diSTriBuTiONS aNd air-Sea FluxeS OF CarBON diOxide

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The Ocean Carbon Cycle in the Western arctic Ocean aBSTr aCT.The Arctic Ocean is a potentially important sink for atmospheric carbon dioxide (CO 2 ) with a recent estimate suggesting that the region contributes from 5 to 14% of the global ocean's net uptake of CO 2 .In the western Arctic Ocean, the focus of this paper, the Chukchi Sea is a strong ocean sink for CO 2 that is partially compensated for by outgassing of CO 2 from the East Siberian Sea shelf.
The Arctic marine carbon cycle and exchange of CO 2 between the ocean and atmosphere appear particularly sensitive to environmental changes, including sea ice loss, warming, changes in seasonal marine phytoplankton primary production, changes in ocean circulation and freshwater inputs, and even the impacts of ocean acidification.In the near term, further sea ice loss, increases in phytoplankton growth rates, and other environmental and physical changes in the Arctic are expected to cause a limited net increase in the uptake of CO 2 by Arctic surface waters.Recent studies suggest that this enhanced uptake will be short lived, with surface waters rapidly warming and equilibrating with the atmosphere.Furthermore, release of large stores of carbon from the surrounding Arctic landmasses through rivers into the Arctic Ocean and further warming over the next century may alter the Arctic from a CO 2 sink to a source over the next century.Serreze and Francis, 2006), seasonal sea ice loss (e.g., Maslanik et al., 2007;Wang and Overland, 2009), and other physical and biological transformations in the terrestrial and marine realms of the Arctic (Wu et al., 2005;McGuire et al., 2006McGuire et al., , 2009)).The Arctic Ocean is also sensitive to atmosphereocean-sea ice forcing and feedbacks and ecosystem transitions associated with warming temperatures and sea ice loss (e.g., Arrigo et al., 2008;Pabi et al., 2008).Because of these rapid environmental changes, the Arctic marine carbon cycle will likely enter a highly dynamic state in the coming decades, with large uncertainties in the exchange of atmosphere-ocean CO 2 (Anderson and Kaltin, 2001;Bates et al., 2006a;Bates and Mathis, 2009;Cai et al., 2010;Jutterström and Anderson, 2010) in response to sea ice loss and other climate-change-induced processes.
Furthermore, the Arctic marine carbon cycle and marine ecosystems are also vulnerable to ocean acidification that results from the uptake of anthropogenic CO 2 from the atmosphere (Orr et al., 2005;Steinacher et al., 2009;Bates et al., 2009;Yamamoto-Kawai et al., 2009).
In this article, we review the present However, the presence of multiyear ice in the central basin and thinner seasonal sea ice (1 to 2 m) across the Arctic shelves has declined dramatically since the 1990s and in particular since 2007 (Comiso et al., 2008).Thus, sea ice loss reinforces surface warming due to reduced surface reflectivity and increased heat absorption (Perovich et al., 2007), which in turn impact Arctic Ocean chemistry and biology.
The expansive coastal seas of the Arctic Ocean (e.g., Barents, Laptev, Kara, East Siberian, Chukchi, and Beaufort Seas) comprise approximately 53% of its total area (Macdonald et al., 2010) and completely surround a deep central basin (Eurasian and Canada Basins;  et al., 1975;Roach et al., 1995;Woodgate et al., 2005).The physics and biogeochemistry of the Chukchi Sea is highly influenced by this inflow, and its shelf can be characterized as an "inflow" shelf (Carmack and Wassmann, 2006;Bates and Mathis, 2009) (Carmack and Wassmann, 2006), including: "inflow" shelves such as the Chukchi and Barents Seas, "interior" shelves such as the Siberian and Beaufort Seas, and "outflow" shelves (i.e., Canadian archipelago).Nitishinsky et al., 2007).The Beaufort Sea exhibits a general flow eastward along the shelf from the Chukchi Sea and considerable shelf-basin exchanges (e.g., Mathis et al., 2007a).On both of these "interior shelves, " freshwater inputs (from the Mackenzie River into the Beaufort Sea and the Kolyma River into the East Siberian Sea), seasonal sea ice melt and formation, and brine rejection in coastal polynyas are important processes (e.g., Dmitrenko et al., 2008) with high rates of sea ice production in the East Siberian Sea as well as the Laptev Sea (Eicken et al., 2000) compared to other Arctic Ocean shelves.
In the central basin of the western Arctic Ocean, surface waters of the Canada Basin or Beaufort Gyre are influenced by shelf-basin exchanges of water while subsurface waters are relatively isolated from surface waters due to sharp density stratification with depth (e.g., Jones and Anderson, 1986;Wallace et al., 1987).Thus, environmental change due to warming, sea ice loss, and other processes mostly affects surface waters rather than the deep, isolated subsurface waters in the central basin.

