Seasonal development of the properties and composition of landfast sea ice in the Gulf of Finland, the Baltic Sea

JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 109,, doi:10.1029/2003jc001874, 2004 Seasonal development of the properties and composition of landfast sea ice in the Gulf of Finland, the Baltic Sea Mats A. Granskog, 1,2 Matti Leppäranta, 1 Toshiyuki Kawamura, 3 Jens Ehn, 1 and Kunio Shirasawa 4 Received 27 March 2003; revised 22 July 2003; accepted 10 September 2003; published 20 February 2004. [1] The seasonal development of the structure, salinity, and stable oxygen isotopic composition (d 18 O) of landfast sea ice was studied during the winter seasons 1999 2001 in the Gulf of Finland in the Baltic Sea. The main focuses were on the seasonal and the interannual variability in ice properties and composition and on the contribution of meteoric ice to sea ice mass balance. Results provide a first statistical description of the seasonal evolution of sea ice in mild ice climate conditions. The ice has a characteristic structure with an upper granular ice layer, composed almost exclusively of superimposed ice and snow-ice, averaging at 20 35% of the total ice thickness. The remaining is composed of columnar or intermediate granular columnar ice, depending on growth conditions. While salinity shows a uniform profile through the ice, d 18 O shows lower values in the surface because of meteoric ice formation. The thin ice cover is susceptible to changes in atmospheric conditions, and rapid changes in ice salinity are connected to changes in the ice thermal regime and flooding. The contribution of meteoric ice varied from 0 to 35% (by mass), depending on season and year. Superimposed ice formation is a recurring process and significantly contributed to ice growth (up to 20% by mass), especially late in the season during snowmelt-freeze cycles. Liquid precipitation also cause formation of intermittent superimposed ice layers at these latitudes. The contribution of meteoric ice to sea ice mass balance is largely dependent on the amount and timing of snow accumulation and timing of snowmelt-freeze processes, which all showed large year-to-year variation. The conditions presented here may start to occur at higher latitudes if global warming continues. INDEX TERMS: 1863 Hydrology: Snow and ice (1827); 1827 Hydrology: Glaciology (1863); 4504 Oceanography: Physical: Air/sea interactions (0312); KEYWORDS: sea ice, meteoric ice, snow ice, superimposed ice, Baltic Sea Citation: Granskog, M. A., M. Leppäranta, T. Kawamura, J. Ehn, and K. Shirasawa (2004), Seasonal development of the properties and composition of landfast sea ice in the Gulf of Finland, the Baltic Sea, J. Geophys. Res., 109,, doi:10.1029/2003jc001874. 1. Introduction [2] The Baltic Sea belongs to the seasonal sea ice zone, with the ice season lasting from some weeks in the south to about six months in the north. The interannual variability in the ice conditions is considerable [Haapala and Leppäranta, 1997], and there is evidence that suggests that the ice extent in the Baltic Sea is well correlated with large-scale atmospheric circulation patterns [e.g., Tinz, 1996]. 1 Division of Geophysics, Department of Physical Sciences, University of Helsinki, Finland. 2 Now at Arctic Centre, University of Lapland, Rovaniemi, Finland. 3 Institute of Low Temperature Science, Hokkaido University, Sapporo, Japan. 4 Sea Ice Research Laboratory, Institute of Low Temperature Science, Hokkaido University, Mombetsu, Japan. Copyright 2004 by the American Geophysical Union. 0148-0227/04/2003JC001874 [3] Although the salinity of northern Baltic Sea water is as low as 2 7 PSU in the surface layer the ice shows typical sea ice features. This holds down to about 0.6 PSU for the salinity of the parent water, and only in proximity of rivers the Baltic Sea ice shows characteristic freshwater structure [Palosuo, 1961]. To date seasonal time series observations on any aspect of the sea ice cover are sparse in the Baltic Sea, even on landfast sea ice [e.g., Palosuo, 1963; Norrman and Andersson, 1994; Granskog et al., 2003b]. Time series observations are vital for the understanding of the seasonal development of sea ice covers, for physical, biological and chemical studies. Palosuo [1963] studied the seasonal evolution of sea ice salinity and contribution of snow-ice at a number of sites on the Finnish coast, by measuring the upward growth of the ice cover using a stick frozen into the ice sheet. The results showed large interannual variability, from 0 to about 50%, in the contribution of upward growth to the total thickness of landfast ice. In pack ice the composition is rather variable with a considerable granular 1of11

