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 its δ 56 Fe signature. By contrast, for oxygen concentrations less than 50 µM, higher bioturbation decreased the benthic DFe flux and its δ 56 Fe signature. The expressed overall fractionation of the benthic DFe flux relative to the δ 56 Fe 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 −1 to approximately 160 G mol DFe yr −1 and decreases the δ 56 Fe signature of the DFe flux.

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“…In May, we observed small polychaetes at the sediment surface, consistent with the previous observations of sediment recolonization by juvenile macrofauna in late spring [ 9 ]. We hence contend that macrofaunal activity could induce the down-mixing of FeO x through bioturbation [ 43 , 44 ]. A fraction of FeO x in the top layer will be mixed down which will be reduced in deeper layers, thus accelerating the conversion of FeO x into FeS ( figure 7 ) and hence partially weakening the strength of the iron firewall.…”