Abstract. Fe isotope ratios and concentrations of dissolved Fe (Fe_dis, < 0.45 μm) and of suspended particulate Fe (Fe_SPM) were analyzed from a depth profile through the anoxic Eastern Gotland Basin water column, Baltic Sea. Results show a sharp gradient in δ⁵⁶Fe_dis across the ferruginous layer with δ⁵⁶Fe_dis = −0.4‰ in the euxinic deep basin and δ⁵⁶Fe_dis = +0.3‰ in the oxic upper water column. The isotopic gradient overlaps with a strong concentration gradient of Fe_dis, a concentration maximum in Fe_SPM and lower δ⁵⁶Fe_SPM values than δ⁵⁶Fe_dis. These features indicate preferential loss of light Fe isotopes from solution to suspended iron-oxyhydroxides (Fe_IOH) during typical oxidative precipitation across the redox interface. The sign of the overall fractionation, Δ⁵⁶Fe_IOH-Fe(II)(aq) < 0‰, is in contrast to similar, mostly non-marine redox environments, where Δ⁵⁶Fe_IOH-Fe(II)(aq) > 0‰. The difference appears to be the result of isotope exchange dominated by reaction kinetics in the marine water column, rather than equilibrium fractionation generally inferred for oxidative Fe precipitation elsewhere. High residual δ⁵⁶Fe_dis immediately above the oxic–ferruginous interface and throughout the oxic water column suggests that any potential dissolved Fe export from marine reducing waters into the oxic open water column is enriched in the heavy isotopes. In the deep, mildly euxinic water column above the level of Fe sulfide saturation, a decreasing δ⁵⁶Fe_SPM trend with depth and a generally low δ⁵⁶Fe_dis are comparable to trends generally observed in marine anoxic sediment profiles where microbial reductive Fe dissolution occurs. The isotope composition of the redox-cycled Fe inventory in anoxic marine basins mainly reflects the balance between external fluxes, driving the composition towards crustal δ⁵⁶Fe values, and intensity of internal recycling, driving δ⁵⁶Fe towards negative values.