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Abstract

The Southern Ocean plays a prominent role in the global carbon cycle, but the role of Antarctic sea ice is poorly understood. Antarctic sea ice is considered to foster winter export to depth and summer uptake from the atmosphere, forming the sea ice carbon pump. However, large uncertainties on the driving processes and their uncertainty remain. Here, 70 complete seasons of the upper ocean physical and biogeochemical characteristics recorded from available SOCCOM floats in the Antarctic sea ice zone (2014-2022) were collocated with satellite-retrieved ice concentration and ERA5 low-atmosphere reanalysis data. We find that surface ocean physical and biogeochemical properties are tightly in phase with sea ice seasonality. A large amplitude, highly consistent increase in surface water DIC and S is found during the cold season, with three implications. 1) DIC and salt are similarly released to the surface ocean upon sea ice growth. 2) Efficiency of DIC export to depth is weak unlike previously suggested but consistent with model studies, reducing the possible influence of sea ice to deep polar ocean basins. 3) Air-sea carbon exchanges are limited during the ice-covered season. A sharp Chl-a increase during the sea ice decay season is detected, peaking some 10 days after sea ice retreat. The surface water TA-DIC ratio also decreases during the melting season, but weakly so and consistently with a nitrate drawdown signal. Overall, the analyzed dataset suggests that the sea ice carbon pump would mostly result from the combination of reduced air-sea exchanges in winter and biological export in summer.