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Antarctic sea ice carbon dioxide system and controls

2011; American Geophysical Union; Volume: 116; Issue: C12 Linguagem: Inglês

10.1029/2010jc006844

2156-2202

Agneta Fransson, Melissa Chierici, Patricia L. Yager, Walker O Smith,

Methane Hydrates and Related Phenomena

Journal of Geophysical Research: OceansVolume 116, Issue C12 Free Access Antarctic sea ice carbon dioxide system and controls Agneta Fransson, Agneta Fransson agneta@gvc.gu.se Department of Earth Sciences, Oceanography, University of Gothenburg, Göteborg, SwedenSearch for more papers by this authorMelissa Chierici, Melissa Chierici Department of Chemistry, Marine Chemistry, University of Gothenburg, Göteborg, SwedenSearch for more papers by this authorPatricia L. Yager, Patricia L. Yager Department of Marine Sciences, University of Georgia, Athens, Georgia, USASearch for more papers by this authorWalker O. Smith Jr., Walker O. Smith Jr. Virginia Institute of Marine Sciences, College of William and Mary, Gloucester Point, Virginia, USASearch for more papers by this author Agneta Fransson, Agneta Fransson agneta@gvc.gu.se Department of Earth Sciences, Oceanography, University of Gothenburg, Göteborg, SwedenSearch for more papers by this authorMelissa Chierici, Melissa Chierici Department of Chemistry, Marine Chemistry, University of Gothenburg, Göteborg, SwedenSearch for more papers by this authorPatricia L. Yager, Patricia L. Yager Department of Marine Sciences, University of Georgia, Athens, Georgia, USASearch for more papers by this authorWalker O. Smith Jr., Walker O. Smith Jr. Virginia Institute of Marine Sciences, College of William and Mary, Gloucester Point, Virginia, USASearch for more papers by this author First published: 23 December 2011 https://doi.org/10.1029/2010JC006844Citations: 53AboutSectionsPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinked InRedditWechat Abstract [1] In austral summer, from December 2008 to January 2009, we investigated the sea-ice carbon dioxide (CO2) system and CO2 controls in the Amundsen and Ross Seas, Antarctica. We sampled seawater, brine and sea ice for the measurements of total alkalinity (AT), total inorganic carbon (DIC), pH, inorganic nutrients, particulate organic carbon (POC) and nitrogen (PON), chlorophyll a, pigments, salinity and temperature. Large variability in all measured parameters was observed in time and space due to the complex sea-ice dynamics. We discuss the controls of the sea-ice CO2 system, such as brine rejection, biological processes, calcium carbonate (CaCO3) precipitation/dissolution and CO2 exchange. Most (80 to 90%) of the DIC loss was due to brine rejection, which suggests that the sea ice acted as an efficient DIC sink from 0.8 and 2.6 mol m−2 yr−1 (9.6–31 g C m−2 yr−1). The remaining change in DIC was to a large extent explained by net biological production. The AT:DIC ratio in the sea ice was higher than in the under-ice water (UIW), with ratios reaching 1.7, which indicated CaCO3 precipitation and concomitant DIC loss in the sea ice. Elevated AT:DIC ratios and carbonate concentrations were also observed in the UIW, which reflect the solid CaCO3 rejected from the ice during melt. The potential for uptake of atmospheric CO2 in the mixed layer increased by approximately 56 μatm due to the combined effect of CaCO3 precipitation during ice formation, and ice melt in summer. Key Points Sea ice acts as an inorganic carbon pump through brine rejection Unique data set in a climatically sensitive Amundsen Sea, Antarctica Ice-air CO2 exchange is determined by biological processes, CaCO3 precipitation 1. Introduction [2] Ice-covered polar oceans are extremely sensitive to increased temperatures and increased oceanic CO2 levels [Stouffer et al., 1989; Manabe et al., 1994; Flato et al., 2000; Orr et al., 2005]. The Southern Ocean (SO), in particular, is recognized as one of the most important regions for the marine carbon cycle, and its response to climate change, is predicted to have a large impact on atmospheric CO2 [e.g., Le Quéré et al., 2007; Takahashi et al., 2009]. One major uncertainty within future predictions is the lack