Scholarly article on topic 'Ikaite crystals in melting sea ice – implications for <i>p</i>CO<sub>2</sub> and pH levels in Arctic surface waters'

Ikaite crystals in melting sea ice – implications for <i>p</i>CO<sub>2</sub> and pH levels in Arctic surface waters Academic research paper on "Earth and related environmental sciences"

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The Cryosphere

Academic research paper on topic "Ikaite crystals in melting sea ice – implications for <i>p</i>CO<sub>2</sub> and pH levels in Arctic surface waters"

The Cryosphere, 6, 901-908, 2012 doi:10.5194/tc-6-901-2012 © Author(s) 2012. CC Attribution 3.0 License.

The Cryosphere

Ikaite crystals in melting sea ice - implications for pCO2 and pH levels in Arctic surface waters

S. Rysgaard1 2 3 6, R. N. Glud3 4 5, K. Lennert3, M. Cooper2, N. Halden12, R. J. G. Leakey4, F. C. Hawthorne2, and D. Barber1

1 Centre for Earth Observation Science, CHR Faculty of Environment Earth and Resources, University of Manitoba, 499 Wallace Building, Winnipeg, MB R3T 2N2, Canada

2Department of Geological Sciences, CHR Faculty of Environment Earth and Resources, University of Manitoba, 499 Wallace Building, Winnipeg, MB R3T 2N2, Canada

3 Greenland Climate Research Centre, Greenland Institute of Natural Resources, Kivioq 2, 3900 Nuuk, Greenland 4Scottish Association for Marine Science, Scottish Marine Institute, Oban. PA37 1QA, UK 5 Southern Danish University and NordCEE, Odense M, Denmark 6Arctic Research Centre, Aarhus University, 8000 Aarhus C, Denmark

Correspondence to: S. Rysgaard (

Received: 18 February 2012 - Published in The Cryosphere Discuss.: 14 March 2012 Revised: 19 June 2012 - Accepted: 25 June 2012 - Published: 15 August 2012

Abstract. A major issue of Arctic marine science is to understand whether the Arctic Ocean is, or will be, a source or sink for air-sea CO2 exchange. This has been complicated by the recent discoveries of ikaite (a polymorph of CaCO36H2O) in Arctic and Antarctic sea ice, which indicate that multiple chemical transformations occur in sea ice with a possible effect on CO2 and pH conditions in surface waters. Here, we report on biogeochemical conditions, microscopic examinations and x-ray diffraction analysis of single crystals from a melting 1. 7 km2 (0.5-1 m thick) drifting ice floe in the Fram Strait during summer. Our findings show that ikaite crystals are present throughout the sea ice but with larger crystals appearing in the upper ice layers. Ikaite crystals placed at elevated temperatures disintegrated into smaller crystallites and dissolved. During our field campaign in late June, melt reduced the ice floe thickness by 0.2 m per week and resulted in an estimated 3.8ppm decrease of pCO2 in the ocean surface mixed layer. This corresponds to an air-sea CO2 uptake of 10.6mmolm-2 sea ice d-1 or to 3.3 ton km-2 icefloe week-1. This is markedly higher than the estimated primary production within the ice floe of 0.3-1.3 mmolm-2 sea ice d-1. Finally, the presence of ikaite in sea ice and the dissolution of the mineral during melting of the sea ice and mixing of the melt water into the surface oceanic mixed layer accounted for half of the estimated pCO2 uptake.

1 Introduction

Recent work suggests that sea ice may play a central role in the control of the air-sea CO2 flux in high latitude areas (Rysgaard et al., 2011; Loose and Schlosser, 2011; Geil-fus et al., 2012). During winter, as sea ice grows, reduced air temperatures cause the brine volumes of surface sea ice to contract and permeability to decrease, effectively stopping air-sea ice gas exchange (Loose et al., 2011). At the same time brine volume contraction further increases brine salinity, CO2 concentration, and supersaturation with respect to a suite of minerals, including several polymorphs of calcium carbonate (CaCO3) (Assur, 1958). Crystallization of CaCOs [Ca2+ + 2HCO- ^ CaCO3 (s) + H2O + CO2] increases the concentration of CO2 in the brine during growing sea ice conditions. Calcium carbonate crystals can be trapped in the ice matrix during brine rejection. This is equivalent to total alkalinity (TA) retention within the sea ice in the form of CaCO3. In comparison, the total dissolved inorganic carbon (TCO2) is more efficiently lost from the sea ice along with the rejected brine. This can lead to a higher TA: TCO2 ratio in the sea ice brine upon CaCO3 dissolution than the TA: TCO2 ratio in the surface waters. This was observed recently in both Arctic and Antarctic sea ice (Rys-gaard et al., 2007; Fransson et al., 2011). The observation of

Published by Copernicus Publications on behalf of the European Geosciences Union.

