| Summary: | Iron limits primary productivity in more than 30% of the oceans, including the Southern Ocean. Antarctica is characterized by seasonal ice, which represents a significant source of Fe during its melting. Observed Fe concentrations in the sea ice can be up to 2 orders of magnitude higher in the sea ice and in the open ocean seawater. Biological activity varies geographically, seasonally and within the sea ice (surface, internal and bottom layers), and can be dominated by either autotrophic or heterotrophic activity. We hypothesized that these biological activities can play a crucial role in the cycling of Fe and can induce contrasted redox conditions within the brine pockets and channels, with impacts on both particulate and dissolved iron isotopic composition.Dissolved and particulate Fe isotopic compositions of brines, seawater and sea ice have been determined on samples collected during ARISE (Sept-Nov 2003, 64-65°S/112-119°E), ISPOL (Nov 2004-Jan 2005, 67-68°S/54-55°W) and SIMBA (Oct 2007, 69-71°S, 90-95°E) cruises. Values of particulate d56Fe up to +1.2 ‰ were observed in the bottom ice, where large diatoms dominate the microbial community (corresponding to ?56Fep-d = +1.6‰). Lowest particulate d56Fe down to -3.4 ‰ were obtained in brines where bacteria were dominant (corresponding to ?56Fep-d = -2.1‰). These results appear to reflect the autotrophic and heterotrophic activities of the microbial communities and suggest that those activities play a pivotal role in the cycling of Fe in the sea ice.
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