ice contribution [Omstedt, 1985; Weeks et al., 1990]. Oxygen isotopic data of landfast sea ice in the Baltic Sea indicate a substantial contribution from meteoric ice to the overall mass balance of sea ice [Kawamura et al., 2001; Granskog et al., 2003a]. The term meteoric ice is used for that part of the sea ice cover which contains atmospherically derived ice [e.g., Lange et al., 1990], i.e., precipitation transformed into ice. Snow-ice and superimposed ice layers contain meteoric ice. Snow-ice forms of a mixture of snow and seawater (slush), and a heavy snow load that suppresses the ice surface below the water level is one, but not the only, prerequisite for snow-ice formation. Superimposed ice forms solely of snowmelt water [e.g., Haas et al., 2001; Kawamura et al., 1997], or from liquid precipitation (as will be shown later). [4] In a Finnish-Japanese cooperative project, entitled Ice climatology of the Okhotsk and Baltic Seas, the seasonal evolution of the properties of landfast sea ice in the Baltic Sea was studied during three winters, 1999 2001. The main objective was to study the seasonal cycle of landfast sea ice development, and to relate its evolution to atmospheric and oceanographic conditions. The ice data collection included stratigraphy, crystal structure, salinity, and stable oxygen isotopes. Special attention was paid to get reliable estimates on meteoric ice contribution to sea ice mass balance. Preliminary results from winter 1999 showed snow-ice and superimposed ice layers up to half of the total ice thickness [Kawamura et al., 2001]. This paper reports the final results of this landfast ice structure and properties mapping. 2. Material and Methods 2.1. Sample and Data Collection [5] Sea ice samples were collected weekly throughout the ice seasons in winters 1999, 2000, and 2001 in Santala Bay, located on the northern side of the Hanko peninsula in the Gulf of Finland, the Baltic Sea (Figure 1). The number of sampling days was 36 (13 + 12 + 11), and additional work was done during some intensive periods. Because of its sheltered location and shallowness (mean depth about 6 m) Santala Bay freezes over every winter. The ice season starts on average in the beginning of January and lasts to mid- April. Only the very early and late stages of the ice seasons are not covered with our data. [6] Ice cores were collected using a stainless steel ice auger, with a diameter of 13 cm, put into plastic tubing, and kept frozen at 20 C until analysis. Immediately after coring the ice temperature was measured from small holes drilled into the core at about 5 cm intervals. Under-ice water samples were collected into 25 ml vials for salinity and stable oxygen isotopic analysis. Snow samples were collected directly into polyethylene (PE) ziplock bags. Snow depth and the freeboard of the ice were also recorded. [7] An automated weather station (SQ-1202, Grant Instr., UK) was installed on a platform anchored near the center of Santala Bay to monitor the wind speed and direction, solar radiation, humidity, and air temperatures at 1-hour intervals at 2 m height. After the ice had grown to a thickness of about 20 cm, a thermistor chain was installed through the snow, ice and top layer of under-ice water. A 3-D electromagnetic current meter (ACM32M, Alec Electronics Co. Ltd, Japan) Figure 1. (top) Location of study area in the Gulf of Finland, the Baltic Sea. (bottom) The study site Santala Bay, in the center of the circle, on the northern side of the Hanko peninsula and the Tvärminne Zoological Station (TZS) is also shown (triangle). and a conductivity-temperature recorder (Alec MDS-CT, Alec Electronics Co. Ltd, Japan) were installed at about 2 m depth. The data cover the period from December through June in winters 1999 2000 and 2000 2001, but for winter 1998 1999 a short period in March only. Additional weather data (air temperature, wind speed and direction, snow depth, and daily precipitation) was routinely obtained from the weather station at the Tvärminne Zoological Station of the University of Helsinki (hereafter TZS), located about 10 km southeast from the observation site (Figure 1). 2.2. Sample Analysis [8] The ice cores were first examined for ice structure in a coldroom ( 20 C), at the Technical Research Centre of Finland, Espoo, or at the TZS. Each core was split lengthwise to obtain a 1 cm thick section, which was further cut into 10 to 20 cm long sections. These were attached to glass plates, planed to a thickness of about 5 mm to observe the gas bubble content and then further to about 1 mm thickness for the crystal structure in polarized light. The ice cores were divided into textural classes; granular ice, intermediate granular columnar ice (hereafter g/c ice), and columnar ice [see Eicken and Lange, 1989]. The remainder of the core was divided into sections along distinct structural boundaries for salinity and stable oxygen isotopic analysis. A 100 200 ml sample of each section was taken from the central part of the core and put into a ziplock PE bag. The pieces were melted at room temperature, and immediately after melting 25 ml vials were filled completely and closed tightly. Also the snow samples were melted in the PE bags at room temperature and filled into 25 ml vials. [9] The melted ice and snow, and the water samples were analyzed for stable oxygen isotopes at the Institute of Geology at Tallinn Technical University using a Finnigan- MAT Delta-E mass-spectrometer. Because relative concen- 2of11

