In oxic waters, dissolved Fe exists dominantly as Fe(III) complexes with strongly coordinating, siderophore-like ligands. In this study, we have determined an equilibrium Fe isotope fractionation of 0.6% (∆56Fe) between inorganic Fe(III) and Fe(III) siderophore complexes using the siderophore desferrioxamine B as a model compound. The 57Fe tracer experiments show that the Fe isotopes ofthe siderophores exchange readily with dissolved inorganic Fe. The results indicate that organic ligands are likely to be important in the generation ofFe isotope signatures in oxic environments. For example, the isotopic composition ofmarine Fe-Mn nodules may largely be due to the presence of strongly coordinating ligands.

Ferrihydrite and goethite are amongst the most important substrates for the sorption of contaminants in soil and other environmental media. Isotopic studies of the transition elements, particularly those that exhibit more than one oxidation state and show pH- and/or redox-sensitive behaviour at low temperatures, have been shown to be potentially useful present-day and past proxies for redox (or palaeoredox) conditions. We have made preliminary investigations of Fe isotope fractionation that take place during the formation of FeIII (oxy)hydroxides (FeIII ox) from an aqueous FeIII(NO3)3 solution (FeIII aq) under laboratory conditions. We have attempted to keep the chemical system simple by excluding 'vital effects' and major changes in redox through the maintenance of abiotic conditions and use of FeIII aq. Isotopic measurements (56Fe/54Fe, 57Fe/54Fe) of the FeIII(NO3)3 stock solution, the original ferrihydrite and the mixed ferrihydrite/goethite-supernatant FeIII aq 'pairs' were carried out using a double focusing multicollector inductively coupled plasma mass spectrometer. The results reveal an apparent systematic variation indicating larger ΔFeIII aq—FeIII ox with decrease in the ferrihydrite:goethite ratio, which reflects the time allowed for isotopic exchange. These values range from virtually zero (0.03%) after 24 h to 0.30% after 70 h. In each FeIII ox-FeIII aq 'pair' the lighter Fe isotope is partitioned into the FeIII ox, leaving the FeIII aq isotopically heavier. The observed fractionation reflects isotopic exchange of Fe between the FeIII ox and FeIII aq upon at least a two step transition of ferrihydrite to goethite.