Abstract Chemical dissolution investigations of baseline aerosols have demonstrated that much of the 55Fe associated with aerosols input to the oceans is present as either an amorphous iron oxide or as extremely small particulate species attached to the surfaces of the large aerosol particles. Nearly all of the stable iron is bound in the mineral phase of these aerosol particles. This difference in the chemical and physical forms of the radioactive and stable iron isotopes causes the 55Fe to be more biologically available to marine organisms than is the stable iron. This differential biological uptake of the radioactive element and its stable element counterpart emphasizes that natural levels of elements in the marine environment may not be useful in predicting the effects of anthropogenic inputs. The effectiveness of dilution by natural sources is dependent upon the chemical and physical forms of materials in both the source terms and the receiving environments.