pump in the Atlantic sector of the Southern Ocean. Yet, while fecal pellets produced by salps had high sinking speeds and low microbial degradation rates, only about one third and one tenth of the produced pellets were captured in sediment traps placed at 100 m and 300 m, respectively (Iversen et al., 2016, this issue). This may point to an important but extremely poorly quantified process leading to break-up and loosening of the pellets during their sinking in the top layers, possibly by the surface layer zooplankton (Iversen et al., 2016, this issue). 4. Conclusions The Eddy-Pump campaign of Polarstern cruise ANT-XVIII/3 enabled the collection of a suite of interrelated physical, chemical and biological variables from various biogeographic provinces, harbouring different phyto- and zooplankton assemblages, during one single austral summer in the Atlantic sector of the ACC. A remarkable result common to all investigated provinces is that light limitation due to deep mixing was not hampering primary production and the build-up of phytoplankton biomass in mixed layers reaching down to 100 m depth (Hoppe et al., 2015, this issue). Rather, in mixed layers shallower than 60 m an excess of light for the phytoplankton was suggested by production of photoprotective pigments (Cheah et al., 2016, this issue). The likely explanation for these observations is the low light adaptation of phytoplankton species in the ACC, caused by the wind driven deep mixed layers that prevail all year round around Antarctica. The different areas investigated during Eddy-Pump varied drastically in terms of mixed layer chlorophyll concentration, ranging between less than 0.1 and more than 2.5 mg m−3 (Fig. 1). Most striking is the chlorophyll concentration difference between the station at 52°S, 8°W and those adjacent to the west in the large-scale bloom area above the West Mid-Atlantic Ridge. The low chlorophyll concentrations at 52°S, 8°W coincide with the highest density of salps found during the cruise (Pakhomov and Hunt, 2017, this issue) suggesting that either the bloom accumulation was prevented by salps' grazing activities, or that this "low biomass" system was more conducive to salp development and growth. Lateral advection should also have played an essential role for the observed large-scale chlorophyll distribution. While one possible advection path to the bloom above the West Mid-Atlantic Ridge originates from the westward-located island of South Georgia and its iron-rich shelf, another source region for this bloom is also possible. Based on satellite maps of the large scale chlorophyll concentration, trajectories of drifters and floats, and general knowledge of the surface Ekman transport and drift patterns of sea ice and icebergs south of the ACC in the Atlantic sector of the Southern Ocean, Strass et al. (2016, this issue) argue that the bloom above the West Mid-Atlantic Ridge originated from the north-western Weddell Sea and likely was stimulated by iron released from melting ice. Salps together with other zooplankton were found to influence the downward transport of organic carbon in various, previously unknown ways (Iversen et al., 2016, this issue; Pakhomov and Hunt, 2017, this issue). Interactions between the zooplankton and the phytoplankton species assemblage evidently modified the downward carbon export flux from the upper 100 m, the export efficiency and the flux attenuation at greater depths (Puigcorbé et al., 2016, this issue; Roca-Martí et al., 2015, this issue). Due to the combined actions of the biological and physical carbon pumps, the ocean uptake of atmospheric CO2 was found to vary horizontally on scales of mesoscale eddies (Jones et al., 2015, this issue) and temporally between years (Tanhua et al., 2016, this issue). Taken together, the Eddy-Pump project provided valuable new insights on processes relevant for the carbon cycle in the Southern Ocean and, therefore, its effect on the future global climate development. Acknowledgements We are grateful for the support by Uwe Pahl, captain of RV Polarstern, and his crew. 