Distribution of surface fCO 2 and air–sea fluxes in the Southwestern subtropical Atlantic and adjacent continental shelf

Abstract

Measurements of the fugacity of CO 2, salinity, in situ temperature, nutrients and chlorophyll- a were made in November 1997, January/February and June 1998, during the “Ocean Circulation in the Southwest Atlantic Region” program (COROAS). The surface water fugacity of CO 2, fCO 2(sw), in the open ocean was 362.8 μatm, corresponding to a fugacity difference between surface water and atmosphere (Δ fCO 2) of +3.2 μatm in November; 425.5 μatm (Δ fCO 2 = +75.0 μatm) in January/February, and 350.6 μatm (Δ fCO 2 = −4.0 μatm) in June, respectively, for the latitudes south of 25.3°S. These fCO 2(sw) values suggest that only during winter does this oceanic area absorb any atmospheric CO 2, whereas in the remaining seasons CO 2 is released into the atmosphere. North of 25.3°S CO 2 was released into the atmosphere during all the seasons, with highest Δ fCO 2 at the continental shelf. The contribution of biological and temperature effects to the seasonal fCO 2(sw) variations was calculated by dividing the region sampled into three subsections: the continental shelf, the continental slope and the open ocean. These calculations were compared with normalized fCO 2(sw) and it was concluded that the main contributions to the fCO 2(sw) gradients in the open ocean are the in situ temperature and the exchange of CO 2(g) with the atmosphere. In the slope and shelf zones the biological regenerative processes must also be considered. The estimated net sea–air CO 2 fluxes ranged between −1.1 and 0.1 mmol m −2 day −1 for the open ocean, 0.4 and 3.7 mmol m −2 day −1 for the slope and 0.3 and 9.8 mmol m −2 day −1 for the shelf zone, using the Wanninkhof 92 formula. It is reasonable to assume that the study area is a source of CO 2 for the atmosphere and the sea–air CO 2 fluxes over the continental shelf and slope could lead to refinements regarding the calculations of continental shelf pump.