trations of the isotopes can be measured much more precisely than absolute ones [e.g., Krouse, 1974], the ratio R of the concentrations of the heavy and light isotopes ( 18 O/ 16 O) in a sample is always measured in terms of its deviation from the ratio in a standard. The samples were measured against laboratory internal standard water, which has been calibrated on the V-SMOW/SLAP scale using the international reference materials V-SMOW (Vienna Standard Mean Oceanic Water) and SLAP (Standard Light Antarctic Precipitation) from the IAEA (International Atomic Energy Agency). Data is reported in the d 18 O notation, as follows; d 18 O ¼ R sample 1 1000 R V SMOW where R sample and R V SMOW are the sample and the V- SMOW standard ratio, respectively. Reproducibility of replicate analyses is generally better than ±0.1%. Salinity was determined from the same melted samples using a conductivity meter (Radiometer CDM 83 or Schott handylab LF 1) and United Nations Educational, Scientific, and Cultural Organization (UNESCO) [1983] algorithms accurate to 0.01 PSU. 2.3. Determination of the Meteoric Ice Fraction [10] As mentioned above the ice sections were first divided into three textural classes. We assume that only granular ice layers may have a meteoric contribution. We identify granular ice layers with values of d 18 O lower than the parent seawater [e.g., Jeffries et al., 1994] as having a meteoric contribution. Those granular ice layers are either; (1) snow-ice or (2) superimposed ice. Superimposed ice layers are identified on the basis of the structural and d 18 O properties [see Kawamura et al., 2001]. Superimposed ice has a distinctive (large) polygonal granular structure [e.g., Haas et al., 2001; Kawamura et al., 1997, 2001], with d 18 O values close to that of snow because it is solely of meteoric origin. It usually has a very low salinity [Haas et al., 2001; Kawamura et al., 1997]. However, if superimposed layers are flooded with seawater their salinity can increase, and this might destroy the texture of the ice, making its origin hard to recognize [Haas et al., 2001]. [11] Snow-ice is a two-component mixture of snow and seawater. The contribution of the components in the ice can be determined from snow accumulation and ice growth data, or from oxygen isotopic data [e.g., Lange et al., 1990]. The snow fraction, that is the meteoric ice fraction in the snow-ice layers, can be estimated from stable oxygen isotope data using the mass balance equations [Jeffries et al., 1994]; f s þ f sw ¼ 1 f s d s þ f sw d sw ¼ d where f s is the meteoric ice fraction (by mass) and f sw is the seawater fraction (by mass) of the sample, d s and d sw are the d 18 O values for snow and seawater, respectively, and d is the d 18 O value for the sample. [12] We use the three different values for d sw to provide a minimum, an intermediate and a maximum estimate for the ð1þ ð2þ ð3þ meteoric ice fraction. Laboratory experiments [e.g., Craig and Hom, 1968; Lehmann and Siegenthaler, 1991; O Neil, 1968] show that the maximum fractionation during freezing of seawater is almost 3%, when the growth rate of the ice approaches zero. [13] The maximum estimate here is obtained by assuming that the fractionation is the same as during columnar ice growth, therefore we use the average d 18 O value for columnar ice in each single core as the d sw value. Our data suggest that the fractionation during columnar ice growth is about 2% (i.e., d 18 O of columnar ice is 2% higher than that of the parent seawater). This is probably an overestimate [see Jeffries et al., 1997], because the consolidation of snow-ice is faster than that of columnar ice. The minimum estimate is obtained by assuming that no fractionation occurs, and the d 18 O value of under-ice seawater is used as d sw. The intermediate estimate is between the two extrema. We assume that the fractionation during freezing of the seawater component in snow-ice formation is half of that during columnar ice growth, i.e., 1%. [14] In the Baltic Sea the isotopic properties of snow and seawater can vary substantially [e.g., Punning et al., 1991; Granskog et al., 2003a]. For seawater this variation is mostly due to changes in evaporation and freshwater input [Punning et al., 1991], whereas atmospheric conditions cause large variability in snow d 18 O [e.g., Kawamura et al., 2001; Granskog et al., 2003a]. [15] The choice of a representative d 18 O value for snow is therefore crucial. The properties of snow vary considerably, and therefore a range is often used [e.g., Lange et al., 1990; Eicken et al., 1994; Jeffries et al., 1997]. In our case the d 18 O value for snow is known on a weekly basis throughout the ice season and utilization of this information is likely to provide more accurate estimates for the meteoric ice fraction ( f s ). However, only in winter 1999 was meteoric ice formed throughout most of the season, and changes in the snow properties had a notable impact on the meteoric ice fraction estimates. Furthermore we assume that both liquid and solid precipitation have the same d 18 O composition during the winter months (O. Matsubaya, unpublished data, 2003). [16] A direct method to evaluate the meteoric ice fraction values is to compare the mass of meteoric ice within the ice and the mass of snow on top of the ice sheet to the total precipitation since the beginning of the ice season. This gives a simple budget condition; h si f s r si þ h s r s P tot r w where h, f, and r are the thickness, fraction and density, respectively. Subscripts si, s, and w denote snow-ice and superimposed ice, snow (i.e., meteoric ice), and water, respectively. P tot is the accumulated precipitation at the TZS weather station. 3. Results and Discussion [17] Time series observations may provide vital information for sea ice studies, because one seldom samples long enough to address temporal development. Our observations, as is always the case in ice time series sampling, are contaminated by the short-range spatial variability of sea ð4þ 3of11

