Bioavailable iron in the Southern Ocean: the significance of the iceberg conveyor belt

Iron is believed to limit phytoplankton productivity in the Southern Ocean [1-3], where insufficient Fe is supplied by upwelling from deep waters to surface waters to allow phytoplankton to utilise all the nitrate. Hence any additional Fe sources to the surface waters of the Southern Ocean have the potential to stimulate extra primary productivity [1,3,4]. Most research to date has focused on the addition of Fe to surface waters by dissolution from aeolian dust [2-5]. Other important but more localised sources are derived from melting sea ice [6,7] and the resuspension of shelf sediments [6,8]. However recent work indicates that Fe derived from iceberg-hosted sediment may be an important, and hitherto unrecognised, source of bioavailable Fe [9,10]. These findings are significant because the input of icebergs to the Southern Ocean is increasing [11] and any accompanying productivity increase has the potential to enhance carbon export and provide a new negative feedback loop to mitigate fossil fuel emissions (but see [3,12]).Potential bioavailable inputs from glacial sediment have been overlooked, primarily because Fe in these sediments was assumed to be too inert for plankton to utilise. Two recent studies present a contrasting view. Firstly, high resolution microscopy [9] has shown that sediments (mainly proglacial and supraglacial) associated with glaciers (in Antarctica and elsewhere) contain nanoparticulate iron oxyhydroxides (~5–10 nm diameter). Only a small fraction of the iron oxyhydroxide pool is present as nanoparticulates, but this fraction is biogeochemically reactive and potentially bioavailable. This study [9] only reported on one sample of glacial sediment enclosed in ice and did not attempt to quantify the nanoparticulate Fe contents (although measurements of Fe present in the whole oxyhydroxide pool indicated that <<1% of this pool need be nanoparticulate and bioavailable to fertilise a significant increase in productivity). Further studies of sedimen