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(this issue),Trans-Atlantic variability in ecology of the pelagic tunicate Salpa thompsoni near the Antarctic Polar Front. Puigcorbé, V., Roca-Martí, M., Masqué, P., Benitez-Nelson, C.R., vd Loeff, M.R., Laglera, L.M., Bracher, A., Cheah, W., Strass, V.H., Hoppema, M., Santos-Echeandía, J., Hunt, B.P.V., Pakhomov, E.A., Klaas, C., 2016. Particulate organic carbon export across the Antarctic Circumpolar Current at 10 E: differences between north and south of the Antarctic Polar Front. Deep Sea Res. Part II: Top. Stud. Oceanogr.. Roca-Martí, M., Puigcorbé, V., Iversen, M.H., van der Loeff, M.R., Klaas, C., Cheah, W., Bracher, A., Masqué, P., 2015. High particulate organic carbon export during the decline of a vast diatom bloom in the Atlantic sector of the Southern Ocean. Deep Sea Res. Part II: Top. Stud. Oceanogr.. Ruff, S.E., Probandt, D., Zinkann, A.C., Iversen, M.H., Klaas, C., Würzberg, L., Krombholz, N., Wolf-Gladrow, D., Amann, R., Knittel, K., 2014. Indications for algae-degrading benthic microbial communities in deep-sea sediments along the Antarctic Polar Front. Deep Sea Res. Part II: Top. Stud. Oceanogr. 108, 6–16. Sachs, O., Sauter, E.J., Schlüter, M., van der Loeff, M.M.R., Jerosch, K., Holby, O., 2009. Benthic organic carbon flux and oxygen penetration reflect different plankton provinces in the Southern Ocean. Deep Sea Res. Part I: Ocean. Res. Pap. 56 (8), 1319–1335. Sallée, J.B., Matear, R.J., Rintoul, S.R., Lenton, A., 2012. Localized subduction of anthropogenic carbon dioxide in the Southern Hemisphere oceans. Nat. Geosci. 5 (8), 579–584. Smetacek, V., Assmy, P., Henjes, J., 2004. The role of grazing in structuring Southern Ocean pelagic ecosystems and biogeochemical cycles. Antarct. Sci. 16 (4), 541–558. Smetacek, V., Klaas, C., Strass, V.H., Assmy, P., Montresor, M., Cisewski, B., Savoye, N., Webb, A., Arrieta, J.M., Bathmann, U., Bellerby, R., Berg, G.M., Croot, P., d'Ovidio, F., Gonzalez, S., Henjes, J., Herndl, G.J., Hoffmann, L.J., Leach, H., Losch, M., Mills, M.M., Neill, C., Peeken, I., Röttgers, R., Sachs, O., Sauter, E., Schmidt, M.M., Schwarz, J., Terbrüggen, A., Wolf-Gladrow, D., 2012. Deep carbon export from a Southern Ocean iron-fertilized plankton bloom. Nature 487, 313–319. http://dx.doi.org/10.1038/nature11229. Strass, V.H., Garabato, A.C.N., Pollard, R.T., Fischer, H.I., Hense, I., Allen, J.T., Read, J.F., Leach, H., Smetacek, V., 2002. Mesoscale frontal dynamics: shaping the environment of primary production in the Antarctic Circumpolar Current. Deep Sea Res. Part II: Top. Stud. Oceanogr. 49 (18), 3735–3769. Strass, V.H., Leach, H., Prandke, H., Donnelly, M., Bracher, A.U., Wolf-Gladrow, D.A., 2016. The physical environmental conditions for biogeochemical differences along the Antarctic Circumpolar Current in the Atlantic Sector during late austral summer 2012. Deep Sea Res. Part II: Top. Stud. Oceanogr.. Tanhua, T., Hoppema, M., Jones, E.M., Stöven, T., Hauck, J., González Dávila, M., Santana-Casiano, M., Álvarez, M., Strass, V.H., 2016. Temporal changes in ventilation and the carbonate system in the Atlantic sector of the Southern Ocean. Deep Sea Res. Part II: Top. Stud. Oceanogr.. Venables, H.J., Meredith, M.P., 2009. Theory and observations of Ekman flux in the chlorophyll distribution downstream of South Georgia. Geophys. Res. Lett. 36 (23). http:// dx.doi.org/10.1029/2009GL041371. Wallace, M.I., Cottier, F.R., Brierley, A.S., Tarling, G.A., 2013. Modelling the influence of copepod behaviour on faecel pellet export at high latitudes. Polar Biol. 36, 579–592. ⁎
Volker H. Strass Alfred-Wegener-Institut Helmholtz-Zentrum für Polar-und Meeresforschung (AWI), Germany E-mail address: [email protected] Dieter Wolf-Gladrow Alfred-Wegener-Institut Helmholtz-Zentrum für Polar- und Meeresforschung (AWI), Germany Evgeny A. Pakhomova,b Department of Earth, Ocean and Atmospheric Sciences, University of British Columbia, Vancouver, BC, Canada Institute for the Oceans and Fisheries, University of British Columbia, Vancouver, BC, Canada Christine Klaas Alfred-Wegener-Institut Helmholtz-Zentrum für Polar- und Meeresforschung (AWI), Germany
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Correspondence to: Alfred-Wegener-Institut Helmholtz-Zentrum für Polar-und Meeresforschung (AWI), Postfach 12 01 61, D-27515 Bremerhaven, Germany.
