Ice in the Environment: Proceedings of the 16th IAHR International Symposium on Ice Dunedin, New Zealand, 2nd–6th December 2002 International Association of Hydraulic Engineering and Research

STUDY ON BRACKISH ICE IN THE GULF OF FINLAND

T. Kawamura1, M.A. Granskog2, J. Ehn2, T. Martma3, A. Lindfors2, N. Ishikawa1, K. Shirasawa4, M. Leppäranta2 and R. Vaikmäe3

ABSTRACT Two winter observations have been performed along a 20-km transect from the mouth of the Gulf of Finland to the inner parts of the Pojo Bay. We collected ice samples together with snow and under-ice water samples at four sites in the area in March 1999 and several times in 2000. The ice samples from the outermost site, whose surface water salinity was 1.3–3.3 psu (practical salinity unit), had a characteristic sea ice structure, while at the three inner sites, where the water salinity was 0.1–0.5 psu, structure similar to lake ice appeared. Thus the fresh water-sea ice boundary was at about 1 psu of water salinity, consistent with previous studies. The ice was two-layered. The δ18O values suggest that the upper ice with granular structure is snow ice, while the lower layer is congelation ice. Snow contributed significantly to the sea ice growth in the area. The δ18O values in the bottom ice layers are 1–2 ‰ higher than in the water, which strongly suggests isotopic fractionation during freezing. INTRODUCTION The Gulf of Finland is a rectangular-shaped basin located in the eastern part of the Baltic Sea. The salinity of the surface water is largely controlled by a combination of inflows of saline water from the North Sea and freshwater from rivers in the region. The Gulf of Finland freezes annually to 30–100 percent of its surface, and the maximum annual thickness of landfast ice is 40–80 cm. Studies on ice structure have been made at both fast ice and pack ice areas in the Baltic Sea. Palosuo (1961) observed transition from brackish to fresh-water ice both in natural and artificial ice, and Palosuo (1963) mapped snow depth and thicknesses of different ice layers and determined the ice structure and salinity at many stations on the Finnish coast. Weeks et al. (1990) studied the ice structure for ice samples from the northernmost basin of the Baltic Sea. The region from the mouth of the Gulf of Finland to the Pojo Bay is particularly interesting for brackish water ice studies since the water salinity gradually decreases from

1 Institute of Low Temperature Science, Hokkaido University 2 Division of Geophysics, University of Helsinki 3 Institute of Geology, Tallinn Technical University 4 Sea Ice Research Laboratory, Institute of Low Temperature Science, Hokkaido University

6 psu down to nearly zero. A field programme was designed to examine the ice in this region in winters 1999 and 2000. This paper describes the results for the spatial change of ice structure and discusses the effect of salt to the ice growth and structure. SAMPLING AREA AND METHODS Figure 1 shows the sampling sites. Based on the results in 1999, in the 2000 winter season we shifted the outermost site to about 5 km upstream. Sea ice samples were collected with an ice corer in March 1999 and several times (at about 2 weeks intervals) in 2000. The samples were packed in plastic bags and kept at –20 ˚C in a stock chamber at Tvärminne Zoological Station. In the cold room work later, each sample was split lengthwise to obtain 0.5 cm thick vertical sections along the entire length. These sections allowed us to examine gas bubble and brine layer distributions under scattered light. Then, the sections were smoothed by planing to a thickness less than 1 mm, and the resulting thin sections were illuminated under polarized light to identify individual grains and their structures. Analysis for c-axis orientation was conducted on grains on horizontal thin sections with a universal stage. The entire length of the sample was cut at about 5–10 cm intervals along distinct structural boundaries. Then the pieces were melted in a zip-lock plastic bag for determining the salinity with a conductivity meter (Schott handylab LF1). The standard seawater formula was used to determine salinity from the conductivity (e.g., Fofonoff, 1985) with an accuracy of 0.01 psu. The oxygen isotope compositions (δ18O) of the samples were determined with a mass spectrometer (Finnigan MAT Delta E, accuracy 0.1 ‰) at the Institute of Geology, Tallinn Technical University, Estonia.

Figure 1: Location map for sampling sites. 1: Poja, 2: Tammmisaari, 3: Leksvall, 4a: Tvärminne (1999) and 4b: Predium (2000). Also shown the location of the Tvärminne Zoological Station (TZS). RESULTS Results of 1999 Vertical thin section photographs of the samples obtained at the four sites are shown in Figure 2(a). At Site 2 the ice is thinner than at the other sites and consists of only granular ice throughout the entire length, probably because the sampling site was located close to a

shipping track, and the ice in the area was broken by waves and refrozen several times during the winter. The ice at the other sites from the top down to 20–30 cm depth consisted of 43–55 % granular ice, and the total thicknesses were close to each other, 45–57 cm. Below the granular ice layer, the ice at Site 1 appears as a quite large single grain throughout the entire length. On the other hand, the underlying layer was composed of columnar ice at both Sites 3 and 4a.

Figure 2: Structural photograph of vertical thin section (a), and profiles of salinity and oxygen isotopic composition of snow, ice and water samples collected at Sites 1, 2, 3 and 4a on March 1999.

Figure 3 shows photographs of horizontal thin sections of the lower ice layer at the three sites. The ice at Site 1 consisted of a large grain and some small grains, whose grain boundaries look very smooth, with a vertical c-axis. The horizontal sections as well as the vertical sections reveal the columnar ice structure with a horizontal c-axis at Sites 3 and 4a. However, there are some differences of the ice structure between these two latter sites; smooth/jagged grain boundaries and absence/presence of sub-structure in the grains.

Figure 3: Photographs of horizontal thin sections of the lower ice layer of the sample collected at Sites 1, 3 and 4a on March 1999. The scale of subdivisions measures 1 mm.