Figure 2. The daily average temperature at Tvärminne Zoological Station (dash-dotted line) and Santala Bay (solid line). Note that in 1999, only a period in March (days 70 90) was covered in Santala Bay. The arrows on the left show the freeze-up time, estimated from seawater temperature observations. The arrows on the right show the ice breakup time, estimated from seawater salinity observations. under-ice water salinity in Table 2) because of snow and ice melt. 3.2. Ice Structure [21] The ice in Santala Bay has a characteristic sea ice structure, i.e., a horizontal c axis in columnar ice, jagged grain boundaries and a substructure within the grains associated with brine inclusions [Kawamura et al., 2001]. The ice sheet was composed of an upper granular layer and a lower columnar ice layer in all three winters (Figure 3). The contribution of different texture classes are shown in Figure 4. Table 1 shows the average contribution from different ice types to the total ice thickness. In 1999 snow accumulation was exceptionally heavy resulting in a thick granular layer, at maximum 43% of the total thickness. [22] In 2000 g/c ice [Eicken and Lange, 1989], an intermediate state resulting from oscillations between the granular and columnar ice growth modes [Weeks et al., 1990], contributed significantly (32 100% of the total ice thickness) to ice growth (Figure 4 and Table 1). In 1999 and 2001 its contribution was negligible. This can be explained by differences in the conditions of year 2000 and the other ice properties [see, e.g., Tucker et al., 1984; Eicken et al., 1991]. At least partly it may explain some of the observed inconsistencies in the data. 3.1. Atmospheric and Oceanographic Conditions [18] Figure 2 shows the air temperature at the observation site and at TZS weather station during the three winters. The air temperature was variable, with alternating warm and cold periods. A simple measure for the severity of the ice season is the sum of negative degree days: it was 337, 168, and 325 C day in 1999, 2000, and 2001, respectively. Finnish weather data indicated that winter 2000 was clearly more dominated by westerly moving warm air masses than either 1999 or 2001. [19] Snow accumulation was much higher in 1999 than normal, with almost 50 cm maximum snow depths reported in the area at land-based stations of the Finnish Meteorological Institute, while at TZS the maximum snow depth was 42 cm. The maximum snow depths at TZS in 2000 and 2001 were 12 cm and 20 cm, respectively. In Santala Bay the snow depth was considerably lower because of wind drift and transformation into snow-ice and superimposed ice (Figure 4). The total precipitation at TZS for the period of day 1 to 110 was 260 mm in 1999, and 170 mm in 2000 and 2001. The percentage of liquid precipitation during that period was estimated as 50%, 70%, and 70% in 1999, 2000, and 2001, respectively. This shows that liquid precipitation is not uncommon during the winter months. [20] The water salinity was rather stable at 2 m depth; 6.0 PSU in 1999 and 2000 and 5.8 PSU in 2001, and changes in salinity occurred shortly after ice break-up, when the ice and snowmelt waters were mixed into the whole water column. The weekly water salinity profiles showed large variability in the topmost 0.5 m (shown as Figure 3. Photographs of vertical thin sections in March 1999, 2000, and 2001. 4of11

Figure 4. The contribution of different ice types (see legend in the middle graph) and the snow thickness (white) in (a) 1999, (b) 2000, and (c) 2001. The classification of ice types has been made according to Eicken and Lange [1989]. The dotted line in the granular ice indicates the boundary between superimposed ice and snow-ice. The zero level indicates the ice-snow interface at the time of sampling. two. In 1999 and 2001 the bay froze over rapidly, resulting in static growth conditions from the beginning, whereas in 2000 freezing was first limited to the shoreline, and the central bay (where sampling was made every year) was open for longer. When g/c ice was formed (days 20 25), strong northerly winds and currents prevailed, and the water column was supercooled down to (at least) 2 m depth. This suggests that the growth conditions were not as static as during freeze-up in 1999 or 2001 when supercooling was not observed. [23] In 2000 the relative contribution of columnar ice to total ice thickness was significantly lower than in 1999 and 2001, while in 1999 it was lower than in 2001 (one-way analysis of variance (ANOVA), P < 0.01, Tukey multiple comparison test (Tukey)). The oxygen isotopic properties show that virtually all the granular ice had a meteoric ice contribution and the amount of granular ice of solely seawater origin was negligible small. 3.3. Ice Salinity [24] The evolution of ice salinity in the three winters is shown in Figure 5. The ice salinities were generally below 1.5 PSU, with large and rapid variations especially in the uppermost parts of the ice cover. The changes were concurrent with snow-ice formation and flushing events [see also Granskog et al., 2003b], and are thus strongly connected to atmospheric conditions. Maximum ice salinities of 3.0 PSU were observed in the early winter of 2001 (Figure 5c). [25] Table 2 shows the seasonal mean values for salinity in different ice types and under-ice water, and the mean bulk ice salinity in winter prior to spring flushing. Averaged over all samples during three seasons, the winter bulk ice salinity was 0.65 ± 0.33 PSU. There were no significant differences in ice salinities between different ice types within the years (ANOVA, P > 0.05). Monthly mean bulk ice salinities did not show any significant differences between the three