of data from the ice covered Southern Ocean; in addition, there is little agreement of the significance of the sea-ice cover in modulating CO2 gas exchange. Biological production and CO2 fluxes in the Southern Ocean are highly dependent on the ice cover and the formation and melting of sea ice [e.g., Fransson et al., 2004; Chierici et al., 2004]. Sea ice has previously been considered to be impermeable for gas exchange between ocean and atmosphere [e.g., Yager et al., 1995; Miller et al., 2002; Fransson et al., 2004]. However, recent studies on gas fluxes between sea ice, ocean and atmosphere, conducted in ice-covered seas in both Antarctica and the Arctic have demonstrated that sea ice is permeable to CO2, and that the sea-ice-air CO2 fluxes should be reconsidered and included in regional and global carbon budgets [Semiletov et al., 2004; Nomura et al., 2006; Delille et al., 2007; Rysgaard et al., 2007; Nomura et al., 2010a, 2010b; Papakyriakou and Miller, 2011]. Most experiments on sea-ice CO2 fluxes have been performed in Arctic sea ice, with relatively few studies in Antarctic sea ice [Ackley and Sullivan, 1994; Delille et al., 2007; Lewis et al., 2010]. In the Weddell Sea during the Ice Station Polarstern (ISPOL) experiment, the focus was on the physical conditions for sea-ice-air gas transport, and it was found that gravity drainage and sea-ice bulk porosity were important in controlling gas exchange [Tison et al., 2008]. [3] During the past two decades, the western Antarctic has experienced significant changes. Within this impacted region, the Amundsen Sea is one of the least studied areas due to its remote location, yet we know from satellites that the area has experienced significant surface warming and decreased summer sea-ice extent [Stammerjohn et al., 2008]. Nearby glaciers, such as Pine Island Glacier [Jacobs et al., 1996; Jenkins et al., 1997; Thoma et al., 2008] are also melting rapidly. In contrast, the Ross Sea shows increasing sea-ice extent and duration. The cause for these changes is likely coupled to marine atmospheric forcing such as the Southern Annular Mode [Stammerjohn et al., 2008]. Changes in sea-ice extent and the melting of glaciers affect the structure of the water column, sea-ice formation, and the marine ecosystem, which all have the capacity to induce changes in the ocean carbon uptake. The CO2-system processes impacted during sea-ice formation and melt are not yet well understood and remain poorly quantified. [4] The effect of biological processes on the variability of carbon transport within the sea ice is also little understood, but has a strong potential to affect the CO2 transport and exchange with the surrounding environment. Sea ice forms a unique habitat for microorganisms such as algae, bacteria, and viruses, which are adapted to large changes in salinity, temperature, light, and nutrients [Lizotte, 2003; Junge et al., 2004; Thomas and Dieckmann, 2010]. Algal biomass can reach substantial levels in some sea ice habitats [Lizotte, 2003], but the spatial distribution of sea ice algae is extremely discontinuous [Dieckmann et al., 1998]. Sea-ice algae begin their growth earlier in the season than phytoplankton, presumably due to the stable irradiance environment provided by the ice. Previous observations have suggested that primary production by sea-ice algae accounts for up to 25% of the total primary production in the Southern Ocean, and potentially plays a significant role in the global carbon budget [Arrigo and Thomas, 2004]. In both the Arctic and Antarctic, studies have suggested that the organic carbon consumption by sea-ice heterogenic activity is high or higher in the melted sea ice than in the water below the sea ice [Rysgaard et al., 2007; Deming, 2010, and references therein]. [5] During an opportunistic transect of Swedish Icebreaker Oden across the South Pacific sector of the Southern Ocean, we investigated the role of coastal Antarctic sea ice in the marine carbon cycle. [6] The objectives of our study were 1) to describe the CO2 system in Antarctic sea ice; 2) understand the quantitative relationships between physical, chemical and biological variables in Antarctic