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

CHEMICAL PROCESSES DURING FORMATION OF BRACKISHWATER ICE IN THE BALTIC SEA

Mats A. Granskog1, Jens Ehn2, Kristiina Virkkunen3, Tonu Martma4,David N. Thomas5 and Harri Kola3

ABSTRACTThe behavior of major elements, stable oxygen isotopes, dissolved organic carbon (DOC),and trace metals was investigated during the freezing of low saline water (3 PSU) in theBaltic Sea. The major element ratios indicated variable behavior, with some elementsshowing relative enrichment (sulfate, calcium and magnesium), conservative (sodium),or depletion (potassium) compared to what would be observed in seawater at the samesalinity. DOC showed consistently enrichment in the ice compared to what would beobserved in seawater at the same salinity. Trace elements, such as iron, showed lessconsistent behavior, both depletion and enrichment was observed in the ice relative toseawater.

INTRODUCTIONThe chemical processes during sea ice formation are less well known and understood thanthe physical processes. Studies that have been performed to understand the processes dur-ing ice formation are rather sparse (e.g. Addison, 1977; Anderson and Jones, 1985; Gi-annelli et al., 2001; Meese, 1989; Reeburgh and Springer-Young, 1983), and often reflectthe conditions after the initial ice formation and some thermal cycling. Ratios of majorelements in the ice remain similar, or at least close (Addison, 1977), to those in seawa-ter, i.e. all major elements are fractionated in a similar manner during sea ice formation,which implies that sea ice does not have an significant impact on oceanic chemistry overlong periods of time (e.g. Meese, 1989). However, the understanding of chemical pro-cesses during sea ice formation could be important to understand the geochemistry andchemical budgets in ice covered seas (Anderson and Jones, 1985; and references therein),at least on short time scales.

In this paper we report some of the preliminary results from quasi-experimental studies,where ice was grown under natural conditions in a pool cut into fast ice in the brackish

1Arctic Centre, P. O. Box 122, FIN-96101 Rovaniemi, Finland2Division of Geophysics, Department of Physical Sciences, P.O. Box 64, FIN-000143Department of Chemistry, P.O.Box 3000, FIN-90014 University of Oulu, Finland4Institute of Geology, Tallinn Technical University, Estonia Blvd 7, Tallinn 10143, Estonia5School of Ocean Sciences, University of Wales-Bangor, Menai Bridge, Anglesey, LL59 5EY, UK

water (3 PSU) Bothnian Bay, in the northernmost Baltic Sea. The objective was to studythe fractionation of a number of substances during initial ice formation processes at lowsalinities. The fractionation of major elements, dissolved organic carbon (DOC), stableoxygen isotopes (δ18O), and some dissolved trace metals (e.g. iron, aluminum) was stud-ied during these experiments. The formation of snow-ice during the experiments allowedus to study the chemical processes during the formation of this rather important ice typein the Baltic Sea as well.

MATERIAL AND METHODSField experimentTo study the chemical processes during ice formation in natural conditions a pool, 1.5 by2 m, was cut into 0.5 m thick ice in the vicinity of the Bothnian Bay Research Station(University of Oulu), on the western tip of the Hailuoto island in the northeastern part ofthe Bothnian Bay. The pool was frozen under natural conditions, and under-ice water andice grown in the pool was sampled for studies of the fractionation of salts, stable oxygenisotopes, and (dissolved) inorganic nutrients and metals during ice formation in brackishwater (salinity 2.95 PSU).

The instrumental setup in the pool is shown in Figure 1. Temperatures in the air, iceand under-ice water were monitored using thermistors and logged onto Gantner IDL100dataloggers. Weather conditions were also monitored at the nearby (500 m) Marjaniemiweather station, of the Finnish Meteorological Institute.

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Figure 1: Schematic diagram of the pool and the instrument setup (top view).

Ice samples were collected by cutting a block of ice, using a stainless steel saw, fromthe pool 22.5, 66.5 and 182 hours after ice formation had started. The sampling wasinfrequent as we wanted to collect as undisturbed ice samples as possible. Because somesnow fell onto the ice after the second sampling occasion it was decided that sampleswould be collected only at the end of the experiment, to make the formation of snow-ice asundisturbed as possible. The collected ice blocks were put immediately into polyethylene

bags, and kept stored at below −20◦C until preparation for analysis. Some samples werecut immediately on the ice for comparison of results from stored samples. Water sampleswere collected directly into sample bottles.

