Circumpolar Deep Water (CDW), locally called Warm Deep Water, (WDW), enters the Weddell Gyre in the southeast, roughly at 25-30°E. In December 2002-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 CO2 air-sea fluxes. Deep CTD sections and surface transects of fCO2 were made along the Prime Meridian, a northwest-southeast section, and along 17-23°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 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 of 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. The CO2 source tendency deriving from the upward movement of pre-industrial CDW is declining, as atmospheric CO2 levels continue to increase and thus the CO2 sink ofthe Weddell Gyre will continue to increase as well (provided the upward movement of WDW does not change much).