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Editorial
Eddy-Pump: Pelagic carbon pump processes along the eddying Antarctic Polar Front in the Atlantic Sector of the Southern Ocean 1. Introduction The Southern Ocean influences earth's climate in many ways. It hosts the largest upwelling region of the world oceans where ~80% of deep waters resurface (Morrison et al., 2015). A prominent feature is the broad ring of cold water, the Antarctic Circumpolar Current (ACC), which encircles the Antarctic continent and connects all other oceans. The ACC plays a major role in the global heat and freshwater transports and oceanwide cycles of chemical and biogenic elements, and harbours a series of unique and distinct ecosystems. Due to the upwelling of deep-water masses in the Antarctic Divergence, there is high supply of natural CO2 as well as macronutrients, leading to the worldwide highest surface nutrient concentrations. Despite the ample macronutrients supply, phytoplankton concentration is generally low, limited either by low micronutrient (iron) availability, insufficient light due to deep wind-mixed layers or grazing by zooplankton, or by the combination of all, varying temporally and regionally. In preindustrial times, the Southern Ocean might have been a source of atmospheric CO2 (Morrison et al., 2015). The contemporary Southern Ocean south of 30°S is currently considered a sink of CO2 mainly due to the increase in atmospheric concentrations from ~280 to over 400 ppm (parts per million). 40% of the anthropogenic CO2 taken up by the oceans entered via the region south of 40°S (Khatiwala et al., 2009). There are indications of a weakening of the CO2 sink in recent decades because of a stronger upwelling and associated outgassing of natural CO2 (Le Quéré et al., 2007), based on both atmospheric CO2 inverse and ocean biogeochemistry simulations. This trend was confirmed by an ocean circulation model with embedded biogeochemistry (e.g. Lovenduski et al., 2008) and by the trends in measured pCO2 (partial pressure of CO2) differences between the ocean and the atmosphere in the Indian sector of the Southern Ocean (Metzl, 2009). On the other hand, doubt arose with respect to the robustness of atmospheric inversion results and performance of the coarse ocean circulation models used, where eddy activity was not included (Law et al., 2008). Moreover, Böning et al. (2008) showed with observational data that the meridional overturning of the Southern Ocean is not very sensitive to the observed increased wind stress in the ACC, a mechanism which Le Quéré et al. (2007) and others held responsible for elevated upwelling and concomitant outgassing of natural CO2. Key to explain the difference in views is the eddy activity of the ACC which might increase along with a strengthened wind stress curl and so be able to counteract the wind-forced steepening of isopycnals (Böning et al., 2008; Meredith et al., 2012). The later finding of Landschützer et al. (2015) that the Southern Ocean carbon sink regained its strength between 2002 and 2012 supports the view of a rather dynamic ocean carbon cycle. Varying levels of eddy kinetic energy associated with changes in the wind forcing may not only affect the meridional overturning and thus the physical carbon pump but also the biological carbon pump. Mesoscale dynamics linked to eddies and front meanders shape the physical and chemical environment of primary production in several ways (Strass et al., 2002). It induces mesoscale upwelling that can supply micronutrients (iron) to the photic zone stimulating carbon fixation through photosynthesis (Hense et al., 2003). The cross-front circulation that connects mesoscale up- and downwelling cells also modifies the water column stratification, mixed layer depths and therefore the light environment in which phytoplankton can thrive. Moreover, the subduction that is linked to the cross-front circulation not only sequesters anthropogenic CO2 as dissolved inorganic carbon (Sallée et al., 2012) but also displaces phytoplankton to depth, possibly inducing its sinking as particulate organic carbon (Strass et al., 2002; Omand et al., 2015), and thus potentially enhances the biological carbon pump. But also the large-scale wind-driven upwelling can bring iron into the surface layer and stimulate phytoplankton growth and consequently the biological carbon pump south of the Antarctic Polar Front (APF), as indicated by a coupled ecosystem-general circulation model and regression