In oxic oceans, most of the dissolved iron (Fe) exists as complexes with siderophore-like, strongly coordinating organic ligands. Thus, the

isotope composition of the little amount of free inorganic Fe that is available for precipitation and preservation in the geological record may

largely be controlled by isotope fractionation between the free and complexed iron.We have determined the equilibrium Fe isotope fractionation

induced by organic ligand activity in experiments with solutions having co-existing inorganic Fe(III) species and siderophore complexes, Fedesferrioxamine

B (at pH 2). The two differently complexed Fe(III) pools were separated by addition of Na2CO3, which led to immediate

precipitation of the inorganic Fe without causing significant dissociation of Fe-desferrioxamine complexes. Experiments using enriched 57Fe

tracer showed that isotopic equilibration between the 57Fe-labelled inorganic species and the isotopically “normal” siderophore-bound Fe was

rapid during the first few seconds and then became slower. Consequently, the data fitted poorly to first and second order reaction equations.

However, with a two-stage reaction, the data fit perfectly with a first order equation for the slower stage, indicating that approximately 40% reequilibration

may take place during the separation of the two pools. To further test if the induced precipitation leads to experimental artefacts, the

fractionation during precipitation of inorganic Fe was determined. Assuming a Rayleigh-type fractionation during precipitation, this experiment

yielded an isotope fractionation factor of a56Fesolution-solid=1.00027. Calculations based on these results indicate that isotopic re-equilibration is

unlikely to significantly affect our determined equilibrium Fe isotope fractionation between inorganically and organically complexed Fe. To

determine the equilibrium Fe isotope fractionation between inorganically and organically bound Fe(III), experiments with variable proportions

of inorganic Fe were carried out at 25 °C. Irrespective of the proportion of inorganic Fe, equilibrium fractionation factors were within

experimental uncertainty, yielding an average fractionation factor, ¿56FeDFOB-inorg of 0.60±0.15‰. The results indicate that equilibrium Fe

isotope fractionation induced by strongly coordinating organic ligands may fractionate Fe isotopes in nature. The fractionation is likely to be

important in oxic, Fe(III)-bearing environments, such as soils and rivers, and may, for example, largely control the Fe isotope composition of

marine Fe–Mn crusts.