sea ice: 3) to investigate the controls that affect inorganic carbon in Antarctic sea ice; and 4) to investigate the potential uptake of atmospheric CO2, using two different scenarios: with CaCO3 precipitation within the sea ice and without, due to the combined effect of CaCO3 precipitation in sea ice and dissolution during ice melt. In this work we contribute unique data on sea ice, under-ice water and brine from field measurements. Our study adds to the understanding of the intricate coupling between biogeochemical processes within the sea ice and chemical compounds of climatically significant gases in poorly investigated ice-covered Antarctic seas. 2. Study Area [7] The Amundsen Sea is located between 69°S and 74°S along the Marie Byrd Land between 100 and 135°W. Due to its high concentrations of multiyear sea ice and remoteness from logistical bases, it has been one of the least studied continental shelf regions in the Southern Ocean. To the west and south is the better studied Ross Sea and its large open-water polynya [e.g., Tremblay and Smith, 2007]. Onboard the Swedish Icebreaker Oden during the Swedish-U.S. collaborative cruise Oden Southern Ocean 2008/2009 (OSO08–09), we sampled 16 stations (Table 1 and Figure 1) across this region for sea-ice cores, sea-ice brine, and seawater profiles. Our sampling occurred during the austral spring and summer 24-h sunlight period, between December 12, 2008 and January 1, 2009. Air temperatures ranged from −1.4° to −9.9°C and wind speeds from 5 to 25 m s−1. Figure 1Open in figure viewerPowerPoint Mean sea-ice concentration for the December 2008 with cruise track for the Oden Southern Ocean expedition 2008/09 (blue line), sampling dates (blue dots), and station numbering. Blue areas show areas with no ice cover, and the polynya is clearly visible. The shaded gray area is area which is sea-ice covered. The area within box (dashed, black line) shows the limits of the Amundsen Sea. The land fast marine out-flowing glaciers along the coast are denoted; GIS for the Getz Ice Shelf; DIS, the Dotson Ice Shelf, and CIS marks the Crosson Ice Shelf. Table 1. A Summary of the Location, Sampling Date, Ice Thickness (h), and Snow Depth for Each Ice Station, Where Complete Ice Cores Were Sampledaa For sea ice coverage and bottom topography at ice station locations, see Figures 1a and 1b. Stations included in the box model are marked with an asterisk. Dash denotes negative freeboard; n.d. means no data. Station UTC Date (Year-Month-Day) Latitude (°S, dec) Longitude (°W, dec) Ice Thickness (cm) Snow Depth (cm) Slushy Layer (cm) Free Board (cm) Observation 1* 2008-12-12 70.65 107.01 100 32 3 – Slush on top 2 2008-12-13 71.13 109.08 200 55 23 – Porous snow stratification 6 2008-12-14 71.07 110.52 110 47 9 – 13 2008-12-16 71.71 112.11 170 38 0 17 16 2008-12-17 71.84 114.11 240 28 9 – Stratified 76 cm 18* 2008-12-18 72.92 115.05 110 55 21* 2008-12-20 72.60 116.02 113 5 n.d – 27 2008-12-21 72.42 115.96 290 47 7, 20–30 – 33* 2008-12-22 71.92 118.38 125 60 18 – Snowy ice top 57 cm 35* 2008-12-23 70.15 119.95 46 18 5 34 Snowy ice 8 cm top 36* 2008-12-26 69.41 125.38 97 36 6 – No colored layer 37 2008-12-27 70.22 133.54 150 71 48 – Strongly colored layer at 53 cm 38 2008-12-28 71.07 138.04 60 15 5 3 Snowy ice top 22 cm 39 2008-12-29 72.51 144.73 179 74 17 – Snowy ice top 9 cm 40 2008-12-31 74.44 150.64 183 70 5 – Top 28 cm colored 41* 2009-01-01 75.40 151.23 125 3 18, 40–60 – Slushy core at several horizons a For sea ice coverage and bottom topography at ice station locations, see Figures 1a and 1b. Stations included in the box model are marked with an asterisk. Dash denotes negative freeboard; n.d. means no data. 2.1. Sea Ice Cover and Extent [8] The Oden broke through 50–100% ice cover and encountered sea-ice thicknesses up to about 3 m. At our stations, sea-ice thicknesses varied between 35–240 cm (rafted ice), with variable snow cover on top of the ice (Table 1). Snow depths ranged from 3–70 cm. Most of our stations exhibited a condition known as "negative freeboard," where the ice-snow surface is below sea level and flooded with seawater. Unlike thinner snow layers in the Arctic, thick Antarctic