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

Abstract

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; ∼ 45 µmol/kg) and fugacity of carbon dioxide ( fCO 2; ∼ 25 µatm), and an increase in stable carbon isotopic composition of DIC ( δ 13C DIC; ∼ 0.5‰), 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 CO 2, and biological carbon production and export. After correction for mixing, we find that DIC decreases by up to 42 ± 3 µmol/kg from winter (July) to summer (February), with δ 13C DIC enriched by up to 0.45 ± 0.05‰ for the same period. The enrichment of δ 13C DIC between winter and summer is due to the preferential uptake of 12CO 2 by marine phytoplankton during photosynthesis. Biological processes dominate the seasonal carbon budget (≈ 80%), while air–sea exchange of CO 2 (≈ 10%) and mixing (≈ 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 CO 2 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 mmol C/ m 2 from July to February. A comparison with nutrient changes indicates that organic carbon export occurs close to Redfield values (ΔP:ΔN:ΔC = 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 CO 2 uptake in the same region and suggests that changes in biological export in the region may have large implications for future CO 2 uptake by the ocean.