Southern Ocean iron enrichment promotes inorganic carbon drawdown

The effect of iron supply on phytoplankton growth and the marine carbon cycle was tested in situ at 61 degreesS 141 degreesE in the Southern Ocean Iron Release Experiment (SOIREE). On 9 February 1999 iron and the tracer sulphur hexafluoride (SF6) were added to the mixed layer with additional iron infusions after 3, 5 and 7 days. A small decrease of the fugacity of carbon dioxide (fCO(2)) and dissolved inorganic carbon (DIC) by iron-induced algal growth was observed 4-5 days after the first infusion. From then onwards fCO(2) and DIC steadily decreased, and the iron-enriched waters became a sink for atmospheric CO2. The region with surface-water fCO(2) drawdown closely matched the shape of the patch, as indicated by SF6. Surface-water fCO(2) and DIC drawdown were relatively constant across the patch, whereas SF6 decreased from the patch centre outwards. This pointed to uniform algal carbon uptake, not limited by iron, in the patch. After 13 days surface-water fCO(2) and DIC in the patch centre had decreased by 32-38 mu atm and 15-18 mu mol kg(-1), respectively. Surface-water fCO(2) outside the patch had increased by 8 mu atm, partly as a result of surface-water warming. The iron-induced fCO(2) change exceeded seasonal fCO(2) variability in this region by a factor of two. From the surface-water fCO(2) distribution we estimate a net DIC drawdown of 1353 t of carbon (+/- 14%) (1 t = 10(6) g) across the patch after 12 days, assuming uniform drawdown in the upper 50 m. Correction for vertical diffusion and air-sea exchange results in a gross DIC drawdown of 1408 t of carbon. The decrease of fCO(2) and DIG, integrated over the mixed layer, was remarkably similar in size after 13 days of SOIREE as changes observed after 6-9 days during IronEx II, if we consider the 4-5 days lag in algal carbon uptake at the Southern Ocean site. SOIREE has demonstrated in situ the occurrence of algal iron limitation and of iron-induced carbon uptake in these Southern Ocean waters. The subsequent fate of the fixed inorganic carbon can only be speculated upon. (C) 2001 Elsevier Science Ltd. All rights reserved.