Sea-ice sampling and sample processing

Sea-ice sampling was performed during March 2009 in Kongsfjorden, Svalbard (Fig. 1; Table 1). Repeated sampling (stations 1, 2 and 4) was performed during four successive days ∼500m from the harbour of Ny Ålesund, accessible within a walk of 15 min from the Arctic Marine Laboratory where part of the analysis was performed. Station 3 was obtained 200 m offshore from the harbour within somewhat thicker ice. Two further sampling sites (5 and 6) were located 10 km to the east and accessed by snowmobiles.

Fig. 1.

Field sampling in Kongsfjorden, Svalbard, during March 2009: (a) sampling locations and (b) meteorological conditions in Ny Ålesund during (approximate) freeze-up and sampling 2–3 weeks later. Dates are day/month.

Table 1.

Sea-ice sampling conditions; superscript numbers indicate the observed variation in the last digit(s). Except for S _Water, these variations are larger than the measurement errors. Stations 1, 2 and 4 (denoted with r ) were located at the same site

Ice cores were obtained with a 7.25 cm diameter coring device (Mark III, Kovacs Enterprises), immediately cut into 3–4 cm thick subsamples and packed in their natural orientation into conical lockable plastic beakers. By this method we also collected the brine at the bottom of the beakers that drained prior to centrifugation (in most cases a small fraction and generally <10% of the total drained plus centrifuged brine).

On a first ice core, temperatures were measured with a penetration probe. These bulk ice samples were later melted for electrolytic conductivity and ion concentration measurements. For a second core, different procedures were applied at different stations. At stations close to Ny Ålesund, the plastic boxes were packed into Styrofoamor small dewars and rapidly (within 20 min) transported to the laboratory, where they were set into temperature-controlled freezers (WAECO Coolfreeze T56). the freezers had been preset close to the sample in situ temperatures estimated from an initial surface temperature measurement. the largely linear temperature profiles of the thin ice, with lowest temperatures at the surface, allowed this simple procedure. Due to the small temperature range of the ice cores (1–1.5 K for the thinnest and 3–4 K for the thicker ice) the preset values in most cases agreed with the actual observations within 0.3– 0.5 K. For the remote stations, the temperature-controlled boxes were transported by snowmobiles to the field sites and either powered by the engines or an aggregate. Samples were then directly placed into the corresponding temperature boxes. the procedure ensured that samples to be centrifuged in the laboratory remained close to their respective in situ temperatures. At every station, at least one core for bulk ice and centrifuging was obtained.

After a maximum of 1–2 days of storage in the laboratory, the samples were centrifuged at their respective temperatures. After cooling the centrifuge (Sarstedt LC 1K with minimum –30 ^˚ C) for 30 min, specially prepared plastic beakers were set into the stainless-steel beakers of the rotor and centrifuged for 15 min. Since many of our samples were very warm and relatively soft, a value of 10g (10 × gravitational acceleration) was selected. For 3–4 cm thick samples, this corresponds to a pressure of ∼3–4 kPa, well below the lowest tensile strength values (20–50 kPa) observed for natural sea ice (Reference Weeks, Ackley and UntersteinerWeeks and Ackley, 1986). the centrifuged brine was collected and added to the (generally small) amount of brine that had drained during storage in the sample beaker. Its mass was measured and, after equilibration to room temperature, its electrolytic conductivity. the centrifuged ice was immediately packed into plastic bags and put into a –80 ^˚ C freezer.