analysis (Hauck et al., 2014). Productivity in the Southern Ocean varies not only across the ACC with its embedded fronts but also along the ACC axis. In the Atlantic sector of the Southern Ocean highest productivity is usually observed downstream of land masses, in particular the Patagonian shelf, the Antarctic Peninsula and South Georgia - as was also the case during Eddy-Pump (Fig. 1). The plume of high productivity observed downstream of South Georgia extends eastward along the meandering Polar Front where longitudinal bands with high chlorophyll content can also be observed. Benthic oxygen measurements have revealed that both areas show very high rates of carbon export reaching the sediment surface (Sachs et al., 2009). Significant differences between these two regions, however, have been observed both in the magnitude, inter-annual variability and composition of benthic fluxes as well as in chlorophyll a concentrations and sediment composition (Abelmann et al., 2006; Sachs et al., 2009; Venables and Meredith 2009). These differences have been attributed to differences in magnitude and mode of iron supply (Venables and Meredith, 2009). Apart from its physical and chemical drivers, the biologically mediated carbon export critically depends on phytoplankton species assemblages and food web structure (e.g. Longhurst and Harrison, 1988, Smetacek et al., 2004; Assmy et al., 2013; Wallace et al., 2013). Dominance of phytoplankton and zooplankton key species and food webs in the ACC change strongly with the meridional gradients across fronts but also zonally along the ACC (Abelmann et al., 2006, http://atlas.biodiversity.aq/). The relationship between plankton composition, productivity and the http://dx.doi.org/10.1016/j.dsr2.2017.02.009
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Fig. 1. Upper panel: Track of Polarstern cruise ANT-XXVIII/3 ‘Eddy-Pump’, superimposed on a map of the chlorophyll a (Chla) concentration monthly composite of February, 2012, obtained from the Ocean Colour Climate Change Initiative dataset, Version 3, European Space Agency (http://www.esa-oceancolour-cci.org/). Lower panels: Chlorophyll a concentrations measured underway during ANT-XXVIII/3.
magnitude of POC export is variable, and the processes responsible for this variability are still poorly understood and quantified. The Eddy-Pump program was designed to identify and better describe the interaction of physical, biological and chemical processes and their combined effects on the carbon flux to the ocean interior and deep sea. With the overarching common goal of improving the current understanding of the Southern Ocean's role in the global climate system, Eddy-Pump integrated the specific disciplines and relevant fields of work. To achieve its goal, Eddy-Pump followed a dedicated survey strategy, concentrating on areas of contrasting environmental conditions along the ACC focussing on evidently most relevant but least understood processes, and applying state-of-the-art methods. 2. Cruise itinerary Following its research strategy, Eddy-Pump concentrated on the band of latitudes between 50°S and 60°S where the upwelled deep-water masses interact with the atmosphere before they are subsequently subducted at the Sub-Antarctic Front. To reveal the effect of carbon pump processes during a significant part of the growth season, process studies were conducted for days to weeks within eddies or meanders, which were tracked before and during the cruise using satellite remote sensing of sea surface height anomalies and ocean colour. The study sites were selected to represent different productivity regimes presumed to be dominated by different plankton assemblages and with contrasting export fluxes. The data for Eddy-Pump were collected 2012 during Polarstern cruise ANT-XXVIII/3, which started January 7 in Cape Town (Fig. 1). The route from Cape Town led southwest until 44°S and 10°E. A transect with hydrographic stations was conducted along 10°E until 53°S, thereby crossing the Subantarctic Front, the Antarctic Polar Front and the Southern Polar Front. Passing stations at 52°S, 8°W in chlorophyll-poor water (Fig. 1) and 52°S, 12°W, the route led to the region around 51°12'S, 12°40'W above the topographic structure of the West Mid-Atlantic Ridge, where we sampled a grid of stations within a phytoplankton bloom for more than 2 weeks. After a short visit of the British Antarctic Survey station on South Georgia we steamed to the Georgia Basin in the northwest of that island. In the region around 50°S, 38°W a grid of 30 stations at intervals of 24 nautical miles was carried out. The station work was finished at March 7 and Polarstern sailed to Punta Arenas, Chile, where the cruise ended on March 11, 2012 (Fig. 1). 2
