Continental Shelf Research 67 (2013) 42–51

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Continental Shelf Research

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Reasearch paper

Assessing seasonal changes in carbonate parameters across smallspatial gradients in the Northeastern Chukchi Sea

Jeremy T. Mathis a,b,n, Jennifer M. Questel a

a School of Fisheries and Ocean Sciences, University of Alaska Fairbanks, Fairbanks, AK 99775-7220, United Statesb NOAA Pacific Marine Environmental Laboratory, 7600 Sand Point Way NE, Seattle, WA 98115, United States

a r t i c l e i n f o

Article history:Received 10 May 2012Received in revised form16 April 2013Accepted 29 April 2013Available online 9 May 2013

Keywords:Western Arctic OceanCarbonate systemCO2 fluxesOcean acidification

43 & 2013 The Authors. Published by Elseviex.doi.org/10.1016/j.csr.2013.04.041

esponding author at: NOAA Pacific Marine Enint Way NE, Seattle WA 98115, United States.206 526 6744.ail address: jeremy.mathis@noaa.gov (J.T. Mat

a b s t r a c t

Observations of the marine carbonate system were made in 2010 in the northeastern Chukchi Sea toconstrain the seasonal progression of carbonate mineral saturation states (Ω) throughout the watercolumn and determine the air–sea flux of carbon dioxide (CO2). As sea ice retreats from the Chukchi Shelf,primary production consumes dissolved inorganic carbon (DIC) in the euphotic zone causing pH andcarbonate mineral saturation states to increase. Throughout the summer and early autumn months of2010, saturation states for calcite and aragonite ranged from 2.5 to 4.0 and 1.5 to 2.5, respectively, wellabout the saturation horizon of 1.0. Much of the organic matter produced during the bloomwas verticallyexported from the relatively small study area leading to an uptake of CO2 from the atmosphere of at least340,000 kg-C. The exported organic matter settled near the bottom and was remineralized back into DIC,causing concentrations to increase sharply, particularly in autumn months, driving down pH to as low as7.75 and suppressing the concentrations of important carbonate minerals to the point that aragonitebecame undersaturated. The data showed a definitive seasonal progression of this process with aragonitebecoming partially undersaturated along the bottom in September, and broadly undersaturated inOctober. While carbonate saturation states would naturally be suppressed by the high rates of exportproduction and the accumulation of DIC near the bottom, the penetration of anthropogenic CO2 into thewater column (ocean acidification) has caused these observed undersaturations, which will likely expandas CO2 levels in the atmosphere continue to rise in the coming decades.

& 2013 The Authors. Published by Elsevier Ltd. Open access under CC BY-NC-ND license.

1. Introduction

Since the Industrial Revolution the oceans have absorbed approxi-mately 1/3 of all anthropogenically produced CO2 (Sabine and Feely,2007). While this has mitigated the increase in atmospheric CO2concentrations by �55% (Sabine et al., 2004), it has changed thecarbonate chemistry of seawater chemical speciation (e.g., Caldieraand Wickett, 2003; Andersson and Mackenzie, 2004; Feely et al.,2004; Lee et al., 2003; Sabine et al., 2002; Orr et al., 2005; Millero,2007) with unknown, but potentially significant impacts to currentand future marine ecosystems (Fabry et al., 2008, 2009; Cooley andDoney, 2009). The absorption of atmospheric CO2 by the ocean hasresulted in a lowering of pH, especially over the last few decades (e.g.,Bates, 2007; Dore et al., 2009; Byrne et al., 2010) as atmospheric CO2levels have risen, with a subsequent decrease in the availability ofcarbonate ions (CO32−) and a suppression of the saturation states (Ω) of

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calcium carbonate minerals, which could result in a reduction ofsuitable habitat for some marine calcifiers. These processes, collec-tively termed “ocean acidification” (OA), have occurred naturally overgeologic time scales (e.g., Zachos et al., 2005) but have been acceler-ated due to anthropogenic emissions from industrial processes andchanges in land use (Feely et al., 2004; Sabine et al., 2004; Orr et al.,2005; Caldiera and Wickett, 2005).

The continental shelves of the western Arctic Ocean play animportant and likely increasing role in the regional carbon budgetwith complex and poorly constrained interactions between sea-ice,the atmosphere, the hydrological cycle and ocean acidification (e.g.,Macdonald et al., 2010; Carmack andWassmann, 2006). Some of theseinteractions could have a significant impact on the global balance ofcarbon dioxide (CO2) and may cause disruptions in marine ecosystemsacross the region. Currently, the Arctic basin is an important sink foratmospheric CO2 with recent estimates suggesting that the regioncontributes between 5% and 14% of the global ocean's net uptake(Bates and Mathis, 2009). The northeastern Chukchi Sea, which is thefocus of this study, provides the strongest seasonal sink for atmo-spheric CO2 in the western Arctic (i.e. Bates et al., 2011).

With these rapid environmental changes, the Arctic marinecarbon cycle will likely enter a transition period in the comingdecades, with large uncertainties in atmospheric-oceanic CO2 fluxes(Anderson and Kaltin, 2001; Bates, 2006; Bates and Mathis, 2009;

D license.