In a further cutting procedure, the ice samples were reduced to 2×2 cm sized cylinders in order to fit them into sample holders for synchrotron-based X-ray microtomography (SXRT) imaging. To minimize the metamorphosis of the centrifuged samples, they were kept near –80 ^˚ C prior to imaging by transport and storage on dry ice. During the imaging process, temperatures were kept below –40 ^˚ C. SXRT then yielded three-dimensional non-destructive images of 1.3–1.6 cm diameter and 1.7 cm height with 11.84 μm voxel resolution. Compared to earlier methods, SXRT has a large potential to improve the understanding of sea-ice physics (Reference MausMaus and others, 2010). the method exceeds the resolution of recent conventional tomography applications (Reference Golden, Eicken, Heaton, Miner, Pringle and ZhuGolden and others, 2007; Reference Pringle, Miner, Eicken and GoldenPringle and others, 2009) by a factor of four in our case. Analysis of these first SXRT images of natural sea ice is ongoing and will be described elsewhere. For the present study, the images have merely been used to distinguish granular from columnar ice (Fig. 2).

Fig. 2.

Horizontal images of 13 cm thick young sea ice, obtained by non-destructive SXRT, at a temperature ≤–40 ^˚ C. Air (revealing centrifuged brine) appears bright, ice is grey and residual salt crystals are dark. Left: granular ice ∼2 cm from the surface; right: columnar ice ∼6 cm from the ice–water interface, showing the well-known lammelar plate structure.

Salinity and conductivity

Electrolytic conductivity measurements were performed on melted bulk ice, melted residual ice samples and the centrifuged brine. Conversion to salinity was made using standard equations (e.g. Reference Millero, Feistel, Wright and McDougallMillero and others, 2008). the WTW Cond340i instruments used, with nominal accuracy of 0.5% for conductivity, were first calibrated with a standard KCl solution and later checked for consistency with a conductivity–temperature–depth (CTD) instrument (SD 204, Saiv A/S). the instrument accuracy was 0.03 for salinity and 0.01 K for temperature (this instrument was also used for profiling the water column below the ice to 30 m depth). Ice and brine salinities obtained in this way have an accuracy of better than 0.2. the CTD-derived sea-water salinities, also used for validating the chemistry measurements, have a relative accuracy of 0.1%.

Chemistry

The melted ice and brine samples were kept dark and at a temperature of 4 ^˚ C before being analysed in the Laboratory of Radiochemistry and Environmental Chemistry of the Paul Scherrer Institute, Villigen, Switzerland. Concentrations of the six major sea-water ions (Cl ⁻ , Na⁺, , Mg²⁺, Ca²⁺ and K⁺) were obtained by standard ion chromatography (IC). Instead of comparing absolute concentrations we investigate the relative deviation of the ratio of these ions to chlorine CX /C _Cl − from that of Standard Mean Ocean Water (SMOW), i.e.

This fractionation measure, first applied by Reference TsurikovTsurikov (1974) to sea ice, makes different salinities and thus the results for ice and brine samples comparable. It also has the advantage of cancelling dilution errors. SMOW ratios are taken from Reference Millero, Feistel, Wright and McDougallMillero and others (2008, Table 2).

From observations by other authors (e.g. Reference Pettersson, ld, Stockholm and BeijersPettersson, 1883; Reference WieseWiese, 1930; Reference Anderson and JonesAnderson and Jones, 1985) we expect that the sea-water ΔX_{/ Cl} − signatures are stable during winter and deviate by <1% from SMOW. To validate our procedure we compare seven sea-water samples resembling winter conditions in Svalbard waters. Four samples were obtained during our fieldwork from 5–20m depth below the ice, while three samples were acquired from Storfjorden.

Figure 3a depicts the relative deviations of absolute concentrations of any ion (including chlorine) from SMOW, and Figure 3b depicts the fractionation (ΔX_{/ Cl} −)_SMOW. Prior to IC analysis we often diluted Ca²⁺ and K⁺, present in normal sea water as ∼2% weight fraction of Cl ⁻ , to concentrations less than 20 times the detection limit (shown in Fig. 3 as thin vertical bars). This dilution may explain the large scatter and a standard deviation of 5–7% for these ions. For SO₄ ²⁻ and Mg²⁺ we found a significant systematic deviation of ΔX_{/ Cl} − from SMOW, which we attribute to an uncertainty of ∼5% in the IC calibration and analysis. We therefore reference all results in the following to our slightly different sea-water measurements in Figure 3b.

