The role of iron sources and transport for Southern Ocean productivity

Iron has been found to limit primary productivity in high nutrient, low chlorophyll regions of the oceans, including the Southern Ocean. Here we assess the relative magnitudes and geographical distributions of the sources of iron (sedimentary, atmospheric, icebergs and sea ice) to the Southern Ocean, and their impact on productivity. We present an iron cycling model, based on the assumptions of iron and light limitation of primary production, which is embedded in an eddy resolving ocean general circulation model. We find that the injection depth of the various iron inputs determines their availability for driving production because dissolved iron may be scavenged prior to it entering the illuminated mixed layer where it can drive primary production. The model suggests that production is predominantly regulated by sediment-derived iron sources rather than icebergs, sea ice or atmospheric dust. We note non-linear response in productivity to changes in the strength of one or more iron sources due to scavenging. Sea ice influences productivity by modifying the timing of iron supply to the euphotic zone. We also show that in the Scotia Sea the majority of productivity is driven by sediment-sourced iron from the Antarctic Peninsula, with additional local hotspots driven by island sources.