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Fig. 1. Map of the Chukchi Sea showing the generalized current system in the region. The three study sites (Klondike, Burger, and Statoil) indicate the locations wereobservations made in 2010. Each study site is 900 nautical miles2.

J.T. Mathis, J.M. Questel / Continental Shelf Research 67 (2013) 42–51 43

Cai et al., 2010; Jutterstrom and Anderson, 2010) in response to sea-ice loss, warming temperatures and alterations in the magnitude ofphytoplankton primary production. Furthermore, the marine carboncycle and marine ecosystems of the Arctic are especially vulnerableto OA (Orr et al., 2005; Steinacher et al., 2009; Bates et al., 2009;Yamamoto-Kawai et al., 2009) largely because the waters in thewestern Arctic Ocean are preconditioned to have low carbonatemineral saturation states due ocean circulation patterns and colderwater temperatures (Fabry et al., 2009).

There have been numerous studies that have shed new light on thecontrols on the carbon biogeochemistry of the region, but many ofthese have suffered from a lack of spatial or temporal resolution. Here,we will present new carbonate chemistry data, collected at anunprecedented spatial resolution near three oil and gas prospects inthe northeastern sector of the Chukchi Sea (Fig. 1) during the openwater season (July–September) of 2010. Each of the three study areas(Klondike, Burger and Statoil) exhibit differing physical and biogeo-chemical characteristics (Day et al., 2013), although they are alldominated by classical pelagic–benthic coupling that is largely con-trolled by the annual advance and retreat of sea ice over the region.The Klondike study area borders the eastern edge of the CentralChannel, a northerly current pathway, and functions as a more pelagic-dominated ecosystem, whereas the Burger study area lies south ofHanna Shoal and functions as a more benthic-dominated ecosystem.The Statoil study area has both pelagic and benthic attributes, althoughit is more like Burger than like Klondike. The high spatial-resolution ofthe study allowed us to determine if these variations in benthic–pelagic coupling are large enough to yield differences in the carbonateparameters at each study area and better elucidate the controls oncarbonate mineral saturations states in the region.

2. Background

The western Arctic Ocean is a highly dynamic environmentwhere sea ice cover plays a major role in controlling the carbon

cycle through vertical homogenization of the water column. Inthe Chukchi Sea, seasonal atmospheric warming and the inflowof warm, lower salinity waters from the Pacific Ocean (viathe Bering Sea) and river water from western Alaska (e.g., theYukon River) leave the broad, relatively shallow shelf nearly sea-ice free for most of the summer months when solar radiation is atits peak.

The inflow of nutrient-rich water through Bering Strait(Codispoti et al., 2005) supports an intensive period of marinephytoplankton photosynthesis as sea ice retreat allows solarradiation to penetrate the surface layers (Cota et al., 1996; Hilland Cota, 2005). Because of the nutrient supply from the BeringSea/North Pacific, rates of phytoplankton production in the Chuk-chi Sea are much higher than in other regions of the western ArcticOcean (i.e. Beaufort Sea) where nutrients are more limited. Ratesof phytoplankton primary production on the Chukchi Sea shelf canexceed Z300 g C m2 y−1 or 0.3–2.8 g C m2 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) andthis intense period of seasonal growth supports a large zooplank-ton biomass (e.g., copepods; see Questel et al., 2013) that inturn supports diverse pelagic and benthic ***ecosystems(i.e. bivalves; Blanchard et al., 2013).

The intense period of phytoplankton production during thebloom results in the formation of high concentrations of particu-late organic carbon (POC) in the water column (Bates et al., 2005b;Moran et al., 2005; Lepore et al., 2007) and relatively high rates ofvertical export of organic carbon to shelf, slope and basin sedi-ments (Moran et al., 2005; Lepore et al., 2007). The Chukchi Sea isdominated by large species of phytoplankton (e.g., Cota et al.,1996; Grebmeier et al., 2008) that produce a substantial amount oforganic matter, which is rapidly exported to the bottom watersover the shelf. This leads to an uncoupling between primaryproducers and grazer. Mathis et al. (2007) showed that as muchas 75% of the total biomass produced from phytoplankton primary

J.T. Mathis, J.M. Questel / Continental Shelf Research 67 (2013) 42–5144

production is exported from the mixed layer due to the delayedarrival of grazers.

2.1. Air-sea fluxes of CO2

The high rates of export production from the surface waters ofthe Chukchi Sea shelf condition the region to be a strong seasonalsink for atmospheric CO2. Numerous studies (Semiletov, 1999;Pipko et al., 2008; Murata and Takizawa, 2003; Bates et al., 2005a;Bates, 2006; Chen and Gao, 2007; Fransson et al., 2009; Andreevet al., 2010)) of surface water partial pressure of CO2 (pCO2) haveshown that values can range from 4100 matm to near equilibriumwith the atmosphere (�390 matm). After the initial ice algae/phytoplankton bloom, pCO2 values are typically in the range of50–200 matm lower than atmospheric values, creating a strongpotential for air–sea exchange. Previous estimates of the rates ofair–sea CO2 exchange during the sea-ice free period in thesummertime have ranged from �−20 to −90 mmol CO2 m−2 d−1(Wang et al., 2003; Murata and Takizawa, 2003; Bates, 2006;Fransson et al., 2009; Cai et al., 2010; Bates et al., 2011) making thisarea the strongest CO2 sink in the western Arctic. The annualocean CO2 uptake for the Chukchi Sea shelf has been estimated tobe 11–53 Tg C y−1 (e.g., Bates, 2006; Bates et al., 2011).