Fig. 3.

Ionic composition of winter sea-water samples from Svalbard: (a) relative deviations from SMOW concentrations for each ion and (b) fractionation or relative deviation of ionic ratios X _ion ^/ Cl ⁻ from SMOW. the average of seven samples, each processed twice, is shown with ±1 standard deviation (large symbols and thick bars), observations as dots and detection limits as thin bars. the salinity is scaled such that the ticks marking S = 35 for each ion are spaced by 5 units. Two samples with highest salinities are from a laboratory experiment.

Sea-ice growth conditions

The meteorological conditions during and prior to sampling are summarized in Figure 1b. We used daily ice charts produced by the Norwegian Meteorological Institute (mainly based on observations from different satellites), information on freeze-up by the local station staff, our classification of granular/columnar ice as well as other information in Table 1 to propose the following ice-growth history.

Kongsfjorden was ice-free during the exceptionally warm January of 2009. After 2 weeks of persistent low temperatures, some fast ice originated in the inner fjord by mid-February. A warming event with 4 cm of precipitation (17–21 February) was then followed by strong winds and moderate cooling (–14 to –15 ^˚ C). This likely created a slushy snow ice cover in many places which then froze under calm cold conditions (–15 to –19 ^˚ C) during 25–28 February. on 1 March, temperatures rose again to –14 ^˚ C and the first 2mmw.e of snow were deposited on the ice; temperature subsequently rose again (to –10 to –4 ^˚ C). Another centimetre of precipitation fell during the following week. the daily average temperature then rose over 2 days to a maximum of –2.4 ^˚ C on 9 March. on the first two days of our sampling programme (10 and 11 March), temperature had decreased to –4 to –5 ^˚ C. the temperature then fell again and remained mostly between –19 and –15 ^˚ C during the other sampling days.

The largest thickness of granular ice from all cores found from the surface downwards was 23 cm (almost the entire core) at station 3 closest to the harbour. This was similar in a bay east of the harbour, likely reflecting the wind-enhanced onshore accumulation of snow slush 2 weeks prior to sampling. the ice at stations 5 and 6 in the inner fjord (Fig. 1a) was thicker. In the structural core of the easternmost station 5, the upper 12 cm of the 32 cm was granular with large grains, followed by an 8 cm transition layer with both columnar and granular ice. At the slightly more western station 6, both the granular ice thickness of 6–7 cm and the total thickness of 22 cm were lower. From the mentioned 4 cm of precipitation between 17 and 21 February, we would expect (assuming a solid packing fraction of 0.4–0.5) an average snow-ice thickness of ∼7–9 cm. Higher thicknesses may accumulate near barriers and coasts. In this sense, the largest eastern granular ice thickness was consistent with the proximity of this station to a shallow called Breskjera. Taking this aspect into account, we suggest that despite varying in thickness the ice at stations 3, 5 and 6 formed at the same time (i.e. 2–3 weeks prior to sampling).

The repeated site (stations 1, 2 and 4) was 300m further offshore than station 3. It only showed 3–4 cm of granular ice at the surface of thinner (13–14 cm) ice. Near this site, which was close to the bottom slope towards the 200m deeper central Kongsfjorden, we observed thin ice and permanent leads. the near-surface water temperatures of –1.5 ^˚ C (0.4K above the freezing point) and higher values of –0.9 to –1.0 ^˚ C at 20m depth highlight the role of upwelling Atlantic Water in retarding ice growth.

Hydrographic conditions measured by CTD at stations 1–4, which indicate a weakly stratified water column with salinities of 34.3 and 34.4 near the surface and at 20m depth, respectively, favour such upwelling of heat. In contrast, the 20m temperatures were much lower (–1.5 to –1.7 ^˚ C) at the eastern stations. Lower near-surface temperatures also indicate that such effects are limited. Considering the precipitation record also, we suggest that this ice has very likely formed 2–3 weeks prior to sampling but has grown less rapidly.