snow cover [Thomas and Dieckmann, 2010] acts to depress the snow-ice interface. We observed slushy layers (described by Perovich et al. [2004]) ranging from 3–60 cm thick. We observed significant rafting and ridging on the thicker sea-ice floes. We also observed large areas of colored ice, so-called "brown ice" [Ackley and Sullivan, 1994]. In most of the individual cores, we found colored sections both at the top and in the upper to middle part of the cores (from 30 to 80 cm from top, Table 1), suggesting the presence of biological activity. [9] We used the remotely sensed observations from 2008 to investigate the seasonal evolution of the sea-ice cover in the study area. Sea-ice concentrations in our study area were obtained from Advanced Microwave Scanning Radiometer (AMSR-E) daily sea-ice charts downloaded from the Webpage of the University of Bremen [Spreen et al., 2008]. A dramatic change occurred from a nearly completely open Amundsen Sea at the end of February (Figure 2a), to maximum ice extent in August (Figure 2b), with few and small coastal polynyas to significant ice-free areas and open coastal polynyas in the Amundsen and Ross Sea again clearly visible in December 2008 (Figure 2c). Figure 2Open in figure viewerPowerPoint Mean daily sea-ice concentration from the Advanced Microwave Scanning Radiometer (AMSR-E) remotely sensed data in the study area during (a) 21st of February 2008, (b) the 22nd of August 2008, and (c) the 22nd of December 2008. The gray area denotes land, the dark blue areas show the open ocean, and the dark purple areas are areas with 100% ice concentration. The Amundsen Sea is the area within the white box. The daily sea-ice charts were downloaded from the Webpage of the University of Bremen, http://iup.physik.uni-bremen.de/iuppage/psa/2001/amsrop.html [Spreen et al., 2008]. 2.2. Hydrography [10] The Amundsen Sea and Ross Seas both are influenced by polynyas (open areas in an otherwise sea-ice covered region), mainly caused by catabatic winds from the Antarctic continent and upwelling of relatively warm sub-surface water. The inflow of warm Circumpolar Deep Water (CDW) carried by the Antarctic Circumpolar Current (ACC), largely influences the area, and has been the main cause for the increased rate of glacier melt [Jacobs et al., 1996; Thoma et al., 2008]. As the CDW is introduced onto the continental shelf, it mixes with the fresh and cold Antarctic Surface Water (AASW) to form Modified Circumpolar Deep Water (MCDW). In the Amundsen Sea the MCDW enters the shallow shelf along deep troughs [Nitsche et al., 2007]. In our study area, along 114°W, MCDW is found at about 400 m with a temperature maximum and salinity >34.5 (Figures 3a and 3b). The relatively cold and fresh surface layer is due to the mixing of sea-ice meltwater and the cold winter water (WW) to form the Antarctic Surface Water (AASW). The WW is a remnant from winter and exhibits a temperature minimum between 50 and 100 m in our study (white dashed line in Figure 3a). Figure 3Open in figure viewerPowerPoint Section plots of distance (km) and (a) temperature (°C) and (b) salinity in the upper 800 m in the water column from station #1 in the northern end of the Amundsen Sea (right end) to station #41 in the Ross Sea. The white dashed line in the temperature plot denotes the depth of the temperature minimum, which defines the winter water (WW). White area in both plots shows the area of no data. 3. Methods 3.1. Sample Collection [11] Bulk sea ice (hereafter referred to as sea ice), brine, and seawater were collected for the determination of total inorganic carbon (DIC), total alkalinity (AT), pH, inorganic nutrients (phosphate, nitrate and silicate), chlorophyll a (chl), particulate organic carbon (POC) and nitrogen (PON), pigments, heterogenic respiration, salinity, and in situ temperature. Sea ice was collected using two different ice corers. Chemical measurements were made primarily on cores collected by a stainless steel barrel ice auger with polished steel cutting teeth (diameter of 0.12 m). Biological samples were collected using a Kovacs Mark V fiberglass coring system (0.14 m diameter) with stainless steel cutting teeth. Sea-ice