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3. Major findings The physical environment encountered during Eddy-Pump is described by Strass et al. (2016, this issue), who used measurements of pressure, temperature and conductivity and the derived variables salinity and density, of mixed layer turbulence parameters, horizontal current vectors as well as dynamic heights and flow trajectories obtained from surface drifters and submerged floats. This information provides the background for the analyses of biological and chemical processes and of biogeochemical fluxes addressed by other papers in this issue. The section along 10°E between 44°S and 53°S showed a classical ACC structure with well-known hydrographic fronts: the Subantarctic Front (SAF) at 46.5°S, the Antarctic Polar Front (APF) split in two, at 49.25°S and 50.5°S, and the Southern Polar Front (SPF) at 52.5°S. Each front was associated with strong eastward flows, with weaker or even reversed flows interspersed between. The West Mid-Atlantic Ridge Survey, centered around 51°12'S, 12°40'W, showed a weak and poorly resolved meander structure between the APF and the SPF. During the first eight days of the survey the oceanographic conditions at the Central Station at 12°40’W remained reasonably constant. However after that, conditions became more variable in the thermocline with conspicuous temperature inversions and inter-leavings accompanied by increased currents, and a decrease in temperature in the surface layer. At the very end of the period of observation the conditions in the thermocline returned to being similar to those observed during the early part of the period with however the higher mixed layer temperature. The Georgia Basin Survey showed a very strong zonal jet at its northern edge that connected to a large cyclonic meander which joined an anticyclonic eddy in the southeastern quadrant. The water mass contrasts in this survey were stronger than in the West Mid-Atlantic Ridge Survey, but similar to those met along 10°E with the exception that the warm and saline surface water typical of the northern side of the SAF was not covered by the Georgia Basin Survey. The trajectories of surface buoys and subsurface drifters following the large-scale flow through the Atlantic sector revealed a northward deflection of the ACC when crossing the Mid-Atlantic ridge, superimposed by mesoscale eddies. Mixed layers found during Eddy-Pump were typically deep, but varied between the three survey areas; the mean depths and standard variations of the mixed layer along the 10°E section were 77.2 ± 24.7 m, at the West Mid-Atlantic Ridge 66.7 ± 17.7 m, and in the Georgia Basin 36.8 ± 10.7 m. Using transient tracer and carbonate system data collected along the 10°E transect compared with historic cruises in the same area, Tanhua et al. (2016, this issue) analyzed temporal changes in ventilation and in inorganic carbon storage in the southeast Atlantic sector of the Southern Ocean. The transient tracer data suggest a significant speed-up of ventilation in the summer-warmed upper part of Antarctic Intermediate Water (AAIW) between 1998 and 2012, which is consistent with the high storage rate of anthropogenic carbon Cant. A shift of the more prevalent northern Cant storage to more southern storage in near surface waters was detected in the early 2000s. Beneath the AAIW the extended multiple linear regression (eMLR) method did not reveal significant storage of Cant. However, the presence of the transient tracer CFC-12 through the whole water column suggests that some Cant should be present, but at concentrations not reliably quantifiable. The observed temporal variability in the ocean interior seems at a first glance to be out of phase with observed surface ocean Cant fluxes, but this can be explained by the time delay for the surface ocean signal to reach the interior of the ocean. Jones et al. (2015, this issue) investigated the influence of eddy structures on the seasonal depletion of dissolved inorganic carbon (DIC) and on carbon dioxide (CO2) disequilibrium. This study focused on the highly dynamic northern edge of the Georgia Basin, which is impacted by both the Subantarctic Front (SAF) and Antarctic Polar Front (APF). Cyclonic cold