Using contemporary CO2 data from the subsurface Weddell Sea, the source/sink function of this region against the changing atmospheric CO 2 level has been investigated. As in the central Weddell Sea, surface water is supplied by upwelling of subsurface water, the CO2 content is also forced by it. TCO2 data of four cruises were used to determine a robust value for the subsurface Warm Deep Water (WDW). After accounting for biological activity in the surface layer and salinity differences between the subsurface and surface waters, the forcing CO 2 partial pressure (pCO2) was calculated from the TCO 2 of the WDW and the conservative alkalinity as taken from the literature. As the WDW contains negligible anthropogenic CO2, the pCO2 forcing by the WDW has been prevalent both in the pre-industrial and modern Weddell Sea. The calculated pCO2 forcing amounts to 300-310 μatm at a minimum in late winter/early spring and possibly 30 μatm more during spring and summer. This figure does not represent the actual pCO2, but rather the value before air-sea exchange gets effective. Hence, in pre-industrial times when the atmospheric pCO2 was about 280 μatm, the Weddell Sea must have been a relatively strong source of atmospheric CO2. Because of the steadily rising atmospheric CO2 levels to more than the pCO2 forcing by the WDW, the Weddell Sea turned into a CO2 sink in recent times. The storage of anthropogenic CO2 in the Weddell Sea surface layer is estimated to be 4.1 mol C m-2. Applying the WDW forcing method to O2, a steady state O2 uptake from the atmosphere of 3.6 mol O2 m-2 year-1 is computed. © 2003 Elsevier B.V. All rights reserved.