winters (ANOVA, P > 0.05). There were no differences in ice salinity with depth (ANOVA, P > 0.05) in any of the years on a monthly basis, which is shown as quite uniform vertical ice salinity distributions when averaged over all years (Figures 6a 6d). This basically means that the climatological sea ice salinity was constant with depth decreasing with time at a rate of less than 0.001 PSU d 1 during the winter months (average for all years). In individual years there were rapid changes in week-to-week salinity profiles (Figure 5), but major changes in bulk salinity occurred only during the very early and very late stages of the ice season. In between, changes in bulk ice salinity were gradual and statistically insignificant, at least on two week or longer timescales. In spring, however, the bulk ice salinity decreased from typical late winter values of around 0.5 PSU to almost zero within a week or two (Figure 5), with an average fall-off rate of 0.02 ± 0.006 PSU d 1 for all years. This drop in bulk salinity that took place shortly after the average daily air temperatures rose above zero shows how susceptible the thin ice cover is to changes atmospheric conditions. [26] Bulk ice salinities were on average 10 to 20% of the under-ice water salinity (Table 2). At times the ice salinity reached values about half of the parent seawater, especially during flooding and snow-ice formation. In winter the bulk ice salinity was on the order of 15 25% of the under-ice water salinity. [27] Ice salinity is an important factor defining the properties of sea ice, and therefore in studies of geophysics, biology and remote sensing of sea ice. Our salinity observations compares with those of Palosuo [1963] in the same area. In the long run the salinity profile is observed to be constant with depth. Also Weeks et al. [1990] suggested that a representative salinity profile for landfast ice in the Bothnian Bay would be close uniform in the vertical, with slightly higher values at the surface. The relatively thin ice sheet in the Baltic Sea is susceptible to thermal changes, that affect gravity drainage, flushing, and flooding, and as a result rapid changes in ice salinity occur. Spatial variability, which is obviously included in our observations as well, is assumed to be smaller than the observed week-to-week variability in our data [Granskog et al., 2003b], as well as when compared to other studies on spatial salinity variations in sea ice [Tucker et al., 1984; Eicken et al., 1991]. 3.4. Oxygen Isotopic Composition [28] The evolution of the stable oxygen isotopic composition (d 18 O) of the ice during the three winters is shown in Figure 7. The lowest values were observed in the upper parts of the ice cover in granular ice layers. Table 3 shows the seasonal mean d 18 O values for snow, different textural Table 1. Average Contribution of Different Texture Classes to the Total Ice Thickness in Santala Bay a 1999 2000 2001 Columnar 65.2 22.5 80.6 Granular 34.8 28.8 19.4 Intermediate g/c 48.7 a Computed according to the absolute method [Jeffries, 1997] from the data of all cores collected each season. Contributions are in percent. 5of11

Figure 5. Evolution of ice salinity (PSU) in (a) 1999, (b) 2000, and (c) 2001. ice classes, and under-ice water. The d 18 O values for different ice types did not show any significant variations between the winters, or within individual years (ANOVA, P > 0.05). The data show that the granular ice portion had significantly lower d 18 O values than the other ice types (ANOVA, P < 0.05, Tukey), which indicates its meteoric ice origin. There was no difference between g/c ice and columnar ice (paired t test, P > 0.05). Polygonal granular ice layers, identified as superimposed ice, had on average d 18 O values of 11.9 ± 0.8, 12.2 ± 0.8, and 9.5 ± 0.4 in 1999, 2000, and 2001, respectively, suggesting clearly a meteoric ice origin for these layers. [29] The similar d 18 O values of g/c ice and columnar ice suggested a seawater origin for g/c ice, as was proposed by Granskog et al. [2003a]. Eicken and Lange [1989] proposed the formation of this type of ice to be related to (1) incorporation of frazil ice into the growing ice sheet, (2) enhanced growth rates due to changes in atmospheric conditions, and (3) changes in the hydrodynamic regime at the ice-water interface. As our observations of g/c ice formation were made in dynamic conditions in the water column, it is reasonable to take 1 or 3 as possible explanations. Dynamic growth processes were also proposed by Granskog et al. [2003a] to account for g/c ice formation in the Bothnian Bay. [30] The d 18 O of under-ice seawater showed some variability (Table 3). During snow and ice melt an under-ice meltwater layer formed with d 18 O values down to 10% at times [Granskog et al., 2003b]. In 1999, when snow accumulation occurred through most of the ice season, d 18 O values of snow showed a clear seasonal trend, from 20% early in the season increasing gradually to 12% in spring. In 2000 and 2001 the snow values were relatively stable throughout the winter. 6of11