Brine, bulk and residual ice salinity

Profiles of bulk ice salinities for the repeated station (1, 2 and 4) of primarily columnar ice are shown in Figure 4a. an interesting aspect to be discussed is that the ice salinity increased in a stage of warming and slight melting from 10 to 11 March, and then decreased again.

In addition to the bulk salinity, centrifuging provides (1) the hydrodynamically accessible salt fraction and (2) the salt fraction that is hardly extractable, located in disconnected voids or dead ends. the residual salt fraction that remained in the ice after centrifugation as a function of the total volume fraction of brine in the bulk ice is shown for all samples in Figure 4b.

Fig. 4.

(a) Bulk ice salinity profiles for the ∼14 cm thick ice during the course of three days at the repeated site (stations 1, 2 and 4) and (b) residual salt fraction (for all stations 1–6) after centrifugation as a function of the overall brine volume fraction in the bulk ice.

For measured salinity and mass of brine (S _b and m _b) and residual centrifuged ice (S _r and m _r), the bulk ice salinity is given as

We define the residual salt fraction f _r (non-dimensional) as the ratio of non-centrifugable to total mass of salt in the bulk ice:

Under thermodynamic equilibrium, f _r is equivalent to the ratio of residual to total brine volume.

The residual salt fraction f _r was computed assuming thermodynamic equilibrium with the freezer temperatures, using equations for brine volume from Reference Cox and WeeksCox and Weeks (1983). Our ensemble mean of 0.35 ± 0.09 for the salt fraction that remained in the ice is slightly larger than 0.2– 0.3 obtained by Reference Weissenberger, Dieckmann, Gradinger and SpindlerWeissenberger and others (1992) for older ice in the Southern Ocean, yet comparable to laboratory data of young ice summarized in Reference MausMaus and others (2010). We do not yet know if this difference may be attributed to different ice age and structure or is due to our order-of-magnitude smaller centrifuge acceleration, resulting in the incomplete removal of interconnected brine. the loss of brine would also overestimate the entrapped salt fraction. Since the samples were rapidly placed in beakers to collect brine draining prior to centrifugation, this effect was likely small.

Ion fractionation: brine versus residual ice

We now describe the fractionation of the major sea-water ions with respect to chlorine. Brine, bulk ice and residual ice salinities and ion concentrations scale in our observations typically as S _b : S _i : S _r ∼15 : 3 : 1. We compare them in Figure 5 on a logarithmic scale of the observed Cl ⁻ concentration. Brine samples appear as triangles at the high end, residual ice samples as circles at the low end and bulk ice samples as squares in between. For every observation a bar shows either the detection limit or standard error from the sea-water sample calibration in Figure 3, whichever is larger. We also show snow samples and indicate upper ice (3–4 cm) samples exposed to the lowest temperature.

Fig. 5.

Ion fractionation for all samples from stations 1–6, comparing brine, bulk ice and residual ice after centrifugation. the vertical bars are the detection limits or one standard error from the sea-water calibration, whichever is larger. Note the different scales for the ions. Surface ice samples are indicated as black dots, and snow samples are shown as stars. A cross on the abscissa indicates the Cl ⁻ concentration 19 353 ppm of standard sea water (SMOW) with salinity 35.

Statistics of these groups are summarized in Table 2. In addition, we compute statistics of the difference between brine and residual ice for the two warmest ice cores 1 and 2 (see Table 1).

Table 2.

