CHANGES OF CARBON-DIOXIDE IN SURFACE WATERS DURING SPRING IN THE SOUTHERN-OCEAN

The fugacity of CO2 (fCO(2)) and the content of chlorophyll alpha in s urface-water were determined during consecutive sections between 47 de grees and 60 degrees S along 6 degrees W in austral spring, October-No vember 1992. In the Polar Frontal region, the fCO(2) of surface-water decreased from slightly below the atmospheric value to 50 mu atm below it. This was accompanied by the development of diatom blooms. Seasona l warming of 1.2 degrees C and air-sea exchange partly compensated the decrease of fCO(2) by biological activity. Meanders of the Polar Fron tal jet and a mesoscale eddy were reflected in spatial variability of fCO(2) and chlorophyll a. Systematic observations indicated relationsh ips between fCO(2) and chlorophyll a, albeit changing with time. The c ombination of biological CO2-uptake with formation of Antarctic Interm ediate Water (AAIW) makes the Polar Front a site of combined biologica l/physical CO2-drawdown from the atmosphere. In the southern part of t he Antarctic Circumpolar Current (sACC) and the Southern Frontal regio n, fCO(2) increased 7-8 mu atm due to surface-water warming of 0.5 deg rees C. A sharp rise of surface water fCO(2) of 13 mu atm occurred sou th of the southern Frontal jet. As the ice-cover disappeared, the Boun dary between the ACC and the Weddell Gyre released significant amounts of CO2. The Weddell Gyre would become a strong CO2-source after the i mminent retreat of the ice. Clearly mechanisms behind changes of fCO(2 ) in surface waters differ for the hydrographic regions. Interstitial brines of sea-ice had fCO(2) as low as 100 mu atm and had been deplete d in nutrients. The summation of significant sources and sinks in the different regions indicates an overall minor oceanic CO2-sink of 0.3 m mol m(-2) day(-1) throughout the cruise, on the basis of the Wanninkho f relationship at in situ wind speed without skin effect. Uptake of CO 2 increased to 1.0 mmol m(-2) day(-1), when a uniform cold skin temper ature difference of 0.2 degrees C was assumed. The skin temperature di fference derived from the physical model by Soloviev and Schlussel (19 94a,b) had an average value of 0.2 degrees C, leading to an uptake of CO2 of 1.2 mmol m(-2) day(-1). The measured skin temperature differenc e exceeded the calculated value. These assessments underline the uncer tainty in the estimated air-sea exchange of CO2 due to the thermal shi n effect, the chosen parametrization of the gas transfer velocity, and the selected length of the wind speed interval. Limited understanding of the mechanistics of gas exchange, as well as large seasonal and sp atial variability of the air-sea flux, still preclude a reliable estim ate of the basin-wide annual flux for the Southern Ocean. (C) 1997 Els evier Science Ltd.