[1] We report on the discovery of the mineral ikaite (CaCO 3 Á6H 2 O) in sea-ice from the Southern Ocean. The precipitation of CaCO 3 during the freezing of seawater has previously been predicted from thermodynamic modelling, indirect measurements, and has been documented in artificial sea ice during laboratory experiments but has not been reported for natural sea-ice. It is assumed that CaCO 3 formation in sea ice may be important for a sea ice-driven carbon pump in ice-covered oceanic waters. Without direct evidence of CaCO 3 precipitation in sea ice, its role in this and other processes has remained speculative. The discovery of CaCO 3 Á6H 2 O crystals in natural sea ice provides the necessary evidence for the evaluation of previous assumptions and lays the foundation for further studies to help elucidate the role of ikaite in the carbon cycle of the seasonally sea icecovered regions.

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The evolution of the main physico-chemical properties of the unflooded 90-cm-thick first-year sea-ice cover at the Ice Station POLarstern (ISPOL) ''clean site'' is described. ISPOL was an international experiment of the German research icebreaker R.V. Polarstern. The vessel was anchored to an ice floe for an observation period of 5 weeks, during the early summer melt onset in the Western Weddell Sea. The ''clean site'' was specially designed and accessed so as to prevent any trace metal contamination of the sampling area. Observations were made at 5-day intervals during December 2004 in the central part of the main floe. Results show the succession of two contrasting phases in the behavior of the brine network (brine channels, pockets, and tubes). Initially, brine salinity was higher than that of sea-water, leading to brine migration and a decrease in the mean bulk salinity of the ice cover. This process is highly favored by the already high bulk porosity (14%), which ensures full connectivity of the brine network. Gravity drainage rather than convection seems to be the dominant brine transfer process.Half-way through the observation period, the brine salinity became lower than that of the sea-water throughout the ice column. The brine network therefore switched to a ''stratified'' regime in which exchange with sea-water was limited to molecular diffusion, strongly stabilizing the bulk mean sea-ice salinity. During the transition between the two regimes, and in areas closer to ridges, slush water (resulting from a mixture of snow meltwater and sea water accumulated at the snow-ice interface) penetrated through the growing ''honeycomb-like structure'' and replaced the downward draining brines. This resulted in a slight local replenishment of nutrients (as indicated by dissolved silicic acid). However, as a whole, the described decaying regime in this globally unflooded location with limited snow cover should be unfavorable to the development of healthy and active surface and internal microbial communities.The switch from gravity to diffusion controlled transport mechanisms within the ice column also should affect the efficiency of gas exchange across the sea-ice cover. The observed late build-up of a continuous, impermeable, superimposed ice layer should further significantly hamper gas exchange.Statistical estimates of the evolution of the ice thickness during the observation period and salinity trends of the under-ice water salinity down to 30 m corroborate model predictions of a moderate bottom melting (5-10 cm) from ocean heat fluxes. r

The origin of dissolved organic matter (DOM) within sea ice in coastal waters of the Baltic Sea was investigated using parallel factor (PARAFAC) analysis of DOM fluorescence. Sea ice DOM had distinctly different fluorescence characteristics than that of the underlying humic-rich waters and was dominated by protein-like fluorescence signals. PARAFAC analysis identified five fluorescent components, all of which were present in both sea ice and water. Three humic components were negatively correlated to salinity and concluded to be terrestrially derived material. Baltic Sea ice DOM was found to be a mixture of humic material from the underlying water column incorporated during ice formation and autochthonous material produced by organisms within the ice. Dissolved organic carbon (DOC) and nitrogen (DON) concentrations were correlated to the humic fluorescence, indicating that the majority of the organic carbon and nitrogen in Baltic Sea ice is bound in terrestrial humic material trapped within the ice. This has implications for our understanding of sea ice carbon cycling in regions influenced by riverine input (e.g., Baltic and Arctic coastal waters), as the susceptibility of DOM to degradation and remineralization is largely determined by its source.