Ion fractionation in young sea ice from Kongsfjorden. Values in parentheses after the class averages are the p-level of the Student’s t test; numbers in italics indicate the 95% level. Superscript a denotes that the t test is passed at 95% but the signal is not larger than the standard error of the sea-water reference measurements from Figure 3b

For sodium ions (Fig. 5a) no dependence on sample type (residual ice, brine, bulk ice, snow) or chlorine concentration was evident. Most observations lie in the range –0.04 to +0.04. We found 0.5–1% fractionation in most samples, differing from 0 at significance levels of 90–95%, with the higher values and significance for the surface and brine samples (Table 2). However, all signals are similar to the uncertainty of our sea-water standard calibration. A few exceptionally low values indicate the method’s reproducability and also appear in Figure 5b and c for the fractionation of Mg²⁺ and Ca²⁺. For magnesium ions (Fig. 5b) the spread is somewhat larger, but a significant difference between the ice types (or surface samples) is not present.

The distribution of sulphate ion fractionation (Fig. 5c; third column in Table 2) indicates two findings. First, almost all surface and snow samples (indicated as black dots) show positive values and thus enrichment in sulphate. Second, the brine samples show a clustering of particularly low fractionation values (depletion) at lowest chlorine concentrations in the range 15 000–20 000 ppm Cl ⁻ . These samples are from the warmest stations 1 and 2 where centrifuged brine salinities were low. Depletion is found to be highly significant in the difference of brine and residual ice at these two stations (p = 0.004 in Table 2).

The calcium ion does not show a significant fractionation in any of the groups (Fig. 5d). Most observations fall between –0.1 and +0.1, a spread comparable to the standard error.

The most noteworthy ion is potassium. In all classes except the bulk ice (with the smallest number of 20 samples) the hypothesis of zero fractionation is rejected at high levels of significance (fifth column in Table 2). In contrast to SO₄ ²⁻ , the most anomalous behaviour is observed in the residual ice which is considerably depleted in potassium (Fig. 5e). A low-salinity snow sample behaves in the same manner. To emphasize this signature, in addition to Δ_K+/ _Cl − the potassium fractionation with respect to Mg²⁺ is also shown in Figure 5f, revealing similar results.

Depth and time dependence

The significant fractionation results for and K⁺ were investigated further by focusing on their depth dependence. the repeated site observations have been averaged at four depths of the 14 cm thick young ice. the mentioned surface enrichment of sulphate in Figure 5c is seen in the ice profile in Figure 6a. Sulphate depletion is seen at all depths below the surface in brine and residual and bulk ice, but is most pronounced in the brine. Figure 6b shows that the depletion of K⁺ in the residual ice was found at all depths. Figure 6a, c and d indicate for , Na⁺ and Mg²⁺ an upwards increase of enrichment with respect to chlorine. the same behaviour is found for the snow on top.

Fig. 6.

Ion fractionation averaged for different depths for the repeated station (1, 2 and 4). the upper values are snow samples, while the sea-water zero fractionation reference is emphasized as a dashed vertical line with its uncertainty as a horizontal bar. Slightly larger error bars for the ice and snow values (see Fig. 7) have been omitted for clarity.

As bulk ice samples consist of brine and residual ice, their signatures should fall between them. This is only the case for K⁺ (Fig. 6b) and consistent with high significance for this ion alone. As shown in Table 2, the results are generally less significant for the relatively small (n = 20) set of bulk ice samples. As these stem from close but different ice cores, the difference may be natural variability. To investigate this question further, we focused on the time dependence of the non-surface (3.5–14 cm) part of the repeated station over the course of three days. In Figure 7a it is seen that while the SO₄ ²⁻ fractionation in the residual ice did not change during this period, the brine signal changed from depletion to the neutral residual ice level. the bulk ice signature follows the residual ice signature. the brine fractionation also approaches the residual ice level that remains depleted for K⁺ (Fig. 7b). Here, however, the bulk ice signature shift is similar to the brine. Figure 7 indicates that signatures in brine and bulk ice are much more variable than in the residual ice.

Fig. 7.

Temporal evolution of fractionation of (a) , (b) K⁺ and (c) Na⁺ averaged over the 3.5–14 cm columnar ice from the repeated stations 1, 2 and 4. Dates are day/month in 2009. For clarity, values for brine, residual ice and bulk ice are shown slightly lagged.