Figure 6. Composite profiles of (a) (d) sea ice salinity and (e) (h) d 18 O averaged for 4-week periods, namely, day of year (DoY) 1 28, 29 56, 57 84, and 85 112, plotted against normalized depth. Each data point represents the mean value, and the horizontal bars represent the standard deviation of all the values in each of five 0.2 normalized depth unit bins. The monthly bulk mean ± s.d. salinity or d 18 O and number (n) of profiles is shown in every panel. [31] The monthly mean sea ice d 18 O profiles showed significant variation with depth (ANOVA, P < 0.05), with lower values in the upper 20 to 40% of the ice cover (Tukey) (Figures 6e 6h). Even with changes in salinity the d 18 O levels were kept basically the same, which implies that d 18 O values were affected almost only by the solid part of the ice and not much by the brine [see also Kawamura et al., 2001]. [32] The isotopic fractionation during columnar ice growth was determined as the difference between d 18 Oin the bottommost columnar ice section and the under-ice water during the growth period. The variation between years was insignificant (ANOVA, P > 0.05), and the mean (± standard deviation) for all three years was 1.9 ± 0.4%. 3.5. Meteoric Ice Fraction [33] The meteoric ice fraction to the total ice mass is shown in Figure 8. The minimum and maximum estimates differ by about 5% units. Changes in meteoric ice fraction values seem inconsistent at times, especially in the mid of the ice season in 2000 (Figure 8b). On day 62, 2000, the surface layer had a somewhat higher d 18 O value than one week before and after (Figure 7b). This gives a considerable lower estimate for the meteoric ice fraction at this date (Figure 8b). The higher d 18 O may be related to lateral variability in sea ice properties, but it may also result from the topmost ice layer composition being affected by liquid precipitation and a slush layer present on the ice. The absolute amount of meteoric ice in the ice cover is shown in Figure 9 (as water equivalents (h si f s r si )) confirms that changes in the relative meteoric ice fraction (Figure 8) are not necessarily related to meteoric ice formation or melt alone. [34] In 1999 meteoric ice formation progressed late into the ice season, with a maximum fraction of about 35% (Figure 8). In 2001, after an initial high snow contribution while the ice was thin, the meteoric ice fraction decreased and remained at about 5% for the rest of the season (Figure 8c), as congelation growth dominated (Figure 4). In 2000, values fluctuated around 10% after initial low values (Figure 8b). f s values of individual snow-ice layers were 0.35 ± 0.21 in 1999, 0.30 ± 0.10 in 2000, and 0.28 ± 0.22 in 2001. These correspond to snow densities of 100 to 450 kg m 3. Superimposed ice layers had f s values between 0.65 and 1.15. f s values larger than unity were assigned a value of 1.00 when computing the meteoric ice fractions in the cores. [35] Earlier results for meteoric ice contribution for Baltic Sea ice are in good agreement with ours [Kawamura et al., 2001; Granskog et al., 2003a]. Observations along the Finnish coast of the Baltic Sea in 2000 indicated that the Table 2. Seasonal Mean ± Standard Deviation of Bulk Ice Salinity (PSU) in Winter, Salinity of Textural Ice Classes, and Under-Ice Water Salinity a Winter 1999 2000 2001 Bulk ice (winter) 0.54 ± 0.20 0.51 ± 0.33 0.88 ± 0.32 Granular ice 0.48 ± 0.34 0.34 ± 0.45 0.95 ± 0.64 Intermediate g/c 0.42 ± 0.42 Columnar ice 0.46 ± 0.24 0.39 ± 0.16 0.83 ± 0.37 Under-ice water 3.67 ± 1.01 3.61 ± 1.42 4.52 ± 1.41 a Mean of all samples collected every season. 7of11

Figure 7. The evolution of ice d 18 O(%) in (a) 1999, (b) 2000, and (c) 2001. snow fraction of the landfast ice cover was 21% on average (intermediate estimate) [Granskog et al., 2003a], about twice as much as in Santala Bay at the same time, their observations also show large variability in space. Palosuo [1963] measured that upward growth of ice could constitute up to half of the total ice thickness, although in some years there was no evidence of upward growth. The year-to-year variations are most likely closely connected to variations in atmospheric circulation and moisture transport, as proposed by Jeffries et al. [2001] to explain variations in snow-ice contribution to growth of Antarctic sea ice. [36] Snow-ice and superimposed ice formation due to high snow accumulation in winter 1999 caused the ice to grow thicker than in 2001, although the severity of these winters was roughly the same. The ice thickness increased considerably when the snow cover started to decay (days 60 85; Figure 4), and at the same time the amount of snow (meteoric ice) incorporated into the ice increased considerably (Figure 9), presumably through formation of superimposed ice, which contributed up to 28% of the total ice thickness [Kawamura et al., 2001]. Observations from the land-based stations of snowmelt and meteorological data, Table 3. Seasonal Mean ± Standard Deviation of d 18 O for Snow, Textural Ice Classes, and Under-Ice Water a Winter 1999 2000 2001 Snow 16.3 ± 3.6 11.4 ± 0.4 12.3 ± 3.2 Granular ice 11.1 ± 1.5 9.5 ± 1.4 9.1 ± 0.7 Intermediate g/c 6.5 ± 0.6 Columnar ice 6.7 ± 0.8 6.3 ± 0.4 6.0 ± 0.4 Under-ice water 8.7 ± 1.0 8.2 ± 0.7 8.0 ± 0.8 a Mean of all samples collected every season. Seasonal mean ± standard deviations are in %. 8of11