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Although salt rejection from sea ice is a key process in deep-water formation in ice-covered seas, the concurrent rejection of CO 2 and the subsequent effect on air-sea CO 2 exchange have received little attention. We review the mechanisms by which sea ice directly and indirectly controls the air-sea CO 2 exchange and use recent measurements of inorganic carbon compounds in bulk sea ice to estimate that oceanic CO 2 uptake during the seasonal cycle of sea-ice growth and decay in ice-covered oceanic regions equals almost half of the net atmospheric CO 2 uptake in ice-free polar seas. This sea-ice driven CO 2 uptake has not been considered so far in estimates of global oceanic CO 2 uptake. Net CO 2 uptake in sea-ice-covered oceans can be driven by; (1) rejection during sea-ice formation and sinking of CO 2-rich brine into intermediate and abyssal oceanic water masses, (2) blocking of air-sea CO 2 exchange during winter, and (3) release of CO 2-depleted melt water with excess total alkalinity during sea-ice decay and (4) biological CO 2 drawdown during primary production in sea ice and surface oceanic waters.

Sea ice brines were collected from a single floe composed of different ice types in the western Weddell Sea in December 2004. The chemical composition of the brines (temperature: 23.4uC to 22.1uC; salinity: 40-63) was examined on seven occasions over 25 days with measurements of dissolved oxygen, dissolved inorganic macronutrients (nitrate plus nitrite, ammonium, phosphorus [DIP], and silicic acid), pH, total alkalinity (A T ), dissolved organic carbon (DOC) and nitrogen (DON), total dissolved inorganic carbon (C T ), and the stable isotopic composition of C T (d 13 C T ). The in situ pH ranged from 8.41-8.82 on the seawater scale, dissolved oxygen from 212-604 mmol kg 21 , nitrate from 0.1-3.1 mmol kg 21 , ammonium 0.1-2.4 mmol kg 21 , DIP 0.4-2.0 mmol kg 21 , silicic acid 4-80 mmol kg 21 , A T 2,690-4,620 meq kg 21 , DOC 115-359 mmol kg 21 , DON 8-26 mmol kg 21 , C T 2,090-3,550 mmol kg 21 , and d 13 C T +2.9%-+6.4%. Compared with the chemical composition of surface oceanic water (salinity of 34), the brines had elevated pH, reduced concentrations of dissolved inorganic macronutrients (including carbon), especially dissolved inorganic nitrogen, and were mostly supersaturated with dissolved oxygen with respect to equilibrium with air, whereas the C T was considerably enriched in 13 C. The chemical composition of the brines was consistent with internal biological productivity, but there was a lack of a distinctive and uniform relationship among the major dissolved inorganic nutrients typically used for describing biological activity. This was interpreted as the result of varying stoichiometry of biological activity within a very small spatial scale. Modification by abiotic processes was a potential contributing factor, such as degassing acting on the dissolved oxygen concentration. Carbonate mineral formation, acting on A T and C T , was not evident in brines from first-year ice but was apparent in brine from second-year ice.Sea ice covers 13% of the surface of the earth at its maximum extent, equivalent to the areal extent of deserts and tundra. The largest expanse of sea ice occurs in the Southern Ocean, totalling 18.8 3 10 16 km 2 of 0.5-0.6 m mean thickness at its maximum extent in September (Comiso 2003). It is an ecologically diverse habitat and has been increasingly recognized for its key role in global biogeochemical cycles, including contribution to the primary productivity of the Southern Ocean (Arrigo 2003;Arrigo and Thomas 2004) and the ventilation of the deep ocean at high latitudes (Francois et al. 1997;Stephens and Keeling 2000).The chemical composition of sea ice is primarily a function of salinity and temperature, but it is modified further by productivity of the internal microbial assemblages recruited from the surface seawater during sea ice formation (Brierley and Thomas 2002;Thomas and Dieckmann 2002;Rysgaard et al. 2007). Primary production is a major carbon sink within sea ice, resulting in the fixation of 30-70 Tg C yr 21 into biomass in the firstand multi-year ice pack of the Sou...

Over the past two decades, with recognition that the ocean's sea-ice cover is neither insensitive to climate change nor a barrier to light and matter, research in sea-ice biogeochemistry has accelerated significantly, bringing together a multi-disciplinary community from a variety of fields. This disciplinary diversity has contributed a wide range of methodological techniques and approaches to sea-ice studies, complicating comparisons of the results and the development of conceptual and numerical models to describe the important biogeochemical processes occurring in sea ice. Almost all chemical elements, compounds, and biogeochemical processes relevant to Earth system science are measured in sea ice, with published methods available for determining