Bioturbation and the δ56Fe signature of dissolved iron fluxes from marine sediments

Abstract

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‰), oxidation (+0.4‰), iron-sulfide precipitation (+0.5‰) and dissolution (−0.5‰) and pyrite precipitation (−0.7‰) that agree with literature values. Results show that for bottom-water oxygen concentrations greater than 50 µM, higher bioturbation increased the benthic DFe flux and itsδ56Fe signature. By contrast, for oxygen concentrations less than 50 µM, higher bioturbation decreased the benthic DFe flux and itsδ56Fe signature. The expressed overall fractionation of the benthic DFe flux relative to theδ56Fe of the iron oxides entering the sediment ranges from −1.67‰ to 0.0‰. On a global scale, the presence of bioturbation increases sedimentary DFe release from approximately 70 G mol DFe yr−1to approximately 160 G mol DFe yr−1and decreases theδ56Fe signature of the DFe flux.