Direct and indirect Contributions of Ice Shelves to Micronutrient Supply to the Surface Waters around Antarctica

Satellite estimates of chlorophyll in the coastal polynyas over the Antarctic continental shelf are strongly correlated with the basal melt rate of adjacent ice shelves. This has led to speculation that the high productivity of coastal polynyas may be related to the release of the limiting micronutrient iron from melting ice shelves. A 5-km resolution ocean/sea ice/ice shelf model of the Southern Ocean is used to examine mechanisms that supply iron to Antarctic continental shelf surface waters. Four sources of dissolved iron are simulated with independent tracers, assumptions about the end member concentrations, and an idealized summer biological uptake. Direct injection of iron from melting ice shelves is important to the total dissolved iron supply to surface waters, providing about 6%. However, the contribution from deep sources of iron on the shelf is much larger at 71%. The relative contribution of dissolved iron supply from basal melt driven overturning circulation within ice shelf cavities is heterogeneous around Antarctica, but at some locations, such as the Amundsen Sea, it is the primary mechanism for transporting deep dissolved iron to the surface. Correlations between satellite-derived estimates of surface chlorophyll a concentration in coastal polynyas around Antarctica and simulated estimates of dissolved iron show that the productivity of the coastal polynyas is linked to the basal melt of adjacent ice shelves. However, rather than being directly influenced by the release of iron from melting ice shelves, the larger impact results from circulation changes driven by ice shelf melt that allow other, deeper, sources of iron to be advected or mixed into the upper waters over the Antarctic continental shelf. These results indicate the potential vulnerability of coastal Antarctic ecosystems to changes in ice shelf basal melt rates.