core as well as anticyclonic warm core eddy structures were found to be hotspots of carbon uptake relative to the rest of the ACC surveyed during Eddy-Pump. Carbon uptake was highest in the cold core where greatest CO2 undersaturation (−78 μatm) and substantial surface ocean DIC deficit (5.1 mol m−2) occurred. In the presence of high wind speeds, the cold core eddy acted as a strong sink (25.5 mmol m−2 day−1) for atmospheric CO2. Waters of the warm core displayed characteristics of the Polar Frontal Zone (PFZ), with enhanced CO2 undersaturation (−59 μatm) and depletion of DIC (4.9 mol m−2). A mechanism proposed for the enhanced carbon uptake across both eddy structures is based on the Ekman eddy pumping theory. First, the cold core is seeded with productive (high chlorophyll-a) waters from the Antarctic Zone and biological productivity is sustained through upwelled nutrients that counteracts DIC inputs from deep waters. Second, horizontal entrainment of due to biological uptake low DIC surface waters from the PFZ, which downwell within the warm core, cause relative DIC depletion in the upper water column. The observations suggest that the formation and northward propagation of cold core eddies in the region of the APF could transfer low DIC waters towards the site of Antarctic Intermediate Water formation and enhance CO2 drawdown into the deep ocean. Sampling during Eddy-Pump also provided first insights into the concentrations of mercury (Hg) species in the Atlantic sector of the Southern Ocean (Canário et al., 2016, this issue). Results showed high spatial variability in the concentrations of total mercury (HgT) and methylmercury (MeHg). HgT (0.93 ± 0.69 ng L−1) and MeHg (0.26 ± 0.12 ng L−1) levels were similar or higher than those reported in previous works in high latitude studies. The highest values were found at a location (53°S, 10°E) south of the South Polar Front, an area with strong currents and horizontal gradients between different water masses. Vertical profiles showed a great variability even for stations sampled at the same location or area dominated by the same oceanographic features. A decrease of HgT accompanied by an increase in MeHg with depth was observed in some sites, suggesting the occurrence of an Hg methylation process, while at other stations a concurrent decrease or increase of both mercury species was observed. In spite of these differences, an overall positive correlation between HgT and MeHg was found. Differences between vertical profiles of Hg species were attributed to favourable environmental conditions for Hg methylation. The highest proportion of MeHg (% of HgT) was observed at sites with low dissolved oxygen or higher estimated remineralization rates. However, the concentrations of MeHg in these areas are more dependent on the environmental conditions than on the total concentration of Hg present in the water. During Eddy-Pump several zones with high phytoplankton biomass and productivity were investigated (Fig. 1). From North to South along 10°E two phytoplankton blooms, defined by chlorophyll a (Chla) concentrations above 1 mg m−3, were crossed at 49.7°S and between 50.3 and 51.3°S in the Antarctic zone (Cheah et al., 2016, this issue, Puigcorbé et al., 2016, this issue). More detailed process studies were carried out further west in another large-scale bloom around 12°W and north of the Georgia Basin around 39°W (Fig. 1; Hoppe et al., 2015, this issue; Roca-Martí et al., 2015, this issue; Iversen et al., 2016, this issue). Despite differences in hydrography and iron supply (Hoppe et al., 2015, this issue) as well as phytoplankton assemblage (Cheah et al., 2016, this issue; Klaas unpublished) for these different locations, results from primary production measurements (Hoppe et al., 2015, this issue) and bio-optical parameters measurements (Cheah et al., 2016, this issue) provided similar results. An important finding was the positive relationship between depth-integrated phytoplankton biomass, productivity and the mixed layer depth (Hoppe et al., 2015, this issue; Cheah et al., 2016, this issue). These results imply that biomass accumulation during diatom-dominated blooms in the Antarctic zone (Cheah et al., 2016, this issue; Klaas et al., unpublished) can occur despite deep mixed layer depths (70 to 100 m depth) as previously observed during an artificial iron fertilization experiment in the region (Smetacek et al., 2012). This was further confirmed by bio-optical and photoprotective pigment measurements (Cheah et al., 2016, this issue) showing that the phytoplankton communities found were shade adapted. 3