The large drawdown in dissolved inorganic carbon (DIC) fromprimary production (Fig. 2) is the major controller of pCO2 valuesin summer (Bates et al., 2005a,b; Bates, 2006; Cai et al., 2010),although changes in surface temperatures can also affect the pCO2as warming in summer drives pCO2 higher, mitigating the biolo-gical effects. The highest seasonal changes in DIC have previouslybeen observed in the vicinity of Barrow Canyon at the northernedge of the Chukchi Sea shelf (Bates et al., 2005a; Hill and Cota,2005), which is close to the study area discussed here. Once thephytoplankton bloom has terminated due to the exhaustion ofnutrients, pCO2 and DIC values begin to return to their pre-bloomconcentrations mainly from the uptake of CO2 from the atmo-sphere and winter mixing once sea ice has returned to the shelf(Omar et al., 2007).

2.2. Ocean acidification

The drawdown of DIC during the spring phytoplankton bloomhas a significant impact on water column pH and the saturationstates (Ω) of the two most important carbonate minerals (calciteand aragonite). As DIC is consumed, pCO2 decreases in the surface

Bering Sea inflow

Chukchi Sea Shelf

50 m

CO2

DIC + nutrients POCphytoplankton PP Export

production

POC

loin

remineralization

Fig. 2. Schematic of the carbonate

layer causing pH and the saturation states of calcite (Ωcalcite) andaragonite (Ωaragonite) to increase (Fig. 2). In response to high exportproduction, the remineralization of organic matter increases theconcentration of DIC and pCO2 in bottom waters and suppressescarbonate mineral saturation states (Fig. 2) to a varying degreeacross the Chukchi Shelf (Bates and Mathis, 2009). In the regionnear the head of Barrow Canyon, export production is highest andthis is where the strongest seasonal suppression of aragonitein subsurface water has been observed (Bates et al., 2009).This suppression of carbonate mineral saturation states correspondsto high apparent inorganic nutrient concentration near the bottom(Questel et al., 2013). The subsurface effects of remineralization canbe especially significant during periods of intense production whensaturation states increases at the surface. These biologically driven,seasonally divergent trajectories of Ω, or the “Phytoplankton-Carbo-nate Saturation State” (PhyCaSS) Interaction, have been observedboth in the Chukchi Sea (Bates et al., 2009; Bates and Mathis, 2009)and the Bering Sea (Mathis et al., 2011a,b), and are likely typical ofhighly productive polar and sub-polar shelves.

On average, OA has already decreased global surface water pHby 0.10 units and reduced the saturation states of calciumcarbonate minerals such as aragonite and calcite (e.g., Byrneet al., 2010), particularly in the North Pacific region. The ArcticOcean is particularly vulnerable to OA due to naturally lowcarbonate mineral saturation states, low surface water tempera-ture, which enhances air–sea exchange of CO2 (Orr et al., 2005;Steinacher et al., 2009; Fabry et al., 2009), low buffer capacity ofsea-ice melt waters (Yamamoto-Kawai et al., 2009), and upwellingalong the shelf-break of both the Chukchi and Beaufort Seas(Mathis et al., 2012).

3. Methods

3.1. Cruise information and water column sampling

Physical, chemical and biological measurements were made inthe study areas (Fig. 1) during three subsequent cruises in August,September, and October of 2010 (Klondike, Burger and Statoil)each measuring �900 nautical miles2. Samples for DIC and totalalkalinity (TA) were collected concurrent with CTD casts(Weingartner et al., 2013) and macro-nutrient sampling (Questelet al., 2013) at every other fixed oceanographic station at all threestudies sites in August and September and only at the Burger areain October. Seawater samples for DIC/TA were drawn from Niskin

summer sea ice extent

Canada Basin

w pCO2creased pH and Ω

DIC high pCO2suppressed pH and Ω

system over the Chukchi Shelf.

J.T. Mathis, J.M. Questel / Continental Shelf Research 67 (2013) 42–51 45

bottles into pre-cleaned �300 mL borosilicate bottles. Thesesamples were subsequently poisoned with mercuric chloride(HgCl2) to halt biological activity, sealed, and returned to thelaboratory for analysis.

3.2. Laboratory analysis and calculation of carbonate parameters

DIC and TA samples were analyzed using a highly precise andaccurate gas extraction/coulometric detection system (Bates,2001). The analytical system consists of a VINDTA 3C (VersatileInstrument for the Detection of Total Alkalinity; http://www.marianda.com) coupled to a CO2 coulometer (model 5012; UICCoulometrics). TA samples were also determined by potentio-metric titration using the VINDTA 3C. Routine analyses of CertifiedReference Materials (CRMs, provided by A.G. Dickson, ScrippsInstitution of Oceanography) ensured that the accuracy of theDIC and TA measurements were within 0.05% (�1.5 mmoles kg−1)and stable over time.

