Sea Ice CO 2 Dynamics Across Seasons: Impact of Processes at the Interfaces

International audience ; Winter to summer CO 2 dynamics within landfast sea ice in McMurdo Sound (Antarctica) were investigated using bulk ice pCO 2 measurements, air‐snow‐ice CO 2 fluxes, dissolved inorganic carbon (DIC), total alkalinity (TA), and ikaite saturation state. Our results suggest depth‐dependent biotic and abiotic controls that led us to discriminate the ice column in three layers. At the surface, winter pCO 2 supersaturation drove CO 2 release to the atmosphere while spring‐summer pCO 2 undersaturation led to CO 2 uptake most of the time. CO 2 fluxes showed a diel pattern superimposed upon this seasonal pattern which was potentially assigned to either ice skin freeze‐thaw cycles or diel changes in net community production. In the ice interior, the pCO 2 decrease across the season was driven by physical processes, mainly independent of the autotrophic and heterotrophic phases. Bottom sea ice was characterized by a massive biomass build‐up counterintuitively associated with transient heterotrophic activity and nitrate plus nitrite accumulation. This inconsistency is likely related to the formation of a biofilm. This biofilm hosts both autotrophic and heterotrophic activities at the bottom of the ice during spring and may promote calcium carbonate precipitation.