The Arctic has the highest warming rates world-wide. Glaciated fjord ecosystems, which are known hotspots of carbon cycling and burial, are predicted to be extremely sensitive to this warming. Glaciers are important sources of iron, an essential nutrient for phytoplankton, to high-latitude marine ecosystems. However, up to 95% of the glacially-sourced iron settles in sediments close to the glacial source. We found that only 0.6-12% of the total glacially-sourced iron is potentially bioavailable. Our results also show that biogeochemical cycling in fjord sediments converts the unreactive glacial iron into more reactive and bioavailable phases, leading to an up to 9-fold increase in the amount of potentially bioavailable iron. Arctic fjord sediments therefore likely are an important source of bioavailable iron. However, once glaciers retreat onto land, the flux of iron from sediments into the water column is reduced, such that glacial retreat could exacerbate iron limitation in polar oceans.