Seawater pCO2, pH and CaCO3 saturation states for calcite(Ωcalcite) and aragonite (Ωaragonite) were calculated from DIC, TA,temperature, salinity, phosphate, and silicate data using thethermodynamic model of Lewis and Wallace [1995]. The carbonicacid dissociation constants of Mehrbach et al. (1973) (refit by(Dickson and Millero, 1987); i.e., pK1 and pK2) were used todetermine the carbonate parameters. The CO2 solubility equationsof Weiss (1974), and dissociation constants for borate (Dickson,1990), silicate and phosphate (Dickson and Goyet, 1994) were usedas part of the calculations. Uncertainty in the calculation of Ωcalciteand Ωaragonite were �0.02.

3.3. Air-sea flux calculations

The net air–sea flux (F) of CO2 was determined by F ¼ ksst�KCO2 � ðpCO2sw−pCO2airÞ, where ksst is the transfer velocity, KCO2 isthe solubility of CO2 at a given temperature, and pCO2sw−pCO2air(ΔpCO2) is the difference between atmospheric and oceanic partialpressures of CO2. The ΔpCO2 sets the direction of CO2 gas exchange(i.e. negative values indicate a flux into the ocean, while positivevalues indicate a flux to the atmosphere), while ksst determines therate of transfer. Here, the quadratic wind dependency (U2) of Hoet al. (2011) was used to determine the gas transfer velocity–windspeed relationships for short-term wind conditions (most watercolumn observations were made over 10–20 day time period).Data for atmospheric pCO2 were collected from the Point Barrowobservation tower (http://www.cmdl.noaa.gov) and subtractedfrom surface seawater pCO2 data to determine ΔpCO2 values. Itshould be noted that using DIC and TA to calculate surface waterpCO2 can introduce up to a 5% error on top of the analytical errorfor these measurements, but direct observations of pCO2 were notpossible during this study. Direct wind speed data was unavailableso daily averaged wind speed values were determined from theNCEP (National Center for Environmental Prediction)/NCAR(National Center for Atmospheric Research) reanalysis 2 dataassimilation model (i.e., NNR data, (http://www.cdc.noaa.gov/cdc/data.ncep.html)) to calculate ksst values. These reanalysisproducts have also been employed to estimate heat fluxes forthe study areas (Weingartner et al., 2013). The spatial resolution ofNNR data is 2.5�2.51 for the Chukchi Sea region. Given therelatively small size of the sampled areas (900 NM2 or�3100 km2 each), one daily averaged wind speed value duringeach month was applied to the Burger, Klondike and Statoil gridsto obtain the total CO2 flux.

3.4. Estimates of seasonal changes in carbonate parameters

In order to determine the monthly changes in carbonateparameters at the surface and near the bottom for each station,we analyzed the differences between the September and Augustoccupations and then the October and September occupations.Using this convention, positive value indicates that a particularparameter increased between the given months, while negativevalues indicate a net loss or reduction in a particular parameter.One weakness of this approach is that it neglects lateral transport,and for the study region the waters are generally moving north-ward at 3–5 cm s−1 (Weingartner et al., 2013). Unfortunately, thereis no way to account for this, although the parameters in anyseason are fairly uniform across the observed spatial gradients. Aswe are particularly interested in the overall change in each studyarea rather than changes at a specific station the advective affectsmay be of limited concern.

4. Results and discussion

4.1. August observations

The most intense period of phytoplankton primary productionhad already occurred when the study area was occupied for thefirst time in August as inorganic nitrogen (nitrate and nitrite)concentrations were near zero at the surface over the entire studyarea (Questel et al., 2013), which is consistent with previousobservations (Cota et al., 1996; Hill and Cota, 2005) in the region.DIC concentrations that had been drawn down in the mixed layerby the bloomwere likely already starting to increase due to air–seaexchange due to the large gradients in CO2 concentrationsbetween the surface waters and the atmosphere (Bates andMathis, 2009). Observations from August in the three study areasshowed that DIC was coupled to salinity (Fig. 3A), as previouslyshown (Bates et al., 2005b; Mathis et al., 2009) with concentra-tions ranging from �2200 mmoles kg−1 near the bottom (i.e.35–45 m), where salinity was 432.5 to a minimum value of�1800 mmoles kg−1 in surface waters (salinity of 28.5). Minimumvalues of TA (�2000 mmoles kg−1) were observed in the surfacewaters and concentrations increased with depth to a maximum of2280 mmoles kg−1 (Fig. 3B) near the bottom (salinity 432.5).

The surface waters at the Klondike site showed a longitudinalgradient fromwest to east with salinity, DIC and TA being lower onthe western side. However, temperatures were warmer on theeastern side. This low salinity, low temperature feature over thatportion of the shelf was likely due to ice-melt (Weingartner et al.,2013). The low DIC and TA concentrations in this water, which areindicative of modifications by primary production and ice-melt(Bates et al., 2009), caused the pH to rise as high as 8.30. Aragonitewas supersaturated throughout the surface layer, but was higheron the western side. Observations in the Burger area showed thesame low salinity feature on the western side of the study areathat was present at Klondike, with subsequently low DIC and TAvalues. This was also likely due to primary production and ice-melt. pH was �8.30 in this feature and aragonite saturation stateswere between 1.5 and 2.0. Below the surface layer, concentrationsof DIC and TA were fairly constant as was temperature and salinity.

