Abstract
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.
Highlights
The Southern Ocean is an area of a large CO2 flux between ocean and atmosphere because of its large surface area and strong regional winds (Sabine and Key, 1998)
The difference between pressure of CO2 (pCO2) cal and pCO2 obs reflects other effects on surface-water pCO2 from winter to summer. This difference was near zero (+0.5 ± 7.5 μatm) between 32 and 40◦ E and as large as +100 μatm (+47.5 ± 16.3 μatm) between 40 and 58◦ E (Fig. 10). These results suggest that the thermodynamic efpCO2 cal – pCO2 obs fect was dominant at 32–40◦ E
For the surface-water pCO2, we demonstrated that pCO2 variations have a thermodynamic origin before the onset of active biological productivity
Summary
The Southern Ocean is an area of a large CO2 flux between ocean and atmosphere because of its large surface area and strong regional winds (Sabine and Key, 1998). McNeil et al (2007) computed a total CO2 uptake of −0.4 Gt C year−1 by the Southern Ocean on the basis of CO2 partial pressure (pCO2) in surface water, calculated from carbonate system parameters in seawater that were parameterized separately for the summer and winter seasons as a function of temperature, salinity and nutrient levels.
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