In the coldroom at the Rovaniemi Research Station (Finnish Forest Research Institute)the ice samples were prepared in the following manner; 1–2 cm of the outer surfaces ofthe blocks were removed using a bandsaw, the remaining was sawed into sections of 10to 20 mm thick (in the vertical), the surfaces of these sections were further scraped offusing a ceramic knife (Boker Baumwerk GmbH, Germany) inside a laminar flow-bench,and put into polyethylene cups (with lids) for melting. Procedural blanks were preparedusing frozen ultrapure water cores which were prepared in exactly the same manner asthe samples.

MeasurementsSalinity was determined using the measured conductivity (Schott handylab LF1 conduc-tometer) and converted to salinity, with a precision of 0.01 PSU (Practical Salinity Unit),using the UNESCO (1983) algorithms. DOC was determined as described in Giannelli etal. (2001), procedural blanks were below the detection limit of the instrument. δ18O wasdetermined using standard methods by an mass spectrometer (Finnigan-MAT Delta-E),to an accuracy of 0.1 per mil (see e.g. Kawamura et al., 2001). Major elements (chloride(Cl), sulfate (SO4), sodium (Na), potassium (K), calcium (Ca), and magnesium (Mg))were determined by ion chromatography following the methods, developed for glacialice, described in Jauhiainen et al. (1999). All samples were diluted with ultrapure waterto a concentration which was within the range of the calibration standards. The effect ofdilution on the precision was estimated by making several dilutions of a sample in everybatch analyzed. The effect was found to be negligible. Trace metals (Fe, Al, Cu andZn) were by measured by inductively coupled mass spectrometry (ThermoElemental X7ICP-MS).

Due to the number of analyses, and consequently the rather high sample volume required,each section the samples were collected with a vertical resolution of 10 mm or higher.

RESULTS AND DISCUSSIONThe conditions during the field experiment changed rapidly from rather calm and coldconditions to stormy and mild weather (Figure 2). During the first phase the air tempera-ture was around−13 ◦C. After about 30 hours of ice growth a low pressure system reachedthe area, and the conditions switched rapidly to rather mild temperatures and windy con-ditions, with some snow accumulation onto the ice, which slowed down ice growth at theice-water interface considerably. On the other hand some snow-ice was formed duringthe second half of the experiment. In the end of the experiment the total ice thickness inthe pool was 21 cm, of which about 11 cm was snow-ice.

Figure 3 shows profiles of salinity, DOC, chloride, and iron in the ice at the end of theexperiment. The topmost 11 cm is snow-ice, whereas the lower part is congelation ice.The incorporation of DOC and chloride quite clearly follows the salinity of the ice, thatapplies also for iron, but seemingly with a somewhat weaker relationship. However, whencomparing the measured DOC values in the ice to those in water diluted to the salinity

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Figure 2: Air temperature (upper) and ice and water temperature (lower). The arrowsindicate sampling times. The legend indicates the depth of the thermistors during initialsetup, after snow-ice formation (at around 72 hours from start) the uppermost thermistorwas about 11 cm from the ice-snow interface.

of the ice there is an excess of DOC in the ice. This implies that during ice formationselective retention of DOC takes place, which has also been observed during artificial seaice formation (Giannelli et al., 2001), however, the data set is limited and further studiesare needed to verify these findings (see also Giannelli et al., 2001). In the case of ironthe results are more variable, with both enrichment and depletion, however, enrichment ismainly observed both in snow-ice and congelation ice. Also the other trace elements showa more variable behavior against chloride than the major elements. However, looking atmetal to iron ratios both zinc and aluminum are depleted in the ice compared to iron. Thiscould be caused by the fact that some iron is bounded to dissolved organic matter, whichseems to be enriched in the ice, based on the observed DOC values. The fractionationof oxygen isotopes (data not shown, see e.g. Lehmann and Siegenthaler, 1991) showedfractionation factors between 1.0012 and 1.0023 during congelation ice growth. The frac-tionation factor is clearly related to ice salinity, which is explained by the relationshipof growth rate on both the initial salinity of the ice and the isotope fractionation duringfreezing.

The results for the major elements were looked at by comparing the ratio of sulfate,sodium, potassium, magnesium, and calcium to chloride in the ice to that in the parentseawater. Deviations in these ratios in the ice from those in the seawater is an indica-tion of selective fractionation during initial freezing, or for older ice by the formation of

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Figure 3: Profiles of a) salinity, b) dissolved organic carbon, c) chloride, and d) ironin the ice at the end of the experiment. The dashed (horizontal) line shows the snow-ice/congelation ice interface.

cryohydrates during thermal cycling which causes selective mobilization or retainment ofsome elements in the brine (e.g. Reeburgh and Springer-Young, 1983). We regard thatour ice had a very short thermal history, compared to natural ice sampled in other studies(e.g. Addison, 1977; Reeburgh and Springer-Young, 1983), however, the role of floodingand snow-ice formation on the element ratios in the underlying ice is hard to assess.

