The global continental margins are important sites of deposition and regeneration of terrestrial organic carbon and of calcium carbonate produced in situ. The material balance of organic carbon in the coastal zone determines to a large extent its role as either a source or sink of CO2 relative to the atmosphere. We explore the role of the continental margin in the global cycling of carbon during the recent geologic past using a new model, TOTEM (Terrestrial Ocean aTmosphere Ecosystem Model). We conclude that during the past 300 yr, the coastal zone has been a site of relatively stable calcium carbonate deposition, as well as a site of greater input, recycling, and storage of terrestrial organic carbon. As a result, more organic carbon in the coastal zone is remineralized than produced in situ by photosynthesis (i.e., the coastal zone is net heterotrophic), and this reduces its sink strength for anthropogenic CO2. Continuation of this trend in the future will weaken the ability of the coastal zone to act as a sink for the rising anthropogenic CO2 in the atmosphere. Reversal of the carbon biogeochemical balance from net remineralization (heterotrophy) to net production (autotrophy) may occur in those continental margin sections where bioproduction is enhanced by elevated inorganic nutrient inputs (e.g., coastal upwelling zones) and/or where there is efficient carbon storage.