Abstract

pH and alkalinity measurements from a coastal upwelling area located near 30°S (Coquimbo, Chile), are used to describe the short-term variations of CO2 air-sea exchanges over a period of one week in summer 1996. A 180 km ocean-coastal transect, together with two almost-synoptic grid surveys off Coquimbo covering approximate 2500 km2 each, showed that during and immediately after a 4 day long southwesterly wind event (24-28 January) a large area of cold surface water (? 14°C), highly supersaturated in CO2 (fCO2 up to 900 ?atm), was located near the coast. Three days after the end of the event, the second grid survey showed that in most of the study area the surface temperature and pH had increased significantly (by 1-3°C and 0.05-0.2, respectively), and that the surface water was no longer supersaturated in CO2. The CO2-supersaturated water observed in the first grid survey was identified as upwelled subsurface equatorial water, a water mass with its core at about 200 m depth: the depth from which the water upwells is a major determinant of the surface water fCO2. Integrated C fluxes within a 20 km wide Coastal strip (1900 km2) indicate a strong outgassing of CO2 from the ocean under upwelling conditions (Grid 1; 121 t C day-1), while the net C exchange was directed to the ocean during the relaxation period (Grid 2; 19 t C day-1). Estimates of CO2 fluxes in upwelling areas based on surface water fCO2 measurements must therefore take into account these short-term variations: reliance on longer-term averages and interpolation will lead to erroneous results.