temperature was measured on site, immediately after the ice core was recovered, at 5-cm intervals using a digital thermistor (Amadigit) with the accuracy of 0.1°C. The holes for the temperature measurement were carefully drilled manually with a stainless steel hand-drill to avoid additional heating from the drill. The sea-ice core for chemical measurements was sliced with a clean stainless steel saw into 10–12 cm horizons, transferred to gas tight bags (Tedlar®) and immediately sealed, from which air was removed using a small vacuum hand pump (Nalgene®). Biological cores were similarly sliced into 10 cm horizons, transferred to acid-washed plastic containers, and diluted to 50% with filtered seawater to prevent salinity shock upon melt. The samples were slowly melted in darkness at 15°C for approximately 24 h. At ice sampling, the thickness of slushy layer, sea-ice freeboard, and sea-ice core length were measured with a plastic measuring stick and visually investigated regarding stratification and color (Table 1). When the sea ice was not submerged, we collected brine in "sackholes" at different depths varying between 40 and 60 cm in the ice (Table 2a), after approximately 15 min, using a syringe for gas samples to minimize the effect of gas exchange with the atmosphere. Larger volumes for biological samples were collected by a small hand pump into sterile glass bottles and kept cold and dark until processed. Before sampling, the sackholes were covered with a plastic lid to avoid CO2 exchange with the atmosphere. Brine temperature (Table 2a) was measured with the digital thermistor in the brine sackholes on site, immediately after sampling. Snow depth was measured in the snow pit as the depth between the slushy layer and the snow surface (Table 1), using a measuring stick. Before CO2-system analysis, the melted sea-ice samples were carefully transferred from the bags to borosilicate glass bottles (250 ml) using tubing to minimize contamination from the atmosphere, and thermostated to 15°C. Table 2a. Properties of the Brine Sampled From Sackholes Drilled at Depths Varying Between 40 and 60 cm in the Iceaa The stations included in the model are marked with an asterisk; "n.d." denotes no data. The fugacity of carbon dioxide (fCO2, μatm) and the carbonate ion concentration ([CO32−]) were calculated using the CO2SYS program [Pierrot et al., 2006]. The average and standard deviations (stdev) are calculated for each parameter and nine measurements. Station Depth (cm) T (°C) S DIC (μmol kg−1) AT (μmol kg−1) AT:DIC fCO2 (μatm) [CO32−] (μmol kg−1) 2 40 −2.3 38.9 2006 2625 1.3 48 418 2 60 −2.3 37.9 2144 2548 1.2 99 273 13 40 −2.5 43.3 2143 2932 1.4 41 515 13 40 −2.5 42.9 2142 2900 1.3 43 536 16 60 −2.4 33.5 2191 2711 1.2 64 369 18* 60 −1.9 33.5 1979 2201 1.1 159 152 21* 40 −2.1 34.3 1463 2357 1.6 9 608 21* 60 −2.1 33 1451 2343 1.6 8 613 33 n.d n.d n.d n.d n.d n.d n.d n.d 35 n.d n.d n.d n.d n.d n.d n.d n.d 36* 40 −1.1 35.3 2204 2426 1.1 210 156 Average ± stdev −2.1 ± 0.4 37 ± 4 1969 ± 300 2561 ± 254 1.3 ± 0.2 76 ± 69 404 ± 180 a The stations included in the model are marked with an asterisk; "n.d." denotes no data. The fugacity of carbon dioxide (fCO2, μatm) and the carbonate ion concentration ([CO32−]) were calculated using the CO2SYS program [Pierrot et al., 2006]. The average and standard deviations (stdev) are calculated for each parameter and nine measurements. [12] For under-ice water sampling we used an electric submersible pump attached to a reticulate pole to allow for undisturbed sampling away from the actual hole, at approximately 0.1–0.2 m below the sea ice, into a borosilicate glass bottle with gas-tight lid, where we immediately measured water temperature. Collection of sub-surface seawater (greater than 5 m) profiles from the ship was co-located with the ice stations, and samples were collected following standard protocols [Dickson et al., 2007] from 12-L Niskin bottles mounted on a General Oceanics 24-bottle rosette equipped with a Conductivity-Temperature-Depth sensor (CTD, Seabird SBE-911 plus). 