The arCTiC mariNe CarBON CYCle
Seawater connections with subarctic entire Arctic Ocean, though there are large uncertainties in these estimates (Macdonald et al., 2010).The Arctic landmasses contain even larger stores of carbon compared to the marine environment, and there are significant river inputs of organic carbon to the Arctic shelves (e.g., Lobbes et al., 2000;Amon, 2004;Rachold et al., 2004;Guo and Macdonald, 2006;Raymond et al., 2007;Holmes et al., 2011).Pan-Arctic river inputs of carbon have been estimated by McGuire et al. (2009) at 33 Tg C yr -1 of DOC and 43.2 Tg C yr -1 DIC, which are 7.1% and 10.6% of their respective total global river fluxes (Cai, 2011).
River inputs of POC and coastal erosion of terrestrial carbon (containing both refractory and labile organic carbon) have been estimated at ~ 12 Tg C yr -1 (e.g., Rachold et al., 2004;Macdonald et al., 2010), at least for the present.Even with transpolar surveys across the deep basin (e.g., Arctic Ocean Section of 1994 [Jutterström and Anderson, 2005;Jones et al., 2008;Tanhua et al., 2009;Jutterström and Anderson, 2010]) and shelf projects such as the Shelf-Basin Interactions (SBI II) program (Grebmeier et al., 2008)