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Differences in light regime might, however, influence bloom temporal dynamics (Hoppe et al., 2015, this issue). Satellite imagery indicated that our study took place in the late stage of the bloom both at 12°W and at 39°W (Hoppe et al., 2015, this issue, Puigcorbé et al., 2016, this issue; RocaMartí et al., 2015, this issue). Hoppe et al. (2015, this issue) however measured high levels of primary production in both areas despite low (subnanomolar) iron concentrations. These results point to the importance of iron recycling (and potentially silicic acid recycling) for productivity, most probably through the action of zooplankton grazers. The low export efficiencies found in all locations from 234Th and sediment trap measurements (Puigcorbé, 2016, this issue; Roca-Martí, 2015, this issue), as well as individual measurements on faecal pellet production and sinking of one of the dominant zooplankton groups in the area (Salpa thompsoni, Iversen et al., 2016, this issue) supported this conclusion. Despite low export efficiencies, flux measurements obtained during Eddy-Pump are among the highest reported for open waters in the Southern Ocean (including results obtained from natural iron fertilization experiments, as discussed in detail by Puigcorbé et al. (2016) and Roca-Martí et al. (2015) both this issue) and found to also influence deep-sea microbial communities (Ruff et al., 2014), indicating the important role of the biological pump for the carbon cycle in the Atlantic sector of the Southern Ocean. Generally, mean abundance and biomass of macro-zooplankton increased from east to west (Pakhomov and Hunt, 2017, this issue; Pakhomov and Hunt, unpublished) from 293 to 788 ind.(1000 m)−3 and from 6.7 to 9.3 mg DW m−3, respectively. The highest densities were recorded in the chlorophyll-poor water at 52°S, 8°W, reaching 3261 ind.(1000 m)−3 and 21.9 mg DW m−3 mainly due to pelagic tunicates (Pakhomov and Hunt, 2017, this issue). While euphausiids, chaetognaths, amphipods and myctophids generally accounted on average for 50 to 80% by both numbers and mass, pelagic tunicates Salpa thompsoni were prominent components of the macroplankton community (Pakhomov and Hunt, 2017, this issue). Novel observations contributing to S. thompsoni ecology were carried out during the Eulerian study at the 12°W location. Rapid growth rates of salps were measured suggesting potentially fast turnover of their populations during the austral summer (Pakhomov and Hunt, 2017, this issue), further pointing to a stronger than expected biological pump in the Atlantic sector of the Southern Ocean. Yet, while fecal pellets produced by salps had high sinking speeds and low microbial degradation rates, only about one third and one tenth of the produced pellets were captured in sediment traps placed at 100 m and 300 m, respectively (Iversen et al., 2016, this issue). This may point to an important but extremely poorly quantified process leading to break-up and loosening of the pellets during their sinking in the top layers, possibly by the surface layer zooplankton (Iversen et al., 2016, this issue). 4. Conclusions The Eddy-Pump campaign of Polarstern cruise ANT-XVIII/3 enabled the collection of a suite of interrelated physical, chemical and biological variables from various biogeographic provinces, harbouring different phyto- and zooplankton assemblages, during one single austral summer in the Atlantic sector of the ACC. A remarkable result common to all investigated provinces is that light limitation due to deep mixing was not hampering primary production and the build-up of phytoplankton biomass in mixed layers reaching down to 100 m depth (Hoppe et al., 2015, this issue). Rather, in mixed layers shallower than 60 m an excess of light for the phytoplankton was suggested by production of photoprotective pigments (Cheah et al., 2016, this issue). The likely explanation for these observations is the low light adaptation of phytoplankton species in the ACC, caused by the wind driven deep mixed layers that prevail all year round around Antarctica. The different areas investigated during Eddy-Pump varied drastically in terms of mixed layer chlorophyll concentration, ranging between less than 0.1 and more than 2.5 mg m−3 (Fig. 1). Most striking is the chlorophyll concentration difference between the station at 52°S, 8°W