TA: TCO2 ratios as high as 2 indicate that calcium carbonate is formed in natural sea ice (Rysgaard et al., 2007, 2009). Ikaite crystals in sea ice have recently been discovered in both hemispheres (Dieckmann et al., 2008, 2010). The specific conditions promoting ikaite precipitation in sea ice are currently poorly understood, but if precipitation occurs in the porous bottom sea-ice layer, where the brine volume is larger than 5% (Weeks and Ackley, 1986; Golden et al., 1998, 2007), the CO2-enriched brine will exchange with seawater via gravity drainage (Notz and Worster, 2009). In contrast, the ikaite crystals could potentially remain trapped within the skeletal layer where they will act as a store of TA, which will become a source of excess TA to the ocean water upon subsequent mineral dissolution when sea ice melts during summer (Rysgaard et al., 2007, 2009, 2011; Nedashkovsky et al., 2009). This will lower the pCO2 of surface waters affected by melting sea ice and cause an increase in the air-sea CO2 flux. More knowledge is, however, needed in order to understand carbonate crystal distribution in sea ice and their fate during summer melt. Here, we report on biogeochem-ical conditions as well as microscopic examination and x-ray diffraction analysis of single ikaite crystals in a melting 1.7 km2 drifting ice floe passing through the Fram Strait in late June 2010.

2 Methods

The research was undertaken in summer 2010 as part of the ICE CHASER II research cruise on the UK icebreaker RRS James Clark Ross (JCR) (cruise JR219). The sampling location is presented in Fig. 1. On 22 June, the JCR was anchored to a 1.7 km2 drifting ice floe at 81° N, 5° E and drifted with the floe to 80° N, 2° E on 30 June 2010. The JCR was used as a laboratory for processing samples collected on the ice floe. Sampling was performed about 0.5 km away from the ship and care was taken not to disturb the coring area that was reached from one side. Sea ice cores (9-cm diameter) were collected with a MARK II coring system (Kovacs Enterprises, USA) on three different sampling dates. We recognize that brine may have been lost upon extraction of the core from the ice (Barber and Yackel, 1999). We estimate brine loss to be approximately 10 percent (±5 %) based on unpublished data collected during the IPY-CFL project in 2008. The measurements were done by replicate samplings (core extraction) from a small area of uniform first-year sea ice (area of about 10 m2). Salinity was measured from these cores, and the variability in the measurements was used to estimate the potential brine loss. Thus, the expectation was that the brine volumes would be approximately equal over this small area, and most of the variability would be due to variable brine drainage. However, brine loss from handling the ice core sections in the laboratory (see below) was minor, as evaluated from the few droplets left in the polyethylene jars and on the clean sheets on which we worked. On each

Fig. 1. Map of the study area. The northern "X" marks the studied sea ice floe position (81° N, 5° E) on 22 June and the southern "X" marks its position (80° N, 2° E) on the 30 June 2010.

occasion, snow and sea ice thickness were recorded. Vertical temperature profiles were measured with a thermometer (Testo, Lenzkirch, Germany, and accuracy 0.1 °C) at 10-cm intervals in the snow and at the center of the cores through 3mm holes drilled immediately after coring. Each sea ice core (one every sampling date) was then cut into 10-cm sections, and each section was transferred to a 1-liter polyethylene jar and kept cold (insulated thermo box) until further processing within an hour in the JCR laboratories. Water samples from 1 m below the sea ice were collected for salinity, temperature, TA and TCO2 determinations with a glass syringe fitted with a gas tight Tygon tube inserted through the ice.

