Southern Ocean iron enrichment promotes inorganic carbon drawdown

Abstract

Abstract The effect of iron supply on phytoplankton growth and the marine carbon cycle was tested in situ at 61°S 141°E 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 (fCO2) and dissolved inorganic carbon (DIC) by iron-induced algal growth was observed 4–5 days after the first infusion. From then onwards fCO2 and DIC steadily decreased, and the iron-enriched waters became a sink for atmospheric CO2. The region with surface-water fCO2 drawdown closely matched the shape of the patch, as indicated by SF6. Surface-water fCO2 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 fCO2 and DIC in the patch centre had decreased by 32–38 μatm and 15–18 μmol kg−1, respectively. Surface-water fCO2 outside the patch had increased by 8 μatm, partly as a result of surface-water warming. The iron-induced fCO2 change exceeded seasonal fCO2 variability in this region by a factor of two. From the surface-water fCO2 distribution we estimate a net DIC drawdown of 1353 t of carbon (±14%) (1 t=106 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 fCO2 and DIC, 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.