Abstract Iron is a key micronutrient for marine biota and potentially one of the main drivers of ocean feedback to changing climate. There is however no consensus on the relative role of different external iron sources to the ocean, hampering our ability to predict how the oceanic iron cycle and biological carbon pump will react to climate change. For the last two decades, stable iron isotopes have been increasingly used in field studies to track contributions of different iron sources and modeling studies started to help interpreting isotope observations. However, measured isotopic compositions of iron sources can vary substantially, and the isotopic signatures of different sources can overlap, leading to high uncertainty in constraining the magnitude of the sources. This study aims to examine the sensitivity of seawater iron isotopes to the uncertainty in the sedimentary source. An existing box model of the marine carbon cycle is extended with a description of the cycle of iron and its stable isotopes. Experiments have been done with variable isotopic end‐member signature and strength of the sedimentary source, and fractionation through biological uptake and binding to organic ligands. The model results reveal the necessity to consider spatially distinct isotopic end‐member signatures for the sedimentary source and fractionations so as to reproduce observed spatial gradients of seawater iron isotopic composition. By assuming a sedimentary input of 7.5–15 Gmol Fe , the model is able to reproduce observed concentrations of dissolved iron and its isotopes in large ocean regions, providing useful constraints for complex global biogeochemistry models. Plain Language Summary Iron is essential for marine life and plays a key role in how