In the laboratory, sea ice density was determined by shaping the ice core section into well defined pieces with planar sides and then measuring the volume and weight of each piece. The pieces were then cut in two. One half was melted within 2 h, and 25 ml was collected for salinity measurements. The salinity of the melted sections (bulk salinity) was determined with a sonde (Knick Konduktometer, Germany) calibrated to a PORTASAL salinometer. Brine volume in sea ice was calculated according to Cox and Weeks (1983) for temperatures below -2 °C, and according to Lepparanta and Manninen (1988) for temperatures within the range 0 ° C to -2 °C. The rest of the meltwater was filtered through a GF/F filter for determination of chlorophyll a (Chl a) concentration. Chl a was extracted from filters for 24 h in 95 % buffered acetone and analyzed by high performance liquid chromatography (HPLC) (Mantoura et al., 1997).

The other half of each sea ice section was used to determine TA and TCO2 concentrations in the following way: The ice segment was placed immediately in a gas-tight laminated (Nylon, ethylene vinyl alcohol, and polyethylene) plastic bag (Hansen et al., 2000) fitted with a 50-cm gas-tight Tygon tube and a valve for sampling. The weight of the

bag containing the sea ice sample was recorded. Cold (1 °C) deionized water (25-50 ml) of known weight and TA and TCO2 concentrations was added together with 50 ^lHgCl2 (saturated solution). The plastic bag was then closed immediately, and excess air and as much deionized water as possible quickly removed through the valve; then the plastic bag was weighed again. The weight of the deionized water accounted for 8-15% of the sea ice weight. The sea ice was melted in the deionized water (at 0 ° C), and the meltwater mixture transferred to a gas-tight vial (12 ml Exetainer, Labco High Wycombe, UK). Deionized water was added in order to remove all the excess air, and as much deionized water as possible was quickly removed through the valve; the plastic bag was then weighed again. The melt water mixture and bubbles released from the melting sea ice were transferred to gas-tight vials. In order to minimize gas exchange between brine and the ambient atmosphere, handling of each sea ice section took few minutes and a sea ice core could be processed within half an hour. Any CaCO3 crystals present in the ice core sections used for this analysis were expected to have dissolved during sampling processing, contributing to the measured TA and TCO2 concentrations. Standard methods of analysis were used: TCO2 concentrations were measured on a coulometer (Johnson et al., 1987), TA by poten-tiometric titration (Haraldsson et al., 1997), and gaseous CO2 by gas chromatography. Routine analysis of Certified Reference Materials (provided by A. G. Dickson, Scripps Institution of Oceanography) verified that TCO2 and TA concentrations (n = 3) could be analyzed within ±1 ^molkg-1 and ±4 ^molkg-1, respectively. Bulk concentrations of TA and TCO2 in the sea ice (Ci) were calculated as:

CmWm - Ca Wa

where Cm is the TA or T CO2 concentration in the meltwater mixture, Wm the weight of the meltwater mixture, Ca the TA or TCO2 concentration in the deionized water, Wa the weight of the deionized water, and Wi the weight of the sea ice.

Following the determination of T CO2 and TA, the bulk partial pressure of CO2 (pCO2) and pH (on the total scale) were computed using the temperature and salinity conditions in the field and a standard set of carbonate system equations, excluding nutrients, with the CO2SYS program of Lewis and Wallace (2012). We used the equilibrium constants of Mehrbach et al. (1973) refitted by Dickson and Millero (1987, 1989).

On 29 June, a sea ice core was collected for visualization and identification of CaCO3 crystals in sea ice. The sea ice core was cut into 10-cm sections and transferred to 1-liter polyethylene jars. The jars were transferred to a temperature controlled laboratory on board the JCR, and each 10-cm section of sea ice was cut into small pieces with a stainless steel knife and allowed to melt slowly over 12 h at -0.2 °C. The jars were then carefully swirled so crystals would concentrate at the bottom center of the jar. A portion of the crys-