3.2. Sample Processing [13] AT was determined by potentiometric titration in an open cell with 0.05 M hydrochloric acid (HCl), according to Haraldsson et al. [1997]. DIC was determined by gas extraction from acidified seawater samples, followed by coulometric titration with photometric detection [Johnson et al., 1985, 1987; Dickson et al., 2007]. The precision of the AT and DIC measurements were obtained by triplicate analysis of one sample, and was estimated to ca. ±2 μmol kg−1 and ±1 μmol kg−1, respectively. The accuracy of AT and DIC was controlled against a certified reference material (CRM, batch #90) supplied by Andrew Dickson (Scripps Institution of Oceanography, San Diego, USA) at the beginning and at the end of 20 samples. [14] pH was determined spectrophotometrically (Diode-array spectrophotometer, HP8452) using a 2 mM solution of the sulphonaphtalein dye, m-cresol purple, as an indicator [Clayton and Byrne, 1993]. Prior to analysis the samples were thermostated to 15°C. Samples were measured in a 1-cm flow cell, where the temperature was measured in the sample upstream of the flow cell using a thermistor (Pt 100). The analytical precision was estimated to ±0.002 pH units, which was determined by triplicate analysis of one sample every day. The pH of the indicator solution was measured daily using a 0.2-mm flow cell. The magnitude of the perturbation of seawater pH caused by the addition of the indicator solution was calculated and corrected for using the method described by Chierici et al. [1999]. The accuracy of spectrophotometric pH values is difficult to assess, since it relies ultimately on the physicochemical characteristics of the indicator solution. Commonly, the overall accuracy is determined by the accuracy of the temperature measurements and the accuracy in the determination of the equilibrium constants of the dye, which has been reported to be approximately ±0.002 pH units [Dickson, 1993]. [15] Samples for inorganic nutrients measurements in melted ice samples were filtered using GF/F glass filters (45 μm) under low pressure prior to analysis. Colorimetric determinations of phosphate (PO4), nitrate (NO3), and silicic acid (Si(OH)4), were performed onboard the ship, immediately after melting of the sample, on an autoanalyzer using routine methods [Grasshof, 1999]. Salinity and conductivity of the melted sea ice, brine, and under ice water were measured using a conductivity meter (WTW Cond 330i, Germany) with a precision and accuracy of ±0.05. Salinity was also measured during DIC analysis by a calibrated SeaBird conductivity meter. [16] Samples for pigment measurements were filtered through 25 mm GF/F filters. For determination of chlorophyll a, the filters were extracted in 90% acetone for 24 h in darkness (T = −10°C), and the resultant extract assessed using fluorescence [United Nationals Educational, Scientific, and Cultural Organization, 1996]. Accessory pigment samples were filtered through GF/F filters, frozen at −80°C, and returned to the laboratory for analysis by high performance liquid chromatography. Full details of the analytical techniques are provided by Smith and Asper [2001] and Smith et al. [2011]. Particulate carbon and nitrogen samples were filtered through precombusted (450°C for 2 h) GFF filters, placed in combusted glass vials and capped with combusted aluminum foil, dried at 60°C, and returned to the laboratory for analysis on a Carlo-Erba 1108 elemental analyzer using acetanilide as a standard [Gardner et al., 2000]. Blanks were filters through which a few mL of filtered seawater had passed and were treated as above. [17] For microbial community respiration rate measurements, under-ice seawater, brine, and biological ice core melt samples from highly colored layers each dispensed aseptically following collecting into six sterile, 200-ml pyrex bottles with ground glass stoppers, sealed, and incubated at −1°C in the dark. Pairs were fixed with 0.2% saturated mercuric chloride solution at 0, 24 and 48 h. Samples were processed shipboard for DIC as described above. Net heterotrophic respiration rate was calculated by linear regression through the six time points. Samples for bacterial abundance were also collected from initial samples, fixed with borate-buffered 2% formalin, and returned home