A seasonal carbon budget for the sub-Antarctic Ocean, South of Australia

Changes from winter (July) to summer (February) in mixed layer carbon tracers and nutrients measured in the sub-Antarctic zone (SAZ), south of Australia, were used to derive a seasonal carbon budget. The region showed a strong winter to summer decrease in dissolved inorganic carbon (DIC; similar to 45 mu mol/kg) and fugacity of carbon dioxide (fCO(2); similar to 25 mu atm), and an increase in stable carbon isotopic composition of DIC (delta C-13(DIC); similar to 0.5 parts per thousand), based on data collected between November 1997 and July 1999. The observed mixed layer changes are due to a combination of ocean mixing, air-sea exchange of CO2, and biological carbon production and export. After correction for mixing, we find that DIC decreases by up to 42 +/- 3 mu mol/kg from winter (July) to summer (February), with delta C-13(DIC) enriched by up to 0.45 +/- 0.05 parts per thousand for the same period. The enrichment of delta C-13(DIC) between winter and summer is due to the preferential uptake Of (CO2)-C-12 by marine phytoplankton during photosynthesis. Biological processes dominate the seasonal carbon budget (approximate to 80%), while air-sea exchange of CO2 (approximate to 10%) and mixing (approximate to 10%) have smaller effects. We found the seasonal amplitude of fCO(2) to be about half that of a study undertaken during 1991-1995 [Metzl, N., Tilbrook, B. and Poisson, A., 1999. The annual fCO(2) cycle and the air-sea CO2 flux in the sub-Antarctic Ocean. Tellus Series B-Chemical and Physical Meteorology, 51(4): 849-861.] for the same region, indicating that SAZ may undergo significant inter-annual variations in surface fCO(2). The seasonal DIC depletion implies a minimum biological carbon export of 3400 mmolC/m(2) from July to February. A comparison with nutrient changes indicates that organic carbon export occurs close to Redfield values (Delta P:Delta N:Delta C = 1:16:119). Extrapolating our estimates to the circumpolar sub-Antarctic Ocean implies a minimum organic carbon export of 0.65 GtC from the July to February period, about 5-7% of estimates of global export flux. Our estimate for biological carbon export is an order of magnitude greater than anthropogenic CO2 uptake in the same region and suggests that changes in biological export in the region may have large implications for future CO2 uptake by the ocean. (C) 2009 Elsevier B.V. All rights reserved.