Winter-to-summer evolution of pCO₂ in surface water and air–sea CO₂ flux in the seasonal ice zone of the Southern Ocean

Abstract. Partial pressure of CO₂ (pCO₂) in surface water and vertical profiles of the carbonate system parameters were measured during austral summer in the Indian sector of the Southern Ocean (64–67° S, 32–58° E) in January 2006 to understand the CO₂ dynamics of seawater in the seasonal ice zone. Surface-water pCO₂ ranged from 275 to 400 μatm, and longitudinal variations reflected the dominant influence of water temperature and dilution by sea ice meltwater between 32 and 40° E and biological productivity between 40 and 58° E. Using carbonate system data from the temperature minimum layer (−1.9 °C < T < −1.5 °C, 34.2 < S < 34.5), we examined the winter-to-summer evolution of surface-water pCO₂ and the factors affecting it. Our results indicate that pCO₂ increased by as much as 32 μatm, resulting mainly from the increase in water temperature. At the same time as changes in sea ice concentration and surface-water pCO₂, the air–sea CO₂ flux, which consists of the exchange of CO₂ between sea ice and atmosphere, changed from −1.1 to +0.9 mmol C m⁻² day⁻¹ between winter and summer. These results suggest that, for the atmosphere, the seasonal ice zone acts as a CO₂ sink in winter and a temporary CO₂ source in summer immediately after the retreat of sea ice. Subsequent biological productivity likely decreases surface-water pCO₂ and the air–sea CO₂ flux becomes negative, such that in summer the study area is again a CO₂ sink with respect to the atmosphere.