mariNe eCOSYSTemS aNd OrgaNiC CarBON OF The WeSTerN arCTiC
In the western Arctic, the different physical setting of each shelf strongly influences its biology.The inflow of nutrient-rich seawater from the Pacific Ocean into the Chukchi Sea (Codispoti et al., 2005), coupled with abundant light and the seasonal retreat and melting of sea ice, supports a brief, but intensive, period of marine phytoplankton photosynthesis and growth compared to other Arctic Ocean shelves where nutrients are limited (Cota et al., 1996;Hill and Cota, 2005).As the foundation for supporting the pelagic/benthic food web, rates of phytoplankton primary production on the Chukchi Sea shelf can be ≥ 300 g C m 2 yr -1 or 0.3-2.8g C m 2 d -1 (e.g., Hameedi, 1978;Cota et al., 1996;Gosselin et al., 1997;Hill and Cota, 2005;Bates et al., 2005a;Mathis et al., 2009;Macdonald et al., 2010).Sea ice algal communities also contribute substantively to early season primary production (e.g., Legendre et al., 1992;Gosselin et al., 1997), with springtime production rates in the Chukchi Sea estimated at ~ 1-2 g C m 2 d -1 (Gradinger, 2009).Intense seasonal growth of marine phytoplankton supports a large zooplankton (e.g., shrimp, copepods) biomass that in turn supports diverse open-water and seafloor ecosystems (Feder et al., 2005;Grebmeier et al., 2008).Both pelagic and benthic ecosystems on the Chukchi Sea shelf support marine mammal (e.g., gray whale, walrus, polar bear), seabird, and human populations in the region.
In the Chukchi Sea, the brief period of high rates of marine phytoplankton primary production results in the formation of high concentrations of suspended POC (sPOC; Bates et al., 2005b) and export of organic carbon to the subsurface and benthos (Moran et al., 2005;Lepore et al., 2007).
During the SBI project in the early  et al., 2005;Lepore et al., 2007;Belicka and Harvey, 2009).In contrast, little seasonal accumulation of DOC due to phytoplankton primary production has been observed (e.g., Davis and Benner, 2005;Mathis et al., 2007b) from springtime to summertime during sea ice retreat (Figure 3).This marine ecosystem appears dominated by large-sized phytoplankton (e.g., diatoms; Grebmeier et al., 2008) that produce a relatively large-size class of organic matter (i.e., as POC rather than DOC).In contrast, in other marine ecosystems dominated by small phytoplankton (i.e., picoplankton) such as the subtropical North Atlantic Ocean, a much larger fraction of DOC is produced seasonally compared to POC (e.g., Carlson et al., 1994).
In the central basins of the Arctic, surface waters are mostly covered by sea ice and have very low nutrient concentrations.As a result, surface waters of the Canada Basin have very low rates of marine phytoplankton growth (e.g., English, 1961;Wheeler et al., 1996;Gosselin et al., 1997;Moran et al., 1997;Pomeroy, 1997;Anderson et al., 2003) and relatively low vertical export of organic matter to the deep seafloor (e.g., Moran et al., 1997;Wassmann et al., 2004;Honjo et al., 2010).Differences in the availability of nutrients, productivity, and sources of organic  The data are plotted using Ocean data View (Schlitzer, 2005).
There are numerous physical and biological controls on the marine carbon cycle with complex interactions between them.Arguably, the most important processes include seasonal cooling and warming of surface waters, exchange of carbon with other basins and shelves, phytoplankton primary production, air-sea transfer of CO 2 , sea ice processes, and inputs of freshwater and terrestrial carbon (Bates and Mathis, 2009).At the air-sea interface, sea ice cover has generally been thought to be a barrier to gas exchange, although there may be minor exchanges in leads and diffusion through the ice (e.g., Gosink et al., 1976;Semiletov et al., 2004;Delille et al., 2007;Nagurnyi, 2008).Recent studies, however, suggest that the exchange of CO 2 through sea ice is much greater than previously thought, with significant release and uptake of CO 2 depending on season and sea ice condition (Rysgaard et al., 2007;Miller et al., 2011;Papakyriakou and Miller, 2011).Within the water column, carbon export via brine rejection during sea ice formation, shelf-basin exchanges of carbon, vertical diffusion, entrainment and detrainment through mixing, vertical export of organic carbon, and remineralization of organic matter to CO 2 in shelf and subsurface waters and sediments are also important processes.
The seasonal changes in DIC have been largely attributed to high rates of summertime phytoplankton primary production or net community production, especially in the vicinity of Barrow Canyon at the northern edge of the Chukchi Sea shelf (Bates et al., 2005a;Hill and Cota, 2005;Mathis et al., 2007b).In summary, seasonal changes in surface pCO 2 on the Chukchi Sea shelf have been largely attributed to cooling of surface waters during the northward transit of waters across the Chukchi Sea shelf (Murata and Takizawa, 2003) and high rates of summertime phytoplankton primary production that act to decrease seawater DIC and pCO 2 (Bates, 2006).These processes produce a dynamic shelf-to-basin carbon pump (Bates, 2006;Anderson et al., 2010).The seasonal rebound of seawater pCO 2 and DIC during wintertime likely results from a continued uptake of CO 2 through gas exchange during sea ice formation and brine rejection (Anderson et al., 2004;Omar et al., 2005), continued transport of Pacific Ocean waters into the Chukchi Sea through Bering Strait, and vertical entrainment by mixing with CO 2 -rich subsurface waters.