Although several months had likely passed since the initiationof the bloom, there were still considerable pCO2 undersaturationsrelative to atmospheric values (390 matm; Fig. 4). In the surfacelayer, pCO2 values ranged from �180 matm to �320 matm (Fig. 4),indicating that the surface waters still had a great deal of potentialto take up CO2 from the atmosphere. The differences in pCO2 at thesurface are a function of biological production (some areas aremore nutrient-rich and allow for greater drawdown of DIC)

http://www.marianda.comhttp://www.marianda.comhttp://www.cmdl.noaa.govhttp://www.cdc.noaa.gov/cdc/data.ncep.htmlhttp://www.cdc.noaa.gov/cdc/data.ncep.html

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Fig. 4. Seasonal distribution of pCO2 (matm) throughout the water column in theKlondike, Burger, and Statoil study sites, northeastern Chukchi Sea, during August,September, and October of 2010.

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Fig. 5. Seasonal distribution of pH (SW scale) throughout the water column in theKlondike, Burger, and Statoil study sites, northeastern Chukchi Sea, during August,September, and October of 2010.

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Fig. 3. Seasonal distribution of (A) dissolved inorganic carbon (mmoles kg−1) and(B) total alkalinity (mmoles kg−1) in relation to salinity in the Klondike, Burger, andStatoil study sites, northeastern Chukchi Sea, during August, September, andOctober, 2010.

Calcite Saturation State (Ω)

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5D

epth

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Fig. 6. Calcite saturation states (Ω) throughout the water column in the Klondike,Burger, and Statoil study sites, northeastern Chukchi Sea, during August, Septem-ber, and October of 2010. The vertical black line show the saturation horizon (Ω¼1)for calcite.

J.T. Mathis, J.M. Questel / Continental Shelf Research 67 (2013) 42–5146

and temperature during the spring and summer. In the deeperwaters, particularly below the well-stratified surface layer, pCO2values increased sharply to 4400 matm, likely in response to theaccumulation of DIC from the remineralization of organic matter

(Fig. 4). However, there was considerable variability in bottomwater pCO2, which is likely a function of the export productionacross the study area, demonstrating that even over small spatialgradients the location and intensity of surface blooms can have alarge impact on carbonate parameters.

The consumption of DIC strongly influenced pH as well, causingan increase at the surface (Fig. 5) after the bloom. pH values inAugust were still higher (less acidic) than “pre-bloom values”(observed during other studies in the region during the pre-icemelt spring of 2010; Mathis unpublished data) ranging from�8.05 to 8.32 at the surface. However, the increased pCO2 atdepth caused pH to be much lower with most of the values fallingbetween 7.95 and 8.10 (Fig. 5).

The changes in pCO2 concentrations at the surface and at thebottom had a definitive effect on the carbonate mineral saturationstates (Figs. 6–8). Both calcite and aragonite were supersaturatedthroughout thewater column in August, but there was a clear gradientbetween the surface and the bottom. Calcite values ranged from �2.5to 3.6 at the surface, with the minimum values (�1.6) occurring in thebottomwaters. Aragonite showed a similar trend (Fig. 9D), with valuesoverall being lower than calcite, but this is due to the greater solubility

J.T. Mathis, J.M. Questel / Continental Shelf Research 67 (2013) 42–51 47

of aragonite in seawater (Ries, 2011). Aragonite values approached thesaturation horizon (Ω¼1.0) near the bottom, but remained above thisthreshold in August (Fig. 7).

There were some spatial differences in the carbonate para-meters in the surface waters during August, but generally theywere fairly uniform. DIC and TA concentrations were lower in thenorthern and western regions of the study area, likely related tolower salinity melt-water in this region (Weingartner et al., 2013).The remaining carbonate parameters exhibited a similar trend asthey are largely controlled by DIC and TA concentrations with pCO2

Aragonite Saturation State (Ω)

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Fig. 7. Aragonite saturation states (Ω) throughout the water column in theKlondike, Burger, and Statoil study sites, northeastern Chukchi Sea, during August,September, and October of 2010. The vertical black line shows the saturationhorizon (Ω¼1) for calcite.

Aug. Sept. 72.5°N

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Fig. 8. Surface plots of the temperature (1C; A–C), pCO2 (matm; D–F), and CO2 flux (mmol C mduring 2010. The black boxes illustrate approximate boundaries of each study site.

being highly undersaturated at the surface relative to the atmo-sphere (Fig. 8D) in the northwestern region of the study area.

In the Statoil area, observations in August showed the watercolumn was horizontally stratified with a small east–west gradientin salinity and temperature, likely from the influence of ice melt onthe eastern side of the study area. DIC and TA concentrationsshowed a similar trend with values being lower in the surfacewaters on the eastern side. Subsequently, both pH and aragonitesaturation states were higher in these locations.

