The effect of iron supply on phytoplankton growth and the marine carbon cyc
le was tested in situ at 61 degreesS 141 degreesE in the Southern Ocean Iro
n Release Experiment (SOIREE). On 9 February 1999 iron and the tracer sulph
ur hexafluoride (SF6) were added to the mixed layer with additional iron in
fusions after 3, 5 and 7 days. A small decrease of the fugacity of carbon d
ioxide (fCO(2)) and dissolved inorganic carbon (DIC) by iron-induced algal
growth was observed 4-5 days after the first infusion. From then onwards fC
O(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 dec
reased 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 increa
sed by 8 mu atm, partly as a result of surface-water warming. The iron-indu
ced 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 patc
h after 12 days, assuming uniform drawdown in the upper 50 m. Correction fo
r vertical diffusion and air-sea exchange results in a gross DIC drawdown o
f 1408 t of carbon. The decrease of fCO(2) and DIG, integrated over the mix
ed 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 ino
rganic carbon can only be speculated upon. (C) 2001 Elsevier Science Ltd. A
ll rights reserved.