SEASONAL AND INTERANNUAL VARIABILITY OF OCEANIC CARBON-DIOXIDE SPECIES AT THE US JGOFS BERMUDA ATLANTIC TIME-SERIES STUDY (BATS) SITE

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 Decembe r 1993, a time series of oceanic measurements of total carbon dioxide (TCO2), alkalinity (TA) and calculated pCO(2) was obtained at the Berm uda Atlantic Time-series Study (BATS) site (31 degrees 50'N, 64 degree s 10'W) in the Sargasso Sea. These measurements constitute the most ex tensive set of CO2 species data collected in the oligotrophic North At lantic. Seasonal changes in surface and water-column CO2 species were similar to 40-50 mu mol kg(-1) in TCO2, similar to 20 mu mol kg(-1) in TA, and similar to 90-100 mu atm in calculated pCO(2). These large ch anges were driven principally by deep convective winter mixing, temper ature forcing and biological activity. TA was well correlated with sal inity (with the exception of a 15-25 mu mol kg(-1) drawdown of TA on o ne cruise resulting from open-ocean calcification). TCO2 and pCO(2) we re well correlated with seasonal temperature changes (8-9 degrees C). Other underlying processes, such as biological production, advection, gas exchange of CO2 and vertical entrainment, were important modulator s of the carbon cycle, and their importance varied seasonally. Each sp ring-to-summer, despite the absence of measurable nutrients in the eup hotic zone, a 35-40 mu mol kg(-1) decrease in TCO2 was attributed prim arily to the biological uptake of TCO2 (evaporation/precipitation bala nce, gas exchange, and advection were also important). An increase in TCO2 during the fall months was associated primarily with entrainment of higher TCO2 subsurface waters. These seasonal patterns require a re assessment of the modelling of the carbon cycle using nutrient tracers and Redfield stoichiometries. Overall, the region is a weak sink (0.2 2-0.83 mol C m(-2) year(-1)) for atmospheric CO2. Upper ocean TCO2 lev els increased between 1988 and 1993, at a rate of similar to 1.7 mu mo l kg(-1) year(-1). This increase appears to be in response to the upta ke of atmospheric CO2 through gas exchange or natural variability of t he subtropical gyre. Copyright (C) 1996 Elsevier Science Ltd