Bioturbation and the delta Fe-56 signature of dissolved iron fluxes from marine sediments

We developed a reaction-transport model capable of tracing iron isotopes in marine sediments to quantify the influence of bioturbation on the isotopic signature of the benthic dissolved (DFe) flux. By fitting the model to published data from marine sediments, we calibrated effective overall fractionation factors for iron reduction (-1.3 parts per thousand), oxidation (+0.4 parts per thousand), iron-sulfide precipitation (+0.5 parts per thousand) and dissolution (-0.5 parts per thousand) and pyrite precipitation (-0.7 parts per thousand) that agree with literature values. Results show that for bottom-water oxygen concentrations greater than 50 mu M, higher bioturbation increased the benthic DFe flux and its delta Fe-56 signature. By contrast, for oxygen concentrations less than 50 mu M, higher bioturbation decreased the benthic DFe flux and its delta Fe-56 signature. The expressed overall fractionation of the benthic DFe flux relative to the delta Fe-56 of the iron oxides entering the sediment ranges from -1.67 parts per thousand to 0.0 parts per thousand. On a global scale, the presence of bioturbation increases sedimentary DFe release from approximately 70 G mol DFe yr(-1) to approximately 160 G mol DFe yr(-1) and decreases the delta Fe-56 signature of the DFe flux.