We studied the temporal variations of CO 2 , O 2 , and dimethylsulfide (DMS) concentrations within three environments (sea-ice brine, platelet ice-like layer, and underlying water) in the coastal area of Adélie Land, Antarctica, during spring 1999 before ice breakup. Temporal changes were different among the three environments, while similar temporal trends were observed within each environment at all stations. The underlying water was always undersaturated in O 2 (around 85%) and oversaturated in CO 2 at the deepest stations. O 2 concentrations increased in sea-ice brine as it melted, reaching oversaturation up to 160% due to the primary production by the sea-ice algae community (chlorophyll a in the bottom ice reached concentrations up to 160 mg L 21 of bulk ice). In parallel, DMS concentrations increased up to 60 nmol L 21 within sea-ice brine and the platelet ice-like layer. High biological activity consumed CO 2 and promoted the decrease of partial pressure of CO 2 (pCO 2 ). In addition, melting of pure ice crystals and calcium carbonate (CaCO 3 ) dissolution promoted the shift from a state of CO 2 oversaturation to a state of marked CO 2 undersaturation (pCO 2 , 30 dPa). On the whole, our results suggest that late spring land fast sea ice can potentially act as a sink of CO 2 and a source of DMS for the neighbouring environments, i.e., the underlying water or/and the atmosphere.Sea ice covers about 7% of Earth's surface at its maximum seasonal extent, representing one of the largest biomes on the planet. For decades, sea ice was assumed to be an impermeable and inert barrier for air-sea exchanges of CO 2 , and global climate models did not include CO 2 exchanges between this compartment and the atmosphere. However, there is a growing body of evidence that sea ice exchanges CO 2 with the atmosphere. While estimating permeation constants of sulfur hexafluoride (SF 6 ) and CO 2 within sea ice, Gosink et al. (1976) stressed that sea ice is a permeable medium for gases. These authors suggested that gas migration through sea ice could be an important factor in winter ocean-atmosphere exchange at sea-ice surface temperature above 210uC. More recently, uptake of atmospheric CO 2 over sea-ice cover has been reported (Semiletov et al. 2004;Delille 2006;Zemmelink et al. 2006) supporting the need to further investigate pCO 2 dynamics in the sea-ice realm and related CO 2 fluxes.Very few studies have been carried out on the dynamics of the carbonate system within natural sea ice. They have generally been aimed at investigating CaCO 3 precipitation or dissolution (Gleitz et al. 1995), or they have focused on measurements of dissolved inorganic carbon (DIC) and total alkalinity (TA) (Anderson and Jones, 1985;Rysgaard et al. 2007) rather than on pCO 2 . As pointed out by 1 Corresponding author (Bruno.Delille@ulg.ac.be).

We report first direct measurements of the partial pressure of CO 2 (pCO 2 ) within Antarctic pack sea ice brines and related CO 2 fluxes across the air-ice interface. From late winter to summer, brines encased in the ice change from a CO 2 large oversaturation, relative to the atmosphere, to a marked undersaturation while the underlying oceanic waters remains slightly oversaturated. The decrease from winter to summer of pCO 2 in the brines is driven by dilution with melting ice, dissolution of carbonate crystals, and net primary production. As the ice warms, its permeability increases, allowing CO 2 transfer at the air-sea ice interface. The sea ice changes from a transient source to a sink for atmospheric CO 2 . We upscale these observations to the whole Antarctic sea ice cover using the NEMO-LIM3 large-scale sea ice-ocean and provide first estimates of spring and summer CO 2 uptake from the atmosphere by Antarctic sea ice. Over the springsummer period, the Antarctic sea ice cover is a net sink of atmospheric CO 2 of 0.029 Pg C, about 58% of the estimated annual uptake from the Southern Ocean. Sea ice then contributes significantly to the sink of CO 2 of the Southern Ocean.

Historical sea ice core chlorophyll‐a (Chla) data are used to describe the seasonal, regional, and vertical distribution of ice algal biomass in Antarctic landfast sea ice. The analyses are based on the Antarctic Fast Ice Algae Chlorophyll‐a data set, a compilation of currently available sea ice Chla data from landfast sea ice cores collected at circum‐Antarctic nearshore locations between 1970 and 2015. Ice cores were typically sampled from thermodynamically grown first‐year ice and have thin snow depths (mean = 0.052 ± 0.097 m). The data set comprises 888 ice cores, including 404 full vertical profile cores. Integrated ice algal Chla biomass (range: <0.1–219.9 mg/m2, median = 4.4 mg/m2, interquartile range = 9.9 mg/m2) peaks in late spring and shows elevated levels in autumn. The seasonal Chla development is consistent with the current understanding of physical drivers of ice algal biomass, including the seasonal cycle of irradiance and surface temperatures driving landfast sea ice growth and melt. Landfast ice regions with reported platelet ice formation show maximum ice algal biomass. Ice algal communities in the lowermost third of the ice cores dominate integrated Chla concentrations during most of the year, but internal and surface communities are important, particularly in winter. Through comparison of biomass estimates based on different sea ice sampling strategies, that is, analysis of full cores versus bottom‐ice section sampling, we identify biases in common sampling approaches and provide recommendations for future survey programs: for example, the need to sample fast ice over its entire thickness and to measure auxiliary physicochemical parameters.