tals was then pipetted into a plankton counting chamber for microscopic examination at 100 and 200 magnification using a Zeiss (Carl Zeiss, Welwyn Garden City, UK) Axiovert S100 inverted microscope fitted with an AxioCam HRc digital camera and AvioVision version 4.7 image analysis software. Ten to fifteen images of crystals were obtained from each section. Using the microscope, we soon realized that it was possible to observe ikaite crystals dissolve when temperature was allowed to increase above 4 °C. Therefore, a dissolution experiment was performed on single crystals collected from the uppermost 0-10 cm section. Images of the crystals were obtained 80 times over 400 min while temperature was allowed to increase above 4 °C. The potential for loss of brine upon extraction from the sea ice may have affected both rates and magnitudes of processes reported. We expected that, in general, our estimates would be proportionally underestimated due to the brine loss. Sections of a parallel sea ice core (entire core) were kept at -18 °C and brought to the x-ray laboratory at the Department of Geological Sciences at the University of Manitoba. There, sea ice was melted as described above, and a small droplet of cold water containing transparent crystals was deposited onto a cold glass slide resting on a chilled aluminum block containing a 1-cm central viewing hole. The crystals were first examined with a polarized light microscope to assess their optical properties and then mounted for x-ray study using a stereo binocular microscope. Selected crystals from 0-10 cm, 10-20 cm, and 40-50 cm sections (distance from top of the sea ice) were dragged across the cold glass slide from the water droplet using a metal probe, and immersed into a drop of special purpose sampling oil, which restricted sublimation. Each crystal was then scooped up with a low x-ray scattering micro-loop and rapidly transferred to the nitrogen cold stream (-20 °C) on the x-ray diffraction instrument with a magnetic coupling goniometer head. The x-ray diffraction instrument consisted of a Bruker D8 three-circle diffractometer equipped with a rotating anode generator (MoKa X-radiation), multi-layer optics, APEX-II CCD detector, and an Oxford 700 Series liquid-N Cryostream. The intensities of more than 100 reflections were harvested from six frame series (each spanning 15° in either m or 0) collected to 60° 29 using 0.6 s per 1° frame with a crystal-to-detector distance of 5 cm. In total, 14 crystals were identified through successful indexing of observed x-ray diffraction maxima onto known characteristic unit cells. Of these, 4 were ikaite and exhibited a distinct morphology, as compared to other crystals identified as feldspar, quartz and corundum. After the discovery onboard JCR that ikaite crystals had a distinct morphology (rhombs, thinner plates) and that they dissolved at elevated temperature, or by adding acid, the dimensions of 127 ikaite crystals, identified by light microscopy, were measured, and the crystals grouped into size categories for describing the vertical size distribution through the ice. The procedure was to take a photo of a cold sample, then warm it up for 5-10 min and take a photo again. Crystals dissolving were assumed

Fig. 2. Average conditions of temperature (open circles), bulk salinity (closed circles), brine volumes, TA (open circles), TCO2 (closed circles), TA : TCO2 ratio and Chl a at the coring site of the sea ice floe sampled during 22 to 30 June 2010. Values for temperature, salinity and brine volume are mean ± standard deviation (n = 10) and for TA, TCO2, and Chl a (n = 3).

to be ikaite. After melting the samples at room temperature for 24 h, all ikaite crystals disappeared and crystals left behind were analyzed on x-ray. They were feldspar and quartz, and a single crystal of corundum. Morphologically, they differed from ikaite by not being crystal clear, having a different shape, and with color. The number of ikaite crystals was very high and by far exceeded any other crystals in the sea ice.

In summary, bulk sea ice temperature, salinity, and brine volume were derived from 10 ice cores; TA, TCO2 and Chl a were obtained from 3 ice cores; and ikaite crystal microscopy and distribution with depth in the ice were conducted on 1 core on 29 June. Mineral phase identification by x-ray diffraction was performed on material extracted from reserved core stored at -18 °C.

3 Results

The average sea ice thickness at the sampling site, as determined from coring, decreased from 100 (±5) cm to 80 (±6) cm from 22 June to 29 June (2-3 cmd-1) while the ice floe was drifting SSW. The decayed ice surface of 1-2 cm thickness on 22 June reduced to a mm thick layer during the 8 day sampling period. Air temperatures and water column temperatures 20 cm beneath the sea ice fluctuated around 0 °C (Table 1). Sea ice temperatures varied from 0 to -1.7 °C, indicating a rapidly melting ice floe (Fig. 2).

Bulk salinity over the study period varied from 0.2 to 6. The high temperatures and low salinities caused brine volume to be high. Average brine volumes during 22, 25 and 29 June were 0.16, 0.21 and 0.19, respectively. Brine volumes > 0.05 indicated that the sea ice was very permeable (Freitag and Eicken, 2003). A slight increase in the average TCO2 and TA conditions was observed over the sampling period (Table 2). Concentrations of T CO2 and TA in the vertical profiles (including dissolved ikaite) varied from 80

Fig. 3. Examples of ikaite crystals found in the sea ice. Scale bar is 100 ^m.

to 435 ^molkg-1 melted sea ice and 210 to 680 ^molkg-1 melted sea ice, respectively. The TA: TCO2 ratio in sea ice was on all sampling dates significantly higher than in the water column (Tables 1, 2). Chlorophyll a values in the sea ice ranged from 0.03 to 2.4 ^gkg-1 melted sea ice with highest concentration in the central part of the sea ice.