In all three study areas, the biologically driven, divergenttrajectories of Ω (PhyCaSS Interaction), were present as expected,confirming that this process is likely the dominate driver ofcarbonate parameters in the Chukchi Sea (Bates et al., 2009;Bates and Mathis, 2009) and typical of highly productive polarand sub-polar shelves.

4.2. September observations

In September, the low salinity feature on the western sideof the Klondike area was absent and DIC concentrations atthe surface were higher (concentrations increased by21–123 mmoles kg−1) across the three study regions than thoseobserved during August (see Supplemental Material Tables 1–4),and were again fairly well correlated with salinity (Fig. 3). DICconcentrations in the bottom waters (salinity 432.5) alsoincreased in some locations, but on average the concentrationswere similar to August observations. The increase in DIC concen-trations at the surface is likely a product of the uptake of CO2 fromthe atmosphere, although a small part of the this increase is likelydue to remineralization of dissolved organic carbon (DOC) in thesurface layer (Mathis et al., 2005, 2007) or suspended particlesthat have very low sinking rates (Bates et al., 2005b).The combinations of these two processes, along with an increase

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μatm

1.6

1.4

1.2

1.0

0.8

1.8

0.6

Ar

Klondike

Burger

Statoil

Fig. 9. Bottom water plots of pCO2 (matm; A–C) and aragonite saturation states (Ωaragonite) in the Klondike, Burger, and Statoil study areas, northeastern Chukchi Sea, during2010. The black boxes illustrate approximate boundaries of each study site.

J.T. Mathis, J.M. Questel / Continental Shelf Research 67 (2013) 42–5148

in average surface temperatures (0.45–2.52 1C increase betweenAugust and September) caused pCO2 at the surface to increase (33–105 matm) (Fig. 10) and at some locations the water becamesupersaturated with respect to atmospheric CO2 (Figs. 4 and 8E).In fact, the highest pCO2 values observed during all three cruisesoccurred during September in the Klondike and western Statoilstudy areas (Fig. 10). Near the bottom, pCO2 values also increasedsharply (40–109 matm) compared to August observations, and atsome locations exceeded 600 matm (Figs. 4 and 9B).

The occupation of the Burger study area showed that the lowsalinity feature that was present in August was gone and the watercolumn had horizontally stratified with only a small, lowersalinity/temperature feature present around 165.251W. Concentra-tions of DIC and TA were also stratified and increased with depth,with values at the surface and near the bottom higher thanmeasurements made in August. The saturation state for aragonitewas still 41.50 in the upper 20 m, but in the bottom water adecrease was apparent due to the increase in DIC concentrations.

Ocean pH and the saturation states of calcite and aragoniteresponded to these changes in pCO2 at the surface and near thebottom (Figs. 5–7) between August and September. The pH valuesranged between 8.00 and 8.10 and aragonite saturation stateswere between 1.5 and 2.0 in the upper 20 m of the water column.Along the bottom, pH and aragonite saturation states were �8.00and �1.5 below 25 m. Both calcite and aragonite saturation statesdecreased but remained well above the saturation horizon at thesurface (Figs. 6 and 7). The accumulation of DIC at depth in somelocations and the subsequent increase in pCO2 caused the saturationstates to drop sharply near the bottom with aragonite becomingundersaturated (Ωo1.0) at a few locations (Figs. 7 and 9E), whilecalcite remained above the saturation horizon at all locations (Fig. 7).

In September, the water column in the Statoil area remained highlystratified, but there was still some evidence of warm, low salinitywater closer to the coast, where pH and aragonite saturation stateswere slightly higher. However, the bottomwaters in this area showedthe influence of the increase in DIC concentrations and aragonitesaturation states were near undersaturation (Fig. 9E).

4.3. October observations

Although the October cruise was limited to only the Burger site,a number of important observations were made showing the

continuation of the seasonal progressions that drives the carbo-nate system in the Chukchi Sea. Temperature and salinity profilesshowed that the water column was still relatively horizontallystratified. However, concentrations of DIC and TA had decreased too1900 and o2100 mmoles kg−1, respectively in the upper 20 m,but DIC had increased in the bottom waters to around2200 mmoles kg−1. Observations of DIC showed a divergence fromsalinity (Fig. 3), particularly in the upper 30 m as concentrationswere drawn down by 475 mmoles kg−1 at some locations. Thedrawdown in DIC at the surface indicates that some level ofproductivity had occurred between September and October.Although there is no other supporting evidence at this time (i.e.increases in dissolved oxygen concentrations) it is likely that therewas a moderate “late season” bloom that was stimulated bynutrient entrainment into the surface waters. These nutrientscould have been mixed from depth due to increased storm activityor transported laterally from other parts of the shelf. Near thebottom, DIC concentrations increased by as much as55 mmoles kg−1 between September and October.

In response to the drawdown of DIC and decreasing watertemperatures at the surface (down 2.56 1C), pCO2 exhibited someof the lowest values (Figs. 4 and 10) in October again illustratingthe strong potential sink for atmospheric CO2 that the Chukchi Seaprovides. However, pCO2 values near the bottom increased by�147 matm compared to September, as the remineralization pro-cess continued to add DIC at this depth. pCO2 near the bottom wasabove 500 matm at most locations and exceeded 600 matm at somestations (Fig. 8F). This increase in pCO2 caused pH to drop below7.90 at most locations below the mixed layer (Fig. 5), that inducedbroad undersaturations for aragonite from 30 m to the bottomover most of the study area (Figs. 7 and 9F). Calcite valuesremained supersaturated, but did show a seasonal decrease ofalmost 0.4 units compared to September.

