Physical speciation of iron in the Atlantic sector of the Southern Ocean along a transect from the subtropical domain to the Weddell Sea Gyre.

Distributions of total dissolvable iron (TDFe; unfiltered), dissolved iron (DFe; 0.2 μm filtered), and soluble iron (SFe; 0.02 μm filtered) were investigated during the BONUS‐GoodHope cruise in the Atlantic sector of the Southern Ocean (34°S/17°E-57°S/0°, February-March 2008). In the mixed layer, mean values of 0.43 ± 0.28 and 0.22 ± 0.18 nmol L−1 were measured for TDFe and DFe, respectively. In deeper waters, TDFe and DFe concentrations were 1.07 ± 0.68 and 0.52 ± 0.30 nmol L−1, respectively. DFe concentrations decreased from the north (subtropical waters) to the south (Weddell Sea Gyre). In the subtropical domain, dusts coming from Patagonia and southern Africa and inputs from the African continental margin may explain high DFe and TDFe concentrations in surface and intermediate waters. Results from numerical models gave support to these hypotheses. In the Antarctic Circumpolar Current domain, estimation of the median advective time of water masses suggests that sediment inputs from the Antarctic Peninsula, South America margin, and/or South Georgia Islands could be an important source of Fe. Except in the subtropical domain where 0.4-0.6 nmol L−1 of SFe were observed in the upper 1500 m, all stations exhibited values close to 0.1-0.2 nmol L−1 in surface and 0.3-0.5 nmol L−1 in deeper waters. For all stations, colloidal Fe (CFe) was a minor fraction of DFe in surface waters and increased with depth. Colloidal aggregation, sinking of CFe, and assimilation of SFe, followed by rapid exchange between the two fractions, are suspected to occur.