AIR-SEA CARBON-DIOXIDE EXCHANGE IN THE NORTH PACIFIC SUBTROPICAL GYRE- IMPLICATIONS FOR THE GLOBAL CARBON BUDGET

The role of the ocean as a sink for anthropogenic carbon dioxide is a subject of intensive investigation and debate. Interest in this proces s is driven by the need to predict the rate of future increase of atmo spheric carbon dioxide and subsequent global climatic change. Although estimates of the magnitude of the oceanic sink for carbon dioxide app ear to be converging on a value of approximately 2 (Gt) C yr-1 for the 1980s, a detailed understanding of the temporal and spatial variabili ty in the rate of exchange of carbon dioxide between the ocean and the atmosphere is not available. For example, recent modeling work and di rect measurements of air-sea carbon dioxide flux produce very differen t estimates of the air-sea flux in the northern hemisphere. As a conse quence, it has been suggested that a large unidentified oceanic carbon dioxide sink may exist in the North Pacific. As a part of our time se ries observations in the North Pacific Subtropical Gyre, we have measu red dissolved inorganic carbon and titration alkalinity over a four-ye ar period. These measurements constitute the most extensive set of obs ervations of carbon system parameters in the surface waters of the cen tral Pacific Ocean. Our results show that the ocean in the vicinity of the time series site is a sink for atmospheric carbon dioxide. On the basis of these observations, we present a mechanism by which the Nort h Pacific Subtropical Gyre can be a potential sink for approximately 0 .2 Gt C yr-1 of atmospheric carbon dioxide. Although our observations indicate that the North Pacific Subtropical Gyre is a sink for atmosph eric carbon dioxide, the magnitude of this oceanic sink is relatively small. Our data and interpretations are therefore consistent with the argument for a relatively large sink during the 1980s in northern hemi sphere terrestrial biomass. Another possibility is that the net releas e of carbon dioxide to the atmosphere owing to land use activities in tropical regions has been overestimated.