and those adjacent to the west in the large-scale bloom area above the West Mid-Atlantic Ridge. The low chlorophyll concentrations at 52°S, 8°W coincide with the highest density of salps found during the cruise (Pakhomov and Hunt, 2017, this issue) suggesting that either the bloom accumulation was prevented by salps' grazing activities, or that this "low biomass" system was more conducive to salp development and growth. Lateral advection should also have played an essential role for the observed large-scale chlorophyll distribution. While one possible advection path to the bloom above the West Mid-Atlantic Ridge originates from the westward-located island of South Georgia and its iron-rich shelf, another source region for this bloom is also possible. Based on satellite maps of the large scale chlorophyll concentration, trajectories of drifters and floats, and general knowledge of the surface Ekman transport and drift patterns of sea ice and icebergs south of the ACC in the Atlantic sector of the Southern Ocean, Strass et al. (2016, this issue) argue that the bloom above the West Mid-Atlantic Ridge originated from the north-western Weddell Sea and likely was stimulated by iron released from melting ice. Salps together with other zooplankton were found to influence the downward transport of organic carbon in various, previously unknown ways (Iversen et al., 2016, this issue; Pakhomov and Hunt, 2017, this issue). Interactions between the zooplankton and the phytoplankton species assemblage evidently modified the downward carbon export flux from the upper 100 m, the export efficiency and the flux attenuation at greater depths (Puigcorbé et al., 2016, this issue; Roca-Martí et al., 2015, this issue). Due to the combined actions of the biological and physical carbon pumps, the ocean uptake of atmospheric CO2 was found to vary horizontally on scales of mesoscale eddies (Jones et al., 2015, this issue) and temporally between years (Tanhua et al., 2016, this issue). Taken together, the Eddy-Pump project provided valuable new insights on processes relevant for the carbon cycle in the Southern Ocean and, therefore, its effect on the future global climate development. Acknowledgements We are grateful for the support by Uwe Pahl, captain of RV Polarstern, and his crew. We thank Tilman Dinter for providing satellite-based ocean colour graphs of the Atlantic sector of the Southern Ocean during our cruise; ocean colour information has been extremely helpful for making decisions about the exact cruise track during our expedition. References Abelmann, A., Gersonde, R., Cortese, G., Kuhn, G., Smetacek, V., 2006. Extensive phytoplankton blooms in the Atlantic sector of the glacial Southern Ocean. Paleoceanography 21 (1). Assmy, P., Smetacek, S., Montresor, M., Klaas, C., Henjes, J., Strass, V.H., Arrieta, J.M., Bathmann, U., Berg, G.M., Breitbarth, E., Cisewski, B., Friedrichs, L., Fuchs, N., Herndl, G.J., Jansen, S., Krägefsky, S., Latasa, M., Peeken, I., Röttgers, R., Scharek, R., Schüller, S.E., Steigenberger, S., Webb, A., Wolf-Gladrow, D., 2013. Thick-shelled, grazer-protected diatoms decouple ocean carbon and silicon cycles in the iron-limited Antarctic Circumpolar Current. PNAS, (www.pnas.org/cgi/doi/10.1073/pnas.1309345110). Böning, C.W., Dispert, A., Visbeck, M., Rintoul, S.R., Schwarzkopf, F.U., 2008. The response of the Antarctic Circumpolar Current to recent climate change. Nat. Geosci. 1 (12), 864–869. Canário, J., Santos-Echeandia, J., Padeiro, A., Amaro, E., Strass, V., Klaas, C., Hoppema, M., Ossebaar, S., Koch, B.P., Laglera, L.M., 2016. Mercury and methylmercury in the Atlantic
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Volker H. Strass Alfred-Wegener-Institut Helmholtz-Zentrum für Polar-und Meeresforschung (AWI), Germany E-mail address: [email protected] Dieter Wolf-Gladrow Alfred-Wegener-Institut Helmholtz-Zentrum für Polar- und Meeresforschung (AWI), Germany Evgeny A. Pakhomova,b Department of Earth, Ocean and Atmospheric Sciences, University of British Columbia, Vancouver, BC, Canada Institute for the Oceans and Fisheries, University of British Columbia, Vancouver, BC, Canada Christine Klaas Alfred-Wegener-Institut Helmholtz-Zentrum für Polar- und Meeresforschung (AWI), Germany
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Correspondence to: Alfred-Wegener-Institut Helmholtz-Zentrum für Polar-und Meeresforschung (AWI), Postfach 12 01 61, D-27515 Bremerhaven, Germany.
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