In the East Siberian and Beaufort
Seas, surface water pCO 2 conditions appear highly variable during the sea ice-free period.In the East Siberian Sea shelf (~ 300-500 µatm), surface waters close to or above atmospheric values have been reported (Semiletov et al., 1999(Semiletov et al., , 2007;;Pipko et al., 2008), with much higher values near the outflow of the Kolyma River (~ 500 µatm) that drains into the East Siberian Sea shelf (Semiletov et al., 1999(Semiletov et al., , 2007)).
Furthermore, very high values (~ 500 to ~ 1500 µatm) have been observed in bottom waters of the inner shelf and also in the nearshore bays (e.g., Tiksi Bay) and estuaries of the East Siberian Sea (Semiletov et al., 1999(Semiletov et al., , 2007)).We have also observed seawater pCO 2 values close to equilibrium with the atmosphere in the East Siberian Sea (Figure 5).The high seawater pCO 2 values can be attributed primarily to the remineralization of organic matter introduced from the Siberian Rivers (e.g., Anderson et al., 1990;Cauwet and Sidorov, 1996;Kattner et al., 1999), given that there are low rates of summertime phytoplankton primary production (~ 6-12 g C m -2 yr -1 ; Macdonald et al., 2010).In the western Chukchi Sea near Long Strait, summertime seawater pCO 2 conditions were observed to be close to equilibrium with the atmosphere (Fransson et al., 2009; also Figure 5), presumably reflecting outflow of  1990).The data are plotted using Ocean data View (Schlitzer, 2005).
In the eastern Beaufort Sea shelf, summertime surface seawater pCO 2 values were generally low (Mucci et al., 2008) or close to equilibrium with the atmosphere, particularly in the vicinity of the Banks Island polynya (Fransson et al., 2009).The data are plotted using Ocean data View (Schlitzer, 2005).
Oceanography | September 2011 195 air-Sea CO 2 FluxeS iN The WeSTerN arCTiC OCeaN The exchange of gases such as CO 2 between the atmosphere and the ocean is primarily controlled by gas concentration differences between air and sea (i.e., air-sea CO 2 disequilibrium or ΔpCO 2 ) and by turbulence in the lower atmosphere (which is commonly parameterized as a function of wind speed; see Wanninkhof, 1992).In the earliest study of the inorganic carbon cycle in the Arctic Ocean, Kelley (1970) observed that surface waters in the Barents Sea had lower pCO 2 values than the atmosphere.In the last two decades, more precise and accurate carbon data have been collected in the Arctic Ocean, allowing better assessments of its sink or source status.
In the Chukchi Sea, early season observations under near complete sea ice cover also indicate that Chukchi Sea shelf "winter" surface waters were not as undersaturated with respect to the atmosphere (ΔpCO 2 values of ~ -20 to -60 µatm, with negative values indicating direction of gas exchange toward the ocean) compared to the summertime sea ice-free period (Bates, 2006).In contrast, summertime ΔpCO 2 values are typically in the range of -50 to -200 µatm.Previous estimates of the rates of air-sea CO 2 exchange during the sea ice-free period in the summertime have ranged from ~ -20 to -90 mmol CO 2 m -2 d -1 (Wang et al., 2003;Murata and Takizawa, 2003;Bates, 2006;Fransson et al., 2009), indicating that the surface waters of the Chukchi Sea shelf have the potential to be a strong sink of atmospheric CO 2 (Kaltin et al., 2002), similar to the Barents Sea shelf (Kelley, 1970;Fransson et al., 2001;Kaltin and Anderson, 2005).
However, in regions of the Chukchi Sea shelf where sea ice cover remained high (> 80%) during the summertime, air-sea CO 2 exchange rates were estimated to be generally low (i.e., ocean uptake of < 1 mmol CO 2 m -2 d -1 ; Bates, 2006), while wintertime air-sea CO 2 exchange rates (during complete sea ice cover) were estimated to be minor (i.e., ocean uptake of < 1 mmol CO 2 m -2 d -1 ; Bates, 2006) as sea ice coverage greatly reduces air-sea gas exchange.The annual ocean CO 2 uptake for the Chukchi Sea shelf has been estimated at 2-9 mmol C m -2 yr -1 (Kaltin and Anderson, 2005;Bates, 2006), or approximately 11-53 Tg C yr -1 (Table 1 (Mucci et al., 2008;Fransson et al., 2009, ~ 6  waters.Thus, enhanced uptake of CO 2 through sea ice loss (e.g., Bates et al., 2006a;Jutterström and Anderson, 2010) into newly exposed surface waters of the central basin is likely to be a very short-term phenomenon, and surface waters appear to be rapidly equilibrating with the atmosphere (Cai et al., 2010).
In addition, it should be noted that the

OCeaN aCidiFiCaTiON impaCTS iN The WeSTerN arCTiC OCeaN
The decrease in seawater pH due to the uptake of anthropogenic CO 2 (Bindoff et al., 2007;Bates, 2007) has been termed by the uptake of anthropogenic CO 2 over the last century (Bates et al., 2009).Given the scenarios for pH changes in the Arctic, the Arctic Ocean, and adjacent Arctic shelves, including the western Arctic, will be increasingly affected by ocean acidification, with potentially negative implications for shelled benthic organisms as well as those animals that rely on the shelf seafloor ecosystem.

