Processes governing the carbon chemistry during the SAGE experiment

Measurements of pCO2, pH and alkalinity in the surface waters of an iron fertilised patch of sub-Antarctic water were made during SAGE (SOLAS SAGE: Surface-Ocean Lower Atmosphere Studies Air–Sea Gas Experiment). The iron addition induced a minor phytoplankton bloom, however the patch dynamics were dominated by physical processes which suppressed and masked the biological effects. The Lagrangian nature of the experiment allowed the carbonate chemistry in the patch to be followed for 15.5 days, and the relative importance of the biological and physical factors influencing the surface water pCO2 was estimated. The pCO2 of the surface waters of the patch increased from 327 μatm prior to iron addition to 338 μatm on Day 14, effects of vertical and horizontal mixing offset the 15 μatm drawdown that would have occurred had the induced biological uptake been the sole factor to influence the pCO2. The air–sea carbon flux calculated using the measured skin temperature and a piston velocity parameterisation determined during SAGE (Ho et al., 2006) was 98.5% of the flux determined using conventional bulk temperature measurement and the Wanninkhof (1992) piston velocity parameterisation. The skin temperature alone contributed to an 8% increase in the flux compared with that determined using bulk temperature.