The crystals (10 to 1000 ^m in size) observed in the sea ice were highly transparent with a rounded rhombic morphology (Fig. 3) and showed uniform extinction under cross-polarized light, suggesting that they were simple single crystals. All reflections fitted well to a monoclinic C-centered cell with (a) = 8.806(2), (b) = 8.315(2), (c)= 11.027(2), A P = 110.571(12), and V = 755.9(4). From the general appearance, optical properties and unit-cell determination, the crystals examined were ikaite (Hesse and Kippers, 1983). The

Table 1. Surface water conditions (20 cm below sea ice) of temperature, salinity, T CO2, TA and pCO2 during the field campaign. Data from triplicate samples collected on three separate occasions are shown.

Date TCO2 TA TA : TCO2 T (°C) Salinity pH pCO2

2010 ^mol kg-1 ^mol kg-1 ^atm

25 June 1968.9 2201.5 1.12 -0.08 32.64 8.36 163

1965.0 2196.9 1.12 -0.09 32.62 8.36 163

1985.8 2197.6 1.11 -0.07 32.56 8.31 183

27 June 2005.9 2252.7 1.12 -0.49 33.27 8.38 157

2010.8 2235.8 1.11 -0.04 32.59 8.33 176

2016.2 2243.5 1.11 -0.70 32.64 8.35 169

29 June 1982.4 2173.2 1.10 -0.01 32.19 8.27 202

1984.8 2168.9 1.09 -0.06 32.31 8.26 210

1967.2 2160.8 1.10 -0.09 32.33 8.28 197

Table 2. Sea ice meltwater conditions of temperature, salinity, T CO2, TA and pCO2 during the field campaign. Values are mean ± standard deviation (n = 10) of vertical profile data from sea ice cores collected on the respective dates with pCO2 conditions calculated at 0 dbar and 0 °C.

Date TCO2 TA TA: T (°C) Salinity pH pCO2

2010 ^mol kg-1 ^mol kg-1 TCO2 (at 0 °C) ^atm (at 0 °C)

25 June 142.3 ± 23.9 304.4 ± 62.6 2.1 -1.3 ± 0.4 3.8 ± 1.3 10.1 ± 0.2 0.2 ± 0.1

27 June 229.0 ± 98.8 421.3 ± 126.1 1.8 -1.0 ± 0.5 4.2 ± 1.8 9.9 ± 0.3 0.5 ± 0.4

29 June 291.5 ± 116.5 533.0 ± 137.3 1.8 -1.1 ± 0.5 3.8 ± 1.8 10.1 ± 0.4 0.3 ± 0.5

crystals identified as ikaite have a very distinct morphology, and are easily recognized - the only significant variation we saw was a thinner plate morphology in addition to the more common thicker rhombs. Ikaite crystals were distributed throughout the sea ice with higher proportion of larger (> 100 ^m size) crystals in the surface layers of the ice (Fig. 4). A crystal of ikaite, identified by single crystal x-ray diffraction, was allowed to decompose by raising the temperature above 4 °C over 4h (Fig. 5). This crystal lost its transparency and gradually disaggregated into a multitude of smaller crystal components that resembled ikaite under the optical microscope. Of these, one tiny (~ 20 ^m) fragment was harvested and examined by x-ray diffraction, and was found to be ikaite as well. We have experimental evidence, from the Sea ice Environmental Research Facility (SERF) mesocosm located at the University of Manitoba (Canada) and from a recent winter field campaign to Northeastern Greenland, that ikaite crystals form within hours in new ice but also form in the interior of thicker ice (data to be presented elsewhere). In contrast, the crystals of quartz, feldspar, and corundum also observed inside the sea ice matrix must have been entrained during ice formation.