CONCluSiONS
The continental shelves and central basin of the Pacific sector of the Arctic Ocean generally have lower surface CO 2 content than the atmosphere.At present, although seasonal sea ice cover provides a barrier to atmosphere-ocean gas exchange, the Arctic Ocean is a sink for CO 2 , taking up about 65 to 175 Tg of carbon per year (Bates and Mathis, 2009), contributing perhaps 5 to 14% to the global balance of CO 2 sinks and sources (Takahashi et al., 2002(Takahashi et al., , 2009)).
The Chukchi Sea is a large ocean sink for CO 2 during the brief summertime sea ice-free period and contributes nearly one-third to one-half of the CO 2 sink in the Arctic, with the Barents Sea the other dominant shelf region for air-sea CO 2 exchange.There are, however, localized areas of surface seawater that are highly influenced by sea ice melt and river inputs where the opposite is observed, and these areas are potential sources of CO 2 to the atmosphere (e.g., East Siberian Sea).On the Siberian and Beaufort Sea shelves, river inputs of terrestrial organic carbon contribute to net heterotrophy (e.g., Macdonald et al., 1998;Anderson et al., 2010Anderson et al., , 2011) ) and sustained release of CO 2 to the atmosphere on these Arctic "interior" (e.g., Hansell et al., 2004;Holmes et al., 2008;Alling et al., 2010;Letscher et al., 2011), with a longer-term potential of contributing to reversing the CO 2 sink status of the Arctic.The Arctic Ocean T h e C h a N g i N g a r C T i C O C e a N | S p e C i a l i S S u e O N T h e i N T e r N aT i O N a l p O l a r Y e a r ( 2 0 0 7 -2 0 0 9)

B
Y N i C h O l a S r .B aT e S , W e i -J u N C a i , a N d J e r e m Y T. m aT h i S diSTriBuTiONS aNd air-Sea FluxeS OF CarBON diOxide Oceanography | Vol.24, No.3 186 looking southward from high over the arctic Ocean, NaSa's aqua satellite reveals coastal phytoplankton blooms in the Chukchi Sea along northern alaska (foreground) stretching into Bering Strait in September 2006.Credit: NASA Oceanography | September 2011 187 iNTrOduCTiON The Arctic plays an important and likely increasing role in the global climate system with complex and poorly understood interactions and feedbacks among sea ice, ocean, and atmosphere, the cryosphere-hydrological cycle, and ocean circulation, leading to significant impacts on the global balance of atmospheric greenhouse gases such as CO 2 and methane.Over the last few decades, numerous studies have shown that there are significant warming (ACIA, 2005; state of knowledge about the Arctic marine carbon cycle, exchanges of CO 2 between the atmosphere and the ocean, and the potential physical and biological processes that influence CO 2 sources and sinks in the Pacific-Ocean-influenced Arctic.To illustrate their potential controls on air-sea CO 2 flux in a changing environment, this review also includes a brief treatment of marine ecosystems and organic carbon cycling in the western Arctic; more comprehensive reviews may be found elsewhere, e.g.,Stein and Macdonald, (2004) andMacdonald et al., (2010).Our geographic scope is focused primarily on the western Arctic Ocean shelves (e.g., Chukchi, Beaufort, and of the world's ocean area but < 1% of its volume, is almost completely surrounded by landmasses.It is disproportionately affected by terrestrial fluxes because it receives almost 10% of total global river runoff annually from an extensive system of rivers that drain the watersheds of Siberia and northern North America (McGuire et al., 2006; Cooper et al., 2008).These landmasses contain large stores of freshwater (mostly glacial ice and permafrost) and terrestrial carbon, which, combined with the presence of sea ice in the Arctic Ocean, profoundly influence the hydrological cycle, climate, and biogeochemical dynamics of carbon in the Arctic region.In wintertime, the Arctic Ocean is almost completely covered by sea ice (except for minor areas of open water associated with polynyas and flaw leads).Physical transformations and seasonal sea ice cover together play a major role in controlling shelf water-mass properties through vertical homogenization of the water column by such physical processes as ventilation, brine rejection, and convective mixing.In summertime, seasonal atmospheric warming and the inflow of Pacific and Atlantic Ocean waters, combined with local warming and sea ice melt, leave the Arctic shelves sea ice-free for a relatively brief period.