Figure 8. The evolution of the meteoric ice fraction (%) to the total ice mass in (a) 1999, (b) 2000, and (c) 2001. The maximum, intermediate, and minimum estimates are shown (see text). along with the structural and isotopic properties of the ice, support the formation of superimposed ice in all winters. A simple model of Kuusisto [1980], which calculates the daily snowmelt depth M (cm) by multiplying the number of degree days T ( C day) by the degree day melting factor a (cm C d 1 ), was used to compute the potential snowmelt at Santala Bay every ice season (for days when snow was present). We used a degree day melting factor of 0.35 cm C 1 d 1, which is representative for open areas and a snow density of 300 kg m 3 [see Kuusisto, 1980]. The model indicates that about half of the mass of upward growth from day 60 to 77 (Figure 4a) could be composed of refrozen snowmelt, i.e., 10 12% of the total sea ice mass. Snowmelt can occur at temperatures below 0 C through absorption of solar radiation [e.g., Colbeck, 1989]. Liquid precipitation is not uncommon in winter at Santala Bay, and its potential contribution to superimposed ice formation by infiltration to the bottom of the snowpack can not be neglected. During the period when superimposed ice was formed in 1999 (days 60 85) we estimate that 30 40 mm liquid precipitation accumulated onto the snow and ice. This could add to the formation of superimposed ice. Adding this and the potential snowmelt indicates that at maximum 16 20% of the total sea ice mass could have been of superimposed ice origin. Therefore we can not with certainty prove that superimposed ice formation alone contributed to the upward growth of ice in 1999 as assumed by Kawamura et al. [2001]. The high snow load, together with snowmelt and liquid precipitation, indicates that snow-ice and superimposed ice formation progressed simultaneously in 1999. This is supported by observations of negative freeboard, and ice salinities that are higher than reported for superimposed ice in Antarctica [Kawamura et al., 1997; Haas et al., 2001]. This implies that the superimposed layers were flooded, but did not always loose their characteristic structure or d 18 O properties, as opposed to Haas et al. [2001] who argued that flooding destroys the characteristics of superimposed ice layers. The disagreement is perhaps due to the fact that the water flooding the sea ice here is only 4 6 PSU in salinity much lower than oceanic salinities in Antarctic waters. In 2001 the snowmelt model supported the formation of superimposed ice, however, it coincided at times with snow-ice formation. In 2000 the formation of superimposed ice alone was observed, with ice layers with polygonal granular structure low in both salinity and d 18 O. Both in 2000 and 2001 freezing of liquid precipitation onto the ice surface seems to contribute to the intermittent occurrence of superimposed ice layers. The contribution of superimposed ice to the total ice mass ranged between 0 20%, 0 9%, and 0 9% in 1999, 2000, and 2001, respectively. Superimposed ice formation is a recurrent process at these latitudes and mild ice climate conditions, as seems to be the case in summer Antarctic pack ice and landfast ice as well [Haas et al., 2001; Jeffries et al., 1997; Kawamura et al., 1997]. However, its relative contribution to total ice mass has been considerably smaller elsewhere than reported here. [37] Figure 9 shows the amount of precipitation (or meteoric ice) in the sea ice, the sea ice and snow cover together as water equivalents, and the precipitation since ice formation at TZS. They agree quite well in the early ice season but later the difference increase (Figure 9). This is explained by the loss of accumulated precipitation into the under-ice water (observed as decreased d 18 O values in the water) [see also Granskog et al., 2003b]. Another factor which can cause deviations is aeolian erosion, transport and redeposition of snow [e.g., Eicken et al., 1994]. Our estimates of the amount of precipitation in the ice-snow system are not higher than the actual precipitation, which indicates that our f s estimates are plausible. [38] Interpretation of the data collected by Palosuo [1963] indicates that superimposed ice formation is not an Figure 9. The amount of total precipitation since ice formation (P tot r w ) at TZS and meteoric ice in the ice cover (h si f s r si ) and in the ice and snow cover together (h si f s r si + h s r s ) in Santala Bay in (a) 1999, (b) 2000, and (c) 2001. The two latter estimates are based on the intermediate estimate for the meteoric ice fraction f s (see text), r si = 850 kg m 3 and r s = 350 kg m 3. 9of11

uncommon process in the Baltic Sea, especially in the melting season. However, Palosuo [1963] could not distinguish between snow-ice and superimposed ice formation since he measured only the total upward growth. Evidently these observations warrant a more detailed study onto the occurrence of superimposed ice in the Baltic Sea, especially its formation in late winter and early spring deserves more attention. [39] Our results are an improvement to those provided by Kawamura et al. [2001] for the five early samples in 1999. They used the present maximum estimate for the meteoric ice fraction but the snow sample of each sampling date for d s, which resulted in a meteoric ice fraction of 2 to 26%. Our estimates are consistently lower; for example the maximum of these five samples range between 16.4% and 22.6% (Figure 8; day 47 in 1999). The difference is attributed to the choice of the isotopic fractionation during the freezing of the seawater component in the snow-ice layer, and the choice of representative d 18 O value for snow. [40] The relatively thin ice cover in the Baltic Sea is sensitive to changes in snow accumulation. An increase or decrease in accumulation can increase the total ice thickness if the other climatological conditions are kept constant [Leppäranta, 1983]. At low snow accumulation rates thermodynamic growth at ice-water interface would be favored, while at higher accumulation rates the ice cover could increase in thickness because of snow-ice formation. The landfast sea ice system is highly dynamic with drastic changes in its properties, especially ice salinity and temperature, which are strongly controlled by atmospheric conditions [e.g., Granskog et al., 2003b]. Changes in ice salinity and temperature strongly affect, for example, the fluid transport properties of the ice [Golden et al., 1998], which again affect, among other things, the biology and chemistry of the ice cover and the underlying water as well [Granskog et al., 2003b]. 