Abstract. Circumpolar Deep Water (CDW), locally called Warm Deep Water (WDW), enters the Weddell Gyre in the southeast, roughly at 25° E to 30° E. In December 2002 and January 2003 we studied the effect of entrainment of WDW on the fugacity of carbon dioxide (fCO₂) and dissolved inorganic carbon (DIC) in Weddell Sea surface waters. Ultimately the fCO₂ difference across the sea surface drives air-sea fluxes of CO₂. Deep CTD sections and surface transects of fCO₂ were made along the Prime Meridian, a northwest-southeast section, and along 17° E to 23° E during cruise ANT XX/2 on FS Polarstern. Upward movement and entrainment of WDW into the winter mixed layer had significantly increased DIC and fCO₂ below the sea ice along 0° W and 17° E to 23° E, notably in the southern Weddell Gyre. Nonetheless, the ice cover largely prevented outgassing of CO₂ to the atmosphere. During and upon melting of the ice, biological activity rapidly reduced surface water fCO₂ by up to 100 μatm, thus creating a sink for atmospheric CO₂. Despite the tendency of the surfacing WDW to cause CO₂ supersaturation, the Weddell Gyre may well be a CO₂ sink on an annual basis due to this effective mechanism involving ice cover and ensuing biological fCO₂ reduction. Dissolution of calcium carbonate (CaCO₃) in melting sea ice may play a minor role in this rapid reduction of surface water fCO₂.