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...

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The biogeochemical composition of 2 spatially separate surface gap layers on a single Antarctic sea ice floe during early austral summer was predominantly controlled by the growth of diatoms and, especially, Phaeocystis. These algal communities in and near the gap layers imposed large geochemical changes in the chemical and isotopic composition of the gap waters, typical of intense autotrophic activity. These included a large deficit in all major dissolved inorganic nutrients (dissolved inorganic carbon [C T ], nitrate, soluble reactive phosphorus, silicic acid), O 2 accumulation above air saturation, large pH shifts into the alkaline spectrum, and a large, closely coupled 13 C enrichment of the C T pool and the accumulated particulate organic carbon, in all cases relative to the composition of surface oceanic water. The amount of inorganic carbon removed from the gap water exceeded that which can be predicted from the deficits of dissolved inorganic nitrogen or phosphorus and the elemental composition of the biogenic matter suspended in it or the mean elemental composition of oceanic phytoplankton. This stoichiometric deviation suggests either (1) the operation of the inorganic carbon overconsumption mechanism via the biological production of particular classes of intra-or extracellular carbon rich compounds, or (2) substantial utilisation of ammonium and urea as autotrophic nitrogen sources in addition to nitrate, or (3) both. KEY WORDS: Antarctica. Sea ice. Gap layers. Biogeochemistry. Nutrients. Particulate organic matter. Carbon isotopic composition Resale or republication not permitted without written consent of the publisher

[1] A large-scale geographical study of the ice pack in the seasonal ice zone of the Weddell Sea, Antarctica, took place from September to October 2006. Sea ice brines with a salinity greater than 58 and temperature lower than À3.6 C were sampled from 22 ice stations. The brines had large deficits in total alkalinity and in the concentrations of the major dissolved macronutrients (total dissolved inorganic carbon, nitrate, and soluble reactive phosphorus) relative to their concentrations in the surface oceanic water and conservative behavior during seawater freezing. The concentration deficits were related to the dissolved inorganic carbon-consuming processes of photosynthesis, CaCO 3 precipitation, and CO 2 degassing. The largest concentration deficits in total dissolved inorganic carbon were found to be associated with CaCO 3 precipitation and CO 2 degassing, because the magnitude of the photosynthesis-induced concentration deficit in total dissolved inorganic carbon is controlled by the size of the inorganic nutrient pool, which can be limited in sea ice by its openness to exchange with the surrounding oceanic water.

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