Figure
Figure1).The Arctic Ocean has several important gateways that allow exchanges organic carbon (Pg = 10 15 g).This reservoir is in the form of living organisms and detritus, and includes suspended particulate organic carbon (POC) and dissolved organic carbon (DOC).In addition, there is approximately 25 Pg of dissolved inorganic carbon (DIC) in the forms of bicarbonate C yr -1 (Tg = 10 12 g) in the Rivers thus contribute disproportionately large amounts of freshwater and carbon to the Arctic Ocean compared to river contributions in other ocean basins.Compared to many other open-ocean and coastal environments, relatively few studies of the marine carbon cycle have been conducted in the Arctic.The harsh polar climate and difficult logistical support have limited most studies to opportunistic icebreaker surveys conducted on the Arctic Ocean shelves during the summertime sea ice retreat.
2000s, high concentrations of sPOC were observed (up to 2000 mg C L -1 ; average of ~ 200 -300 mg C L -1 ) across the shelf (Figure 2), with considerable export of sPOC off the shelf into the Canada Basin (Bates et al., 2005b; ~ 2.3-3.5 Tg C yr -1 assuming 0.8 Sv transport during 100 days of active POC production), and relatively high rates of vertical export of organic carbon to shelf, slope, and basin sediments (Moran

Figure 2 .
Figure 2. distributions of suspended particulate organic carbon (pOC) in the Chukchi Sea collected during the 2002 and 2004 Shelf-Basin interactions (SBi) project in the western arctic (Bates et al., 2005b, 2006b,c, http://www.eol.ucar.edu/projects/sbi/all_data.shtml). in the left panel, CTd/ hydrocast stations are shown with different seas denoted: CS = Chukchi Sea.eSS = east Siberian Sea.BS = Beaufort Sea.CB = Canada Basin. in the right panel, pOC data include two sea ice-covered cruises in spring and two sea ice-free cruises conducted in summertime, including: spring 2002 (blue symbols), summer 2002 (green symbols), spring 2004 (yellow symbols), and summer 2004 (red symbols).The data are plotted using Ocean data View(Schlitzer, 2005).

Figure 3 .
Figure 3. Surface distributions of dissolved organic carbon (dOC; µmol l -1 ) across the western arctic Ocean collected during the 2002 and 2004 SBi project in the western arctic (mathis et al., 2005; d.a.hansell and N.r.Bates data available at http://www.eol.ucar.edu/projects/sbi/all_data.shtml),and during the 2009 ruSalCa project (recent work of author Bates and d.a.hansell).in the left panel, CTd/hydrocast stations are shown with different seas denoted: CS = Chukchi Sea.eSS = east Siberian Sea.BS = Beaufort Sea.CB = Canada Basin. in the right panel, surface dOC is plotted from two sea ice-covered cruises in spring (SBi) and three sea ice-free cruises conducted in summer (two SBi and one russian-american long-term Census of the arctic [ruSalCa] cruise).SBi and ruSalCa data sets are differentiated on the figure.The data are plotted using Ocean data View(Schlitzer, 2005).
pCO 2 (~ 200-350 µatm) values were lower than those in the atmosphere (~ 365-380 µatm at the time of observation) during the sea ice-free period.Since then, other studies have observed low seawater pCO 2 conditions on the Chukchi Sea shelf during summertime (~ 150-350 µatm;Pipko et al., 2002; Figure 4. Surface distributions of seawater partial pressure of CO 2 (pCO 2 in µatm) during the summer 2008 China National arctic research expedition (ChiNare;Cai et al., 2010).data were collected using a NOaa atlantic Oceanographic and meteorological laboratory (aOml) underway CO 2 system deployed on the icebreaker Xuelong.The pCO 2 system is described in detail by pierrot et al.(2009).The summer 2008 ChiNare pCO 2 data were collected as a result of collaboration among rik Wanninkhof of NOaa-aOml, uSa; liqi Chen of the Third institute of Oceanography, State Ocean administration, China; and author Wei-Jun Cai.The data are plotted using Ocean data View(Schlitzer, 2005).