4.4. CO2 fluxes

The changes in surface concentrations of DIC and TA, as well asseasonal variability in temperature, salinity and wind speedcaused fluctuations in the flux of CO2 in the three study areas(Fig. 10). Here, we are using the convention that negative valuesindicate a net CO2 flux into the ocean, while positive valuesindicate CO2 outgassing to the atmosphere. In the Burger study area,

Burger Klondike Statoil Burger Klondike Statoil Burger

CO

2 F

lux

(mm

ol C

m-2

d-1

)

-120

-100

-80

-60

-40

-20

0

20

pC

O2

(µ-a

tm)

100

200

300

400

August September October

Fig. 10. The distribution of pCO2 (μatm) and CO2 fluxes (mmol C m−2 d−1) across thethree study areas in August, September and October. The gray boxes indicate thedistribution of the data from the 25–75 percentiles with the back horizontal lineshowing the 50th percentile or the mean. The T-bars descend to the 5th and 95thpercentile, while the black circles indicate data that fell outside of this range.

Table 1Cumulative uptake of CO2 at each of the northeastern Chukchi Sea study areasduring August, September, and October of 2010.

Kg-C Burger Klondike Statoil

August −56,876 −49,417 −67,132September −41,958 −24,242 −27,039October −69,930Total −168,764 −73,659 −94,171

J.T. Mathis, J.M. Questel / Continental Shelf Research 67 (2013) 42–51 49

the average air–sea flux of CO2 was −61716 mmoles C m−2 d−1

during the month of August. However, this flux was reduced to−45716 mmoles C m−2 d−1 in September due largely to a 42 1Cwarming of surface waters. Using the thermodynamic model ofLewis and Wallace (1995) it was determined that a 2 1C increase intemperature can raise the pCO2 at the surface by �10% and inhibitair–sea exchange. As temperatures decreased during October thehighest average CO2 flux (−7575 mmoles C m−2 d−1) was observedin the Burger study area as pCO2 decreased and enhanced air–seaexchange.

A similar trend was observed in the Klondike and Statoil studyareas, where average CO2 fluxes were higher in August (−5373 mmoles C m−2 d−1 and −7274 mmoles C m−2 d−1, respectively)than in September when the average fluxes dropped to −2672mmoles C m−2 d−1 and −29717 mmoles C m−2 d−1, respectively.Although data was not collected in the Klondike and Statoil studyregions in October, it is likely that the CO2 fluxes increased asobserved in the Burger area.

As expected, all three study areas were moderate to strongsinks for atmospheric CO2 when sea ice was not present on theshelf due to the drawdown of DIC from primary production. In3 months, the Burger study region took up more than 168,000 kg-Cfrom the atmosphere, while in August and September the Klondikeand Statoil areas took up �73,000 and 94,000 kg-C, respectively(Table 1). When these measurements are scaled up to the total areaof the shelf then there is a net flux of �31 Tg C y−1 into the region,which is consistent with previous studies during the open waterseason (Bates et al., 2009).

4.5. The impact of anthropogenic CO2

Ideally, the amount of anthropogenic CO2 in a given system canbe estimated by directly calculating the age of the water mass, buta paucity of data in this region prevents this approach. However,

based on the origin of the water on the Chukchi Sea shelf and theobserved density constraints, we can approximate anthropogenicCO2 inventories to evaluate the pre-industrial state of the carboncycle in the region. Sabine et al. (2004) estimated that�35 μmoles kg−1 anthropogenic CO2 has penetrated into watersof the North Pacific Ocean to the 26 kg m−3 isopycnal surface.Because the source waters of the western Arctic Ocean arepartially derived from the Bering Sea Shelf (Mathis et al., 2011a)a conservative assumption can be made that there is at least35 μmoles kg−1 of anthropogenic CO2 in the water column of theChukchi Sea.

To determine the impact of OA due to the uptake of anthro-pogenic CO2, that 35 μmoles kg−1 was subtracted from the DICobservations made during the cruises while keeping the remainingvariables (TA, salinity, temperature) consistent with observations.The remaining carbonate parameters (specifically the saturationstates for calcite and aragonite) were recalculated using thethermodynamic model of Lewis and Wallace (1995). When thiswas done for the 2010 data, the entire water column over the shelfbecame supersaturated with respect to aragonite during all threemonths and in all areas. While there are a number of weaknessesassociated with this first order approximation, the calculationsuggests that OA has resulted in the seasonal aragonite under-saturations that have been observed in the Chukchi Sea and isconsistent with previous estimates of ocean acidification in thisregion (Bates and Mathis, 2009). Similar calculations have beendone along the west coast of North America and have resulted inthe same overall conclusion (Feely et al., 2008; Gruber et al., 2012).As atmospheric CO2 concentrations increase, it is likely that theseundersaturations will spread across the bottom waters over thenext few decades, as the water column inventory of anthropogenicCO2 builds, eventually forcing the western Arctic Ocean to becomeone of the first areas to be perennially undersaturated in aragonite(Steinacher et al., 2009).