The profiles of salinity and δ18O of the samples at the four sites in 1999 are shown in Figure 2. The salinity of water samples, collected both at the water surface and just below the ice bottom, ranged from 0.1 to 0.2 psu at Sites 1, 2 and 3, while the water salinity at Site 4a was 2.5 psu, being one order of magnitude larger than at the others. The δ18O values of snow cover were highly negative, down to –15 ‰ and the values in water ranged between –9 and –11 ‰, higher than in the snow cover but lower than in ordinary sea water (approximately 0 ‰). The result suggests that the water contained fresh water originated from runoff from a river flowing into the Pojo Bay in north, with quite low δ18O values. The ice at Sites 1, 2 and 3 has salinity values of 0.1 psu at most. While the values at Site 4a ranged from 0.4 to 0.8 psu. Results of 2000 Figure 4 shows the time-series observation of snow depth and thickness of different ice types in winter 2000, and Figure 5 demonstrates examples of salinity and δ18O profiles together with structural photograph at the two sites of a selected date (6 Feb.). All the samples, except for some samples on 27 January at the beginning of the ice season, contain granular ice at the upper layer. In contrast, the ice type of the lower layer depends on the site; the ice of the inner three sites looks like a single crystal. At the outermost Site 4b, the ice is composed of columnar ice with jagged grain boundaries. Note the sudden appearance of columnar ice in place of the large crystals at Site 2 on 23 Feb. probably because of slight change of the sampling location or the fact that the ice was broken up and refrozen again between the two sampling occasions. The salinity of ice is nearly zero at the inner three sites, where the water salinity is as low as 0.5 psu. The salinity of the ice at Site 4b has the maximum value of 0.3–0.9 psu on 6 February and decreases gradually to 0.1–0.2 psu on 21 March probably caused by desalination. The water salinity at Site 4b ranged from 1.3 to 3.3 psu. The upper granular ice layers of the samples had lower δ18O values than bottommost layer.

Figure 4: The contribution of snow and different ice types at Sites 1, 2, 3 and 4b in 2000. The value of the under-ice water salinity (psu in unit) is indicated below each column in the figure.

Figure 5: Examples of the profiles of structure, salinity and δ18O of the samples collected at Site 3 and 4b on 6 February 2000.

DISCUSSION AND SUMMARY We have found a clear difference in the ice structure among the sites as described above. Sea ice is characterized by columnar structure with jagged grain boundaries and sub-structure in the grains, whose c-axis orientation is horizontal (Weeks and Ackley, 1982), while ice made of fresh water, e.g. lake ice, has quite different characteristics from sea ice, smooth grain boundaries and absence of the sub-structure. Gow (1986) further showed that lake ice made of pure water has vertical or horizontal c-axes, depending on growth condition. Judging from the criteria of the division between fresh-water ice and sea ice, we can conclude that the fresh water-sea ice boundary is somewhere between Site 3 and Site 4a/4b. Water salinity difference between the two sites is one order of magnitude, 0.1–0.5 and 1.3–3.3 psu. This result is consistent with field observations conducted in the same region (Palosuo et al., unpublished data) and in the Bay of Bothnia (Palosuo, 1961), as well as with a laboratory study of Weeks and Lofgren (1967) showing that the transition from a non-planar to a planar freezing interface is from 1 psu for unstirred to 3 psu for stirred NaCl solutions. There is a marked difference in δ18O levels between upper and lower ice layers at all the samples, especially in 2000. The structure and the δ18O values of snow, ice and water suggest that the upper ice with granular structure is snow ice, while the lower layer is congelation ice. A large amount of snow ice shows the significant contribution of snow to ice growth in the study area (e.g., Kawamura et al., 2001). The δ18O values in the bottom ice layers are 1 to 2 ‰ higher than in the water, which strongly suggests isotopic fractionation during freezing. ACKNOWLEDGMENT We thank the Tvärminne Zoological Station of the University of Helsinki for using their facilities and Ms. Eriko Uematsu for drawing. This work is a part of the 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 Japanese Ministry of Education, Culture, Sports, Science and Technology through Grant-in-Aid for Scientific Research. REFERENCES Fofonoff, N.P. Physical properties of seawater: a new salinity scale and equation of state

for seawater. J. Geophys. Res. 90(C2): 3332–3342 (1985). Gow, A.J. Orientation texture in ice sheets of quietly frozen lakes. J. Crystal Growth

74(2): 247–258 (1986). Kawamura, T., Shirasawa, K., Ishikawa, N., Lindfors, A., Rasmus, K., Granskog, M., Ehn,

J., Leppäranta, M., Martma, T. and Vaikmäe, T. R. Time-series observations of the structure and properties of brackish ice in the Gulf of Finland. Ann. Glaciol. 33, 1–4 (2001).

Palosuo, E. Crystal structure of brackish and fresh-water ice. IASH 54, 9–14 (1961). Palosuo, E. The Gulf of Bothnia in winter. II. Freezing and ice forms. Merentutkimuslait.

julk./Havsforskningsinst. Skr. 209: 42–64 (1963). Weeks W.F. and Ackley, S.F. The Growth, Structure and Properties of Sea Ice. CRREL

Monograph 82-1 (1982) 130p. Weeks, W.F., Gow, A.J., Kosloff P. and Digby-Argus, S.A. The Internal Structure,

Composition and Properties of Brackish Ice from the Bay of Bothnia. CRREL Monograph 90-1 (1990) 5–15.

Weeks, W.F. and Lofgren, G. The effect solution distribution coefficient during the freezing of NaCl solutions. In Physics of Snow and Ice, H.Oura, ed., Inst. Low Temp. Sci., Sapporo, 579–597 (1967).