4. Conclusions [41] The seasonal evolution of landfast sea ice was studied throughout three winters (1999 2001) in the Gulf of Finland, the Baltic Sea, on the basis of weekly sampling. The focus was on the seasonal development of sea ice salinity, d 18 O and structural composition. The role of meteoric ice on sea ice mass balance was of special interest. The three winters were quite different in conditions, as shown by variations in the observed maximum sea ice thickness (52, 28, and 47 cm in 1999, 2000, and 2001, respectively) and maximum snow thickness (42, 12, and 20 cm in 1999, 2000, and 2001, respectively) in the study area. Early March values have been given below since they correspond to the time of maximum ice extent in the Baltic Sea. To summarize the following conclusions are made; [42] 1. The sea ice in Santala Bay has a typical sea ice structure, with horizontal c axis in columnar ice, jagged grain boundaries and a substructure within the grains associated with brine inclusions [Kawamura et al., 2001]. The ice sheet is structurally composed of an upper granular layer, while the remainder is composed of intermediate granular columnar and/or columnar ice depending on the growth conditions. Granular ice, intermediate g/c ice, and columnar ice contributed in early March 33%, 0%, and 67%, respectively, in 1999, 31%, 45%, and 24%, respectively, in 2000, and 20%, 0%, and 80%, respectively, in 2001. [43] 2. The meteoric ice fraction with respect to the total ice mass ranged between 0 and 35% depending on season and year. Early March values (intermediate estimate) were 27%, 13% and 8% in 1999, 2000, and 2001, respectively. Abnormally high snow accumulation took place in 1999 which resulted in a significant meteoric ice contribution (35% at maximum). The highest values reported here are higher than elsewhere for sea ice. However, at times the meteoric contribution can be negligible. Notable are also the large seasonal and year-to-year variations in meteoric ice contribution. [44] 3. Superimposed ice formation is a recurring process at these latitudes, where the temperatures are often sufficiently high for snowmelt to occur during the ice season. Superimposed ice contribution to total ice mass in early March is estimated to be 19%, 2%, and 5% in 1999, 2000, and 2001, respectively. While the maximum contribution during the ice seasons were 20%, 9%, and 9% in 1999, 2000, and 2001, respectively. Formation of superimposed ice from liquid precipitation seems to be important as well, contributing to the episodic occurrence of meteoric ice layers at the ice surface. [45] 4. The remaining meteoric ice is incorporated into the ice sheet during snow-ice formation. Typically snow-ice contributed about 10% of the meteoric ice present in sea ice. Typical meteoric fraction f s values in snow-ice layers are around 0.3 ± 0.2. [46] 5. Winter ice salinities are in the range of 0.3 to 3.0 PSU (0.65 ± 0.33 PSU for all 3 winters), being usually 15 to 25% of the parent seawater salinity. In winter bulk ice salinity changes are insignificant, while in spring the salinity decreases rapidly from winter values to almost zero, with an average fall-off rate of 0.02 ± 0.006 PSU d 1. The sea ice salinity can be regarded as constant with depth on a two week or longer basis, which is probably accounted by the fact that the ice thin cover is susceptible to thermal changes, which ultimately even out the salinity of the thin ice sheet. [47] 6. The d 18 O composition of snow, ice and water seems to be applicable to study the role of snow in sea ice processes in the Baltic Sea. However, one has to keep in mind that the d 18 O composition of the parent seawater and precipitation shows large variability in the Baltic Sea basin both in space and time [see also Granskog et al., 2003a]. [48] 7. The present results give the first statistical description of the evolution of the structure and salinity of landfast sea ice in the Baltic Sea. The location is close to the climatological ice edge and therefore the results represent rather mild ice climate conditions. These conditions may start to appear at higher latitudes if global warming progresses. [49] Acknowledgments. We wish to thank the personnel of the Tvärminne Zoological Station for their assistance and use of facilities. Masao Ishikawa, Antti Lindfors, Kai Rasmus, Sirpa Rasmus and Toru Takatsuka assisted in the field. Tõnu Martma provided the oxygen isotopic analyses. This work is part of project Ice climatology of the Okhotsk and Baltic Seas, financed by the Japanese-Finnish Bilateral Programs with the Japan Society for the Promotion of Science and the Academy of Finland, and also the Finnish Ministry of Trade and Industry and the Japanese Ministry of Education, Culture, Sports, Science and Technology through 10 of 11

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