4 Discussion

The sea ice floe melted rapidly during the investigation period as it drifted SSW in the Fram Strait. This was evident from the ca. 20 cm week-1 decrease in thickness, the reduc-

tion of bulk salinity, and the very high brine volumes in the entire sea ice column. Much lower TA and T CO2 concentrations in sea ice were measured as compared to the underlying water (Tables 1, 2). The TCO2 concentration in the sea ice was ca. 10 % of the TCO2 concentration in the underlying water, and TA concentration in sea ice was ca. 20 % of the TA in the water below. This, together with the higher TA: T CO2 ratio of 1.83 to 2.12 in the sea ice as compared with the underlying water (1.09 to 1.12), is in line with formation of carbonate crystals within the sea ice. Assuming that sea ice is formed from surface water with a TA: TCO2 ratio of 1, the TA: TCO2 ratio of 1.83 to 2.12 observed in the sea ice could be generated by a carbonate crystal concentration of 162.1 to 241.5 ^molkg-1 melted sea ice. This is high compared with estimates from Antarctica (Dieckmann et al., 2008). Ikaite concentrations in our study are 1.5 to 2.2 times higher than the highest concentration reported from Antarctica. On areal basis the ikaite concentration is more than 10 fold higher. Chl a and a diverse microbial community were also observed within the sea ice in several of our ice cores, supporting previous findings of internal sea ice algal communities (Mock and Gradinger, 1999).

Various ikaite crystals were isolated from the sea ice (Fig. 3). These ranged in size from 10 ^m to large mm-size crystals. They were all highly transparent with a rounded rhombic morphology and showed uniform extinction under crossed polarized light, suggesting that they were all well-crystallined simple single crystals. The crystals identified by

x-ray diffraction as ikaite had a morphology similar to the ones reported from Antarctica (Dieckmann et al., 2008) and from Arctic coastal fast ice locations in Svalbard (Dieckmann et al., 2010) and off Station Nord NE Greenland (Rysgaard et al., unpublished results). Furthermore, we observed ikaite crystals throughout the sea ice column with the largest crystals in the upper layers of the sea ice, in agreement with the observations from the other locations. Our ikaite data originating from the offshore Fram Strait thus strongly suggest that ikaite crystals within the sea ice matrix are representative for ice of the Arctic basin.

Ikaite crystals are considered stable at temperatures below 4 °C. At room temperature ikaite is unstable and decomposes into anhydrous calcium carbonate, vaterite and/or calcite and water (Marland, 1975; Shaikh, 1990; Mikkelsenetal., 1999). Furthermore, relative humidity has been identified as an important factor controlling the rate of decomposition of ikaite (Mikkelsen et al., 1999). The observation that a crystal of ikaite, identified by single crystal x-ray diffraction, disintegrated into smaller crystallites and eventually dissolved when temperatures increased above 4 °C over a few hours (Fig. 5) confirms that ikaite crystals are highly unstable. Dissolving could in principle also be related to ikaite's reaction with CO2 in the atmosphere or from interaction with the carbonate system in the water column changing the pH of the ikaite crystals' surroundings. More work is needed to resolve the kinetics of dissolving ikaite crystals in sea ice. Melting of sea ice sections in gas tight bags at 0 °C also resulted in very low CO2 levels in bulk sea ice sections throughout the ice column (Table 2). In addition, pH values increased to 10 in

Fig. 5. Ikaite crystal dissolution during melting of sea ice over 240 min. Arrows point at smaller sub-units of crystals (also ikaite) that emerge during melting. Scale bar is 50 ^m.

the bulk sections. It is likely that ikaite crystals in the lower parts of the sea ice will disintegrate before the ikaite crystals higher up in the sea ice, as they will be in closer contact with the carbonate system of the underlying water due to higher brine volumes in these lower ice layers earlier in the melting season. This would explain why the largest crystals remained in the upper ice layers (Fig. 4). Another explanation for the largest ikaite crystals being found in the top layers of the sea ice is because they had the greatest opportunity (i.e. time) to grow into bigger crystals.