Figure 5 .
Figure 5. Surface distributions of seawater partial pressure of CO 2 (pCO 2 in µatm) calculated from dissolved inorganic carbon (diC) and total alkalinity (Ta) data collected during the summer of 2009 as part of the ruSalCa project (recent work of author Bates).Samples were collected from CTd/hydrocast stations occupied by the icebreaker Professor Khromov.diC and Ta sample analysis is described elsewhere in Bates (2007).Seawater pCO 2 was calculated using CO2calc (robbins et al., 2010) with dissociation constants of mehrbach et al. (1973) as refit by dickson and millero (1987) and KSO 4 of dickson (1990).The data are plotted using Ocean data View(Schlitzer, 2005).
In the central basin, which has been poorly sampled for the marine carbon cycle, there is an emerging picture of mixed surface seawater pCO 2 conditions.In early studies, such as the Arctic Ocean Section (AOS) expedition in 1994, surface waters under sea ice had seawater pCO 2 values of ~ 300-330 µatm (i.e., much lower than the atmosphere;Jutterström and Anderson, 2010).Several repeated sections across the central basin also have shown similar results(Jutterström and Anderson, 2010).In the early 2000s, low seawater pCO 2 values of ~ 240-280 µatm were observed in the Canada Basin off the Chukchi Sea shelf(Bates, 2006;Bates et al., 2006a), and there were even lower surface seawater pCO 2 values of ~ 150-250 µatm in the Makarov Basin of the Canada Basin(Fransson et al., 2009).However, more recently, Yamamoto-Kawai et al.(2009)  showed that some surface areas of the Canada Basin had seawater pCO 2 conditions close to equilibrium with the atmosphere in areas with significant contributions from sea ice melt.After the major summertime sea ice retreat observed in 2007, based on high-resolution underway pCO 2 measurements,Cai et al., (2010) showed that ice-free surface areas of the Canada Basin (mostly the southern part of the basin) had seawater pCO 2 conditions close to equilibrium with the atmosphere, reflecting uptake of CO 2 from the atmosphere and warming during exposure of surface waters as well as strong vertical stratification and low biological production.However, areas with heavy ice cover (mostly the northern part) had lower surface water pCO 2 (Figure4).Thus, ongoing rapid sea ice retreat to the northern basin appears to have resulted in a recent increase in seawater pCO 2 in the Canada Basin that approaches the atmospheric pCO 2 .

Figure 6 .
Figure 6.Surface distributions of seawater partial pressure of CO 2 (pCO 2 in µatm) calculated from diC and Ta data collected during the summer of 2010 as part of the NaSa iCeSCapeS project (recent work of authors Bates and mathis).Samples were collected from CTd/hydrocast stations occupied by the icebreaker uSCgC Healy.diC and Ta sample analysis is described in Bates (2007).Seawater pCO 2 was calculated using CO2calc (robbins et al., 2010) using pKs of mehrbach et al. (1973) as refit by dickson and millero (1987) and KSO 4 of dickson (1990).The data are plotted using Ocean data View(Schlitzer, 2005).
uptake of CO 2 by the Arctic Ocean is small compared to the potential release of land-based carbon to the atmosphere from surrounding Arctic landmasses over the next few centuries as a result of climate change.Finally, potential increases in organic carbon respiration as a result of warming and enhanced terrestrial organic carbon flux due to thawing of permafrost and coastal erosion are difficult to evaluate but will certainly further modify the CO 2 sourcesink terms in the Arctic Ocean under future climate change.
will likely exert greater influence on the global carbon cycle in the coming decades, with the marine carbon cycle and atmosphere-ocean CO 2 exchanges sensitive to both regional and global climate transitions and feedbacks.The capacity of the Arctic Ocean to uptake CO 2 appears to be changing in response to climate and environmental change such as sea ice loss, Arctic warming, and increased inputs of terrestrially derived organic carbon.Finally, in response to increased marine phytoplankton growth and uptake of human-produced CO 2 , the seafloor ecosystem of the Arctic shelves already appears affected by ocean acidification, particularly those species that produce CaCO 3 shells or skeletons.

aCKNOWledgemeNTS
We would like to thank the captains and crews of the icebreakers Xuelong, Professor Khromov, and USCGC Healy, and the scientific participants of the SBI, CHINARE, RUSALCA, and ICESCAPES

Table 1 .
areas, depths, residence times, air-sea CO 2 exchange rates expressed in mmoles C m -2 d -1 , and annual air-sea CO 2 exchange rate expressed in Tg C (10 12 g C).Negative air-sea CO 2 exchange rates indicate ocean uptake of CO 2 (i.e., CO 2 sink).Modified fromBates and Mathis (2009)