The Arctic Ocean plays a critical part in global oceanic circulation, in that it modulates the formation of deep water in the North Atlantic via ice formation. The extent of Arctic sea ice undergoes seasonal changes with an average maximum of 15 × 106 km² and an average minimum of 8 × 106 km² (Gloersen and Campbell, 1991). Due to warming of the world oceans (Levitus et al., 2000) the sea ice cover in the Arctic has decreased by ~10% in extent since the 1970s (Johannessen et al., 1999). Climate change due to the anthropogenic greenhouse emission is expected to be significant and relatively large in the Arctic region compared to other areas in the world and the consensus view is that within 70 years, the average annual temperature of the Arctic is predicted to increase by at least 4°C and in some places by more than 8°C. This increase implies that the sea ice thickness and distribution in the Arctic will decrease markedly and thus alter the heat and mass exchange as well as ocean stratification in the future and hence, research in sea ice areas is urgently needed in order to obtain a basic understanding of the processes and organisms being affected by the expected dramatic alterations in sea ice distribution.

Deep-water formation in the Arctic Ocean and the Greenland Sea contributes to the overflow water in the Denmark Strait and the Faroe Bank Channel (Buch et al., 1996, Hansen et al., 2001), which constitutes the origin of the North Atlantic Deep Water (Reid and Lynn, 1971). In addition, high-latitude areas act as a sink for atmospheric CO₂ and thereby represent a direct pathway for CO₂ exchange between the atmosphere and the deep ocean (Takahashi et al., 2002). The difference between pCO₂ in surface water and the overlying air represents the thermodynamic driving potential for air-sea CO₂ gas exchange, which influences the surface concentrations of dissolved inorganic carbon (DIC). Surface-water pCO₂ levels are regulated by temperature, salinity and DIC and total alkalinity (TA) concentrations. DIC and TA have hitherto been considered to be controlled primarily by biological processes (i.e. photosynthesis, respiration and calcification), upwelling of subsurface water enriched in respired CO₂ and nutrients, and air–sea exchange (Takahashi et al., 2002). However, our recent work suggests that besides oxygen, DIC is also rejected from growing sea ice together with brine (Glud et al. 2002, Rysgaard et al., 2007). In addition, we have observed very low DIC and TA concentrations in Arctic first-year and multi-year sea ice as well as significant fractionation of DIC and TA compared to surface-water conditions (Rysgaard et al., 2007). Downward rejection of CO₂ from growing sea ice together with brine has not been considered previously, and nor has the resulting atmosphere-ice-ocean exchange of carbon resulting from this sea-ice driven DIC pump been quantified. Our first preliminary model calculations show that this DIC pump affects the surface-water pCO₂ significantly in the polar seas and potentially sequesters large amounts of CO₂ in the deep ocean. Brine-mediated DIC transport into the deep ocean may be significant in the global carbon cycle, and variations in sea ice formation under past climatic conditions might help explain variations in atmospheric CO₂ levels.

A number of studies have revealed that ice algae can modify their environment in the sea ice so that they are able to exploit the interior of the sea ice as a habitat. The modifications are presumed to be mediated by the algae synthesizing ice active substances that at least can inhibit the growth of the ice surrounding the algae, but which possibly also widen the spaces by a non-colligative melting of the ice surface around the algae. The substances synthesized by the ice algae have several properties in common with the so called “antifreeze proteins” that are synthesized by a number of cold tolerant ectothermic animals. These proteins can recognize ice surfaces and bind to them, and are thereby able to inhibit the growth of ice crystal. To understand the total CO₂ budget of the sea ice, investigations of these ice associated algae and their physiology is needed. Likewise, future decrease in global sea ice cover may reduce the world oceans’ capacity for taking up atmospheric CO₂. Assessing changes in future sea ice conditions as projected by climate models will allow for a more quantitative analysis of this hypothesis. Thus, more research on this process is urgently needed.

Aim

The aim is to investigate and quantify the importance of sea ice in transporting carbon dioxide from the atmosphere to the ocean in areas with different types of sea ice. Furthermore, our recent discoveries of anoxic conditions and bacterial denitrification/anammox activity in sea ice show that sea ice may play an important role in the removal of nitrogen. More investigations from multi-year sea ice are, however, needed to increase our understanding of its significance in the Arctic and its role in the global nitrogen cycle. Furthermore, sea ice formation may play a far more important role in transporting carbon dioxide (CO₂) from the atmosphere to the ocean than previously assumed. Our preliminary studies on first-year sea ice from northeast Greenland and northern Canada have indicated that ice growth during winter rejects large amounts of CO₂, which sink together with dense brine to intermediate and deep water layers. Subsequent sea ice melt during summer enhances the uptake of CO₂ from the atmosphere, as the resulting meltwater is undersaturated with respect to CO₂. As the transport mechanism is dependent on sea ice formation, future decrease in global sea ice cover may reduce the oceanic capacity for taking up atmospheric CO₂.

During the LOMROG expedition, sea ice samples were acquired by drilling ice cores along the expedition path with the aim of measuring the total concentration of inorganic dissolved carbon (TIC or TCO₂) and total alkalinity (TA). In addition to the physical and chemical measurements of the sea ice, the occurrence of algae in the ice was also investigated. The reason for this is that the role of the algae, concerning the chemical conditions in the context of the CO₂ in the ice, is not well known. Thus it is of importance to investigate the algae and their physiology. Ice algae synthesize some substances that can modify the ice surface and optimise the environment in the ice in which the algae are found. It is therefore also one of the aims of the present study to investigate the algae and the synthesized substances.

Methods

Ice coring

Different types of sea ice were collected using a 7 cm KOVACS ice core drill fitted with a battery driven hand drill. 10 cm pieces from the top, middle and bottom of the ice core were cut out and transferred to plastic containers. Before collecting the 10 cm pieces the temperature along the core was measured at 10 cm intervals.

Physical measurements

The physical parameters air temperature, snow depth, snow temperature (5 cm intervals), ice surface temperature, ice density and total ice salinity (melted ice core) were measured at each drill hole and ice core piece. Density of the core pieces was determined from top, middle and bottom, where possible, by measuring and weighing the core piece.

Preparation of ice cores for measurements of TCO₂, total alkalinity (TA), salinity and chlorophyll a

The collected core pieces were taken to the laboratory and cleaved. One half was transferred to a gas proof plastic bag (Würgler bag (Würgler Hansen et al., 2000) and sealed. The ice was melted in the bag and the melt water transferred to gas tight containers, preserved with HgCl₂ and then brought to the laboratory in Nuuk (Greenland) where the final measurements are to be done.

The other half was melted; some melt water was transferred to a plastic vial for the measurement of salinity and the rest was filtered through a glass filter and the filter was thereafter frozen for the later determination of the chlorophyll a content of the filtrate (for example the ice). In addition to the ice cores, water samples from 10 m depth were acquired from the CTD that was operated by one of the Swedish groups participating on the expedition (see Andersson and Björk).