Melting of sea ice and dissolution of ikaite crystals will affect pCO2 levels and the pH of surface water. We observed pCO2 levels within sea ice of < 1 ^atm, and melting sea ice contributed to the low pCO2 conditions in surface waters (157-210 ^atm) as compared with the 387 ^atm in the atmosphere (Tables 1, 2). In addition, pH in melting sea ice was high (pH = 9.9-10.1) and affected pH of surface waters (pH = 8.3-8.4). Assuming that all ikaite crystals dissolve in the sea ice or in the mixed layer, melting of 0.2 m sea ice - with average temperature (-1.1 °C), salinity (3.9), TA (420 ^molkg-1) and TCO2 (221 ^molkg-1) from Table 2 - into a 20-m thick mixed layer - with average water column characteristics of temperature (-0.2 °C), salinity (32.6), TA (2203 ^molkg-1) and TCO2 (1987 ^molkg-1) from Table 1 - will result in a 3.8 ppm decrease in pCO2 per week. This decrease is calculated from the resultant conditions in a 20-m mixed layer - temperature (-0.20C), salinity (32.2), TA (2186 ^molkg-1) and TCO2 (1970 ^molkg-1) - using the CO2SYS program (see materials and methods). Assuming no CaCO3 crystals, e.g. TA and TCO2 concentrations are both 221 ^molkg-1, the resultant pCO2 decrease will be 2.2 ppm per week. Based on average conditions during the field campaign (Tables 1 and 2), this corresponds to an air-sea CO2 uptake of 10.6mmolm-2 sea ice d-1, or to 3.3 ton CO2 km-2 ice floe week-1 (with CaCO3) and 4.9 mmol m-2 sea ice d-1, or 1.5 ton CO2km-2 ice floe week-1 (without CaCO3). It should be noted that we do not take wind mixing into account, but just consider the resultant CO2 uptake of melting 0.2 m sea ice into a 20-m mixed layer after a return to initial pCO2 conditions. An important finding here is that presence of CaCO3 in sea ice will double the air-sea flux as compared to melting of pure sea ice. This sea ice driven CO2 uptake is higher than the estimated primary production within the ice floe of 0.3-1.3 mmol m-2 sea ice d-1 (Glud et al., work in progress), as assessed from > 100 measurements of primary production incubations using 14C. Thus, sea ice melting plays a more important role in atmospheric CO2 uptake than concurrent primary productivity of sea ice algae. This role is further enhanced by the dissolution of the authi-genic CaCO3 content of the sea ice in the form of ikaite.

In the present study we melted sea ice at 0 0C to isolate ikaite crystals. However, we do not know if other carbonate crystals were present in the sea ice and dissolved by our melting procedure. Nor do we know if ikaite crystal continuously form and dissolve during the season; a process, which could

greatly increase the air-sea CO2 flux. A conceptual model on the processes driving air-sea gas exchange throughout the cycle of sea ice formation and decay was recently provided by Rysgaard et al. (2011). It was suggested that ice growth during winter results in the rejection of CO2 along with salts dissolved in seawater from the ice crystal matrix, which gives rise to dense brine that sinks and is incorporated into intermediate and deep water layers. Subsequent sea ice melt leads to stratification of the surface water column and mixing with melt water already low in TCO2 from biological uptake and from excess TA from the internal sea ice CaCO3 cycle. The net result will be an increase in TA and a lowering of CO2 in the stratified surface waters, enhancing the air-sea CO2 flux. Our present study performed in and below an actively melting ice floe supports the conceptual model and provides evidence that melting sea ice and the dissolution of its authi-genic CaCO3 content indeed lead to an increased CO2 flux from the atmosphere to the ocean. In addition, dissolving carbonate crystals will increase pH in surface waters exposed to melting sea ice.

5 Conclusions

Our study shows that ikaite crystals were present throughout the ice column of a drifting sea ice floe in the Fram Strait. Larger crystals were observed in surface layers. Melting of the sea ice caused extremely low pCO2 conditions within sea ice (< 1 ^atm) and contributed to the low pCO2 conditions in surface waters (157-210 ^atm). In addition, pH in melting sea ice was high (pH = 9.9-10.1) and affected pH of surface waters (pH = 8.3-8.4). Melting of sea ice and the dissolution of its authigenic ikaite content during June was estimated to be responsible for an air-sea CO2 uptake of 10.6 mmol m-2 sea ice d-1. This is higher than the estimated primary production within the ice floe of 0.3-1.3 mmol m-2 sea ice d-1. Finally, dissolution of ikaite crystals doubled the air-sea CO2 uptake as compared with melting of ikaite-free sea ice.

Acknowledgements. The study received financial support from the Danish Agency for Science, Technology and Innovation, the Canada Excellence Research Chair (CERC) program, and the UK Natural Environmental Research Council Oceans 2025 strategic marine research programme. The study is a part of the Greenland Climate Research Center's activities, GCRC6507 ( We are grateful to the captain, officers and crew of the RSS James Clark Ross for their assistance during the study and to Anna Haxen, Louise Melgaard, Thomas Krogh, Debra Brennan, Sian Lordsmith and Tim Brand for technical assistance in the laboratory.

Edited by: H. Eicken


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