Iron is a key micronutrient controlling phytoplankton growth in vast regions of the global ocean. Despite its importance, uncertainties remain high regarding external iron source fluxes and internal cycling on a global scale. In this study, we used a global dissolved iron dataset, including GEOTRACES measurements, to constrain source and scavenging fluxes in the marine iron component of a global ocean biogeochemical model. Our model simulations tested three key uncertainties: source inputs of atmospheric soluble iron deposition (varying from 1.4–3.4 Gmol/yr), reductive sedimentary iron release (14–117 Gmol/yr), and compared a variable ligand parameterization to a constant distribution. In each simulation, scavenging rates were tuned to reproduce the observed global mean iron inventory for consistency. The variable ligand parameterization improved the global model-data misfit the most, suggesting that heterotrophic bacteria are an important source of ligands to the ocean. Model simulations containing high source fluxes of atmospheric soluble iron deposition (3.4 Gmol/yr) and reductive sedimentary iron release (114 Gmol/yr) further improved the model most notably in the surface ocean. High scavenging rates were then required to maintain the iron inventory resulting in relatively short surface and global ocean residence times of 0.83 and 7.5 years, respectively. The model simulates a tight spatial coupling between source inputs and scavenging rates, which may be too strong due to underrepresented ligands near source inputs, contributing to large uncertainties when constraining individual fluxes with dissolved iron concentrations. Model biases remain high and are discussed to help improve global marine iron cycle models.

Iron is a key limiting nutrient for phytoplankton. Continental shelf and slope sediments are important sources of dissolved iron (DFe). Stable iron isotopes (d56Fe) are a particularly useful tool to quantify the DFe sources and sinks in the ocean. The isotopic signature of the sedimentary DFe source is controlled by environmental factors such as bottom water redox conditions, carbon oxidation and bioturbation by burrowing fauna, but the exact relation on a global scale is poorly understood. We developed a reaction-transport model capable of tracing dissolved iron isotope fractionation in marine sediments to quantify the isotopic signature of benthic DFe fluxes under a wide range of environmental conditions. We derived fractionation factors for iron reduction (-1.3 permille), iron oxidation (+0.4 permille), iron sulphide precipitation (+0.5 permille and dissolution (-0.5 permille and pyrite precipitation (-0.7 permille) that were in line with existing literature. At bottom-water oxygen concentrations >50 µM, bioturbation increased the benthic DFe flux and increased the d56Fe signature. In contrast, at bottom-water oxygen concentrations <50 µM, a reduction in bioturbation led to a decrease in the benthic DFe flux and its d56Fe value. On a global scale, a model simulation without bioturbation decreased the sedimentary DFe release from ~158 Gmol DFe yr-1 to ~70 Gmol DFe yr-1, and decreased the variability in the d56Fe signature of the DFe flux. Finally, we find that a decrease in ocean oxygen content by 40 µM can increase global sedimentary DFe release by up to 103 Gmol DFe yr-1.