Mass-dependent fractionation of iron (Fe) isotopes is used to study past and present iron reactions and transport pathways in the ocean and in marine sediments. The overarching themes are 1) iron fluxes and stimulation of the carbon pump (“global warming and feedback mechanisms”) and 2) iron mineral formation/alteration and links to biological activity. In my talk I will introduce you to the methodological background, summarize major achievements of Fe isotope geochemistry, but will also speak about present challenges and limitations. I will present the results of some of our projects where we applied Fe isotope geochemistry in combination with sequential extractions in extreme environments: We studied for example the cycling of Fe in an Antarctic fjord and found that a large fraction of easily reducible Fe oxides in the fjord sediments derives from potentially anoxic subglacial meltwater. Further, we applied stable Fe isotopes to investigated if there are “fingerprints” of microbes reducing iron in deep and hot sediments of the Nankai Trough off Japan.