4.6. Timing of ice retreat

The timing of sea-ice retreat may also have a substantial effecton carbonate chemistry in subsurface waters. Ice retreat exerts asignificant control on the fate of the organic matter producedduring the phytoplankton blooms (Cota et al., 1996; Hill and Cota,2005). Zooplankton grazing of seasonal production is minimalduring blooms associated with colder surface water temperaturesfavoring the benthic ecosystem (Hunt and Stabeno, 2002; Huntet al., 2002). In contrast, warmer years increase zooplanktonproduction by up to 50% (Hunt and Stabeno, 2002; Hunt et al.,2002). Thus, colder waters are expected to be associated withhigher export production to the benthos, and large remineraliza-tion signal will be generated at depth, corresponding to increasesin pCO2 and decreases in carbonate mineral saturation states(Bates and Mathis, 2009). Warmer water blooms will retain carbonin the mixed layer and contribute to increased pelagic productionand reduced bottom water remineralization (Hunt and Stabeno,2002; Hunt et al., 2002).

Variation in the timing of sea-ice retreat could change thetiming and extent of production over the shelf. The earlier retreatof sea ice in recent years indicates that the blooms have beenoccurring in colder water, favoring export production. On the onehand, higher rates of export production should lead to increasedfood supply for benthic organisms and an expansion of biomass(Grebmeier and McRoy, 1989; Grebmeier et al., 2006). However, ifhigh rates of export production coupled to increasing anthropo-genic CO2 inventories over the shelf cause expanded aragoniteundersaturations it could lead to a reduction in suitable habitatsfor some benthic species, especially those with calcareous skele-tons (Fabry et al., 2009).

J.T. Mathis, J.M. Questel / Continental Shelf Research 67 (2013) 42–5150

4.7. Concluding remarks

The carbonate datasets collected during the 2010 high-resolutionspatial surveys of the three study areas in the Chukchi Sea show thedynamic nature of the carbon cycle of the western Arctic Ocean. Brief,but intense periods of primary production in the surface layer in latespring sets the stage for most of the processes that occur for theremainder of the ice-free period. The consumption of DIC fromphytoplankton dramatically lower the pCO2 of the surface waterspromoting broad regions of air–sea exchange and making the ChukchiSea a strong sink for atmospheric CO2 that are consistent with morelarge-scale spatial studies, that were limited in temporal extent. Theremoval of DIC also raises the pH and carbonate mineral saturationstates, counteracting the effects of OA in the surface waters. However,the disconnects between phytoplankton primary production andzooplankton grazing causes large quantities of organic matter to beexported from the mixed layer. When this organic matter is reminer-alized by bacteria, it adds DIC back to the water column, lowering pHand suppressing carbonate mineral saturation states near the bottom.This PhyCaSS interaction could be particularly influential on benthiccalcifiers in the Chukchi Sea because the lowest saturation statescoincide with areas of highest export production. It appears that theexport production, which provides the food source at the bottom, iscausing carbonate mineral suppression that could inhibit shell and testgrowth in calcifying organisms (i.e. bivalves).

The Chukchi Sea is a highly productive region and the reminer-alization of organic matter and the seasonal suppression ofcarbonate mineral saturation states is a natural phenomenon.However, the rising inventories of anthropogenic CO2 in the watercolumn has begun to drive saturation states past the saturationhorizon, particularly for aragonite, and could be detrimental tosome marine calcifiers, particularly the diverse benthic organismsthat dominates the Chukchi Sea. While OA is a global problem, theArctic Ocean will likely experience the physical manifestations andpotential impacts of it much sooner than more temperate regionsand along with coastal upwelling zones provide a bellwether forhow the global ocean will respond. Over the last decade, numer-ous new datasets and insights have elucidated the complexities inthe feedbacks of the Arctic carbon cycle, but they have alsoprovided exciting new insights and enhanced our understandingof the region. It is critical that these investments in infrastructureand acquisition of data continue as the Arctic becomes open formore commercial development.

Acknowledgments

We would like to thank Conoco Phillips, Shell Exploration andProduction and Statoil USA E & P for supporting this study.We thank Olgoonik-Fairweather LLC for their logistic support,the captains and crew members of the R/VWestward Wind, marinetechnicians, and Aldrich Offshore Services. Lab support wasprovided by Natalie Monacci, Kristen Shake, Jessica Cross andStacey Reisdorph.

Appendix A. Supplementary Information

Supplementary data associated with this article can be found inthe online version at http://doi:10.1016/j.csr.2013.04.041.

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Assessing seasonal changes in carbonate parameters across small spatial gradients in the Northeastern Chukchi SeaIntroductionBackgroundAir-sea fluxes of CO2Ocean acidification

MethodsCruise information and water column samplingLaboratory analysis and calculation of carbonate parametersAir-sea flux calculationsEstimates of seasonal changes in carbonate parameters

Results and discussionAugust observationsSeptember observationsOctober observationsCO2 fluxesThe impact of anthropogenic CO2Timing of ice retreatConcluding remarks

AcknowledgmentsSupplementary InformationReferences