| Summary: | The seasonal and interannual dynamics of the oceanic carbon cycle and the strength of air-sea exchange of carbon dioxide are poorly known in the North Atlantic subtropical gyre. Between October 1988 and December 1993, a time series of oceanic measurements of total carbon dioxide (TCO 2 ), alkalinity (TA) and calculated p CO 2 was obtained at the Bermuda Atlantic Time-series Study (BATS) site (31°50"N, 64°10"W) in the Sargasso Sea. These measurements constitute the most extensive set of CO 2 species data collected in the oligotrophic North Atlantic. Seasonal changes in surface and water-column CO 2 species were ~40–50 µmol kg -1 in TCO 2 , ~20 µmol kg -1 in TA, and ~90–100 µatm in calculated p CO 2 . These large changes were driven principally by deep convective winter mixing, temperature forcing and biological activity. TA was well correlated with salinity (with the exception of a 15–25 µmol kg -1 drawdown of TA on one cruise resulting from open-ocean calcification). TCO 2 and p CO 2 were well correlated with seasonal temperature changes (8–9°C). Other underlying processes, such as biological production, advection, gas exchange of CO 2 and vertical entrainment, were important modulators of the carbon cycle, and their importance varied seasonally. Each spring-to-summer, despite the absence of measurable nutrients in the euphotic zone, a 35–40 µmol kg -1 decrease in TCO 2 was attributed primarily to the biological uptake of TCO 2 (evaporation/precipitation balance, gas exchange, and advection were also important). An increase in TCO 2 during the fall months was associated primarily with entrainment of higher TCO 2 subsurface waters. These seasonal patterns require a reassessment of the modelling of the carbon cycle using nutrient tracers and Redfield stoichiometries. Overall, the region is a weak sink (0.22–0.83 mol C m -2 year -1 ) for atmospheric CO 2 . Upper ocean TCO 2 levels increased between 1988 and 1993, at a rate of ~ 1.7 µmol kg -1 year -1 . This increase appears to be in response to the uptake of ...
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