Sulfate was enriched in the sea ice compared to parent seawater (Figure 4a). The enrich-ment being somewhat higher at higher salinities. The results for previous investigationson the sulfate : chlorinity ratio in sea ice has been summed up by Reeburgh and Springer-Young (1983; Table 2), and show that both enrichment and depletion of sulfate occursduring sea ice formation and later on by thermal cycling it is subject to. Sulfate enrich-ment was observed also in the early samples collected, taken prior to any flooding orsnow-ice formation, indicating that selective fractionation occurred already during initialice formation. Enrichment, compared to chloride, was observed also for calcium andmagnesium in the ice in a similar fashion as for sulfate (not shown).

On the contrary to SO4/Cl ratios, K/Cl ratios were slightly smaller than the expectedvalues (Figure 4b), at least for congelation ice, implying depletion of potassium comparedto chloride during ice formation. Addison (1977) observed depletion of both K and Cain a natural sea ice sample. Sodium on the other hand showed consistently conservative

behavior, with Na/Cl ratios very close to that of the parent seawater. This was also thecase in the observations of young natural sea ice by Addison (1977).

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Figure 4: Plot of a) sulfate and b) potassium versus chloride in sea ice after 22.5 (tri-angles), 66.5 hours (crosses), and 188 hours (stars). The solid line represents the aver-age seawater sulfate to chloride and potassium to chloride ratio during the study period,dashed lines show the extreme values observed in seawater during the experiment. Sam-ples with chloride values higher than 0.5 g l−1 are essentially all snow-ice.

The majority of the major element ratios for snow-ice are closer to the ratios in the parentseawater than in congelation ice, perhaps due to different brine retention/rejection pro-cesses during snow-ice formation. For the ice temperatures during the the first phase ofthe experiment some cryohydrates could have been formed, e.g. CaCO3, Na2SO4 andMgSO4 (see e.g. Reeburgh and Springer-Young, 1983; Table 1), which could explain theretention of sulfate observed in the ice.

Assuming that one meter of ice would grow with the same sulfate to chloride ratio as inour observations on average, and the rejected brine would be evenly mixed into a 5 mwater layer, that would change the ratio in the water by around 2 % from the initial value(with the maximum observed ratio up to 4 % change). This is reasonable for large coastalareas in the Bothnian Bay, where landfast ice covers almost a third of the whole basinevery winter. However, further and more detailed studies are needed, especially on theseawater composition in winter in the area to assess the influence on ice growth.

CONCLUSIONSChemical processes were studied during formation of sea ice in brackish water with a

salinity of 3 PSU. The behavior of dissolved organic carbon (DOC), stable oxygen iso-topes, major elements and trace metals was studied during ice formation. The ratios ofmajor elements in the ice and parent seawater indicate differential behavior of elementsduring ice formation. Some are enriched in the ice, such as sulfate, calcium and magne-sium, and some depleted (potassium) compared to seawater at the same salinity. DOCwas enriched in the ice compared to what was expected from the ice salinity. This behav-ior has been observed earlier during artificial sea ice formation (Giannelli et al., 2001),and implies selective retention for DOC during ice formation. As an example of traceelements iron showed more variable behavior, however, with a tendency for enrichmentin the ice compared to seawater with the same salinity.

Chemical processes during sea ice formation are less well reported than the physical orthe biological properties. Even though the impact of sea ice on ocean chemistry has beenshown to be small, at least on longer time scales in the Arctic Ocean (e.g. Meese, 1989),the ocean chemistry may be affected especially during the initial ice formation processes.During ice formation the rejected brines may influence the composition of the under-icewaters, especially in well stratified and shallow waters, particularly if selective retentionoccurs. Even though selective retention does occur, for several chemical species, thereare to sparse observations to make any definite conclusions on the impact of ice formationprocesses on the under-ice water chemistry.

ACKNOWLEDGEMENTThe Bothnian Bay Research Station of the University of Oulu provided facilities for theBaltic Sea field experiment. The Rovaniemi Research Station of the Finnish Forest Re-search Institute provided coldroom and laboratory facilities for sample preparation andanalysis. Funding was granted by the Academy of Finland (through project ’Winter timeprocesses in the Bothnian Bay - effects of an ice cover’).

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