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Analyzing simulations of glacial ocean circulation and radiocarbon distribution I show that increased sea ice cover over the Southern Ocean reduces ventilation and radiocarbon content of the deep ocean. Two simulations, one present-day and one glacial, tuned to have similar rates of North Atlantic Deep Water formation are used. Insulation from air-sea gas exchange due to more extended sea ice cover increases Southern Ocean radiocarbon ages by more than 100 yr. Higher rates of sea ice formation and export from high southern latitudes lead to a salinification of Antarctic Bottom Water (AABW), making it the most saline deep water mass of the glacial world oceans. This increases the density of AABW and hence its rate of formation. Mass and radiocarbon fluxes to the deep ocean are thus decoupled. Both older Southern Ocean waters and a stronger flux of AABW increase radiocarbon ages of glacial bottom waters by up to 300 yr. Available reconstructions of glacial bottom water properties are broadly consistent with the simulation. These results question previous inferences from radiocarbon distributions on glacial deep water formation rates. (C) 2003 Elsevier Science B.V. All rights reserved.