frozen for processing on a flow cytometer [Gasol and De Giorgio, 2000]. [18] We used AT and DIC, measured at 15°C, salinity and temperature together with the CO2-calculation program CO2SYS [Pierrot et al., 2006], to calculate fugacity of CO2 (fCO2), and carbonate ion concentrations [CO32−]. We used the CO2-system dissociation constants (K*1 and K*2) estimated by Roy et al. [1993, 1994], since an internal consistency study showed them to be the most suitable constants for cold and fresher surface waters [Chierici and Fransson, 2009]. The calculations were performed on the total hydrogen ion scale (pHT) using the HSO4− dissociation constant of Dickson [1990]. An internal consistency check for the CO2-system parameters was performed, comparing calculated DIC from AT and pH with measured DIC. The linear regression (zero intercept) of 100 data points between the measured and calculated DIC gave a correlation coefficient (r2) of 0.998, and the root mean standard deviation (rms) of ±11 μmol kg−1. Due to the restricted number of data points of measured nutrient concentrations, we excluded PO4 and Si(OH)4 in the CO2-system calculations. Mean values of PO4 and Si(OH)4 in the bulk sea ice were 0.3 and 7.5 μmol L−1, respectively. By not including PO4 and Si(OH)4, the calculated DIC differed on average by 0.4 μmol kg−1 and a relative error of 0.14% for a mean DIC value of 273 μmol kg−1. All plots were compiled using Ocean Data View (R. Schlitzer, 2002, available at http://www.awi-bremerhaven.de/GEO/ODV). 4. Results and Discussion 4.1. Biogeochemical Properties of Sea Ice, Brine and Under-Ice Water 4.1.1. Variability of Temperature, Salinity and Physical Characteristics [19] Table 1 summarizes information on the characteristics of the ice cores, such as sea-ice thickness, snow depth, slush layer, occurrence and depth of freeboard, and stratification. The measured parameters showed large variability across our study area (Figure 4), reflecting typical sea-ice heterogeneity and dynamics, differences in latitude, and our temporal sequence of sampling. The ice floes generally moved in a north-south direction due to the wind (data not shown). The northern stations (#35 and #36) were located close to the ice edge, in the vicinity of the Polar Front, and they were likely affected by warmer surface water and wave action. Figure 4Open in figure viewerPowerPoint Variability in the sea-ice core (cm) of (a) temperature (°C) and (b) salinity, from station #1 (distance 0 km, right end) to station #41 (left end). Station numbers are shown at the top of the plot with reference to Figure 1 and Table 1 for location. [20] A common feature of Antarctic sea ice is deep snow cover, which was found in our study. Snow cover limits the irradiance to the ice and underlying water, affecting primary production and CO2 uptake. The mean snow depth (and standard deviation) was 38 cm (±22 cm). Minimum snow depth of 3 cm was found at station Sta. 41 (#41), whereas Sta. #39 and #40 had the deepest snow layers of 74 and 70 cm, respectively (Table 1). Most of the stations had negative freeboard due to the snow weighing down the snow-ice interface, causing flooding of the upper part of the sea ice. Flooding by seawater affects the properties of the ice, and consequently the processes controlling the CO2 system. [21] All ice cores, except Sta. #13, had a slushy layer between the top 5 and 20 cm, which at times had visible coloring by algal pigments. The ice core at Sta. #13 was the coldest, and covered with a dry, cold, and hard snow layer. Station #13 was also one of three stations with a positive freeboard of 17 cm. This suggests that seawater could not infiltrate the sides to form a slush layer at the snow-ice interface, protecting the sea ice from interaction with underlying seawater. The cores at Sta. #13 and #16 were stratified at several places in the core, and within these we noted a decrease in temperature and salinity, indicating a more frozen layer within the core. The thickest ice cores at Sta. #16 and #27 were likely due to rafting of floes soon after formation or ridging at a later stage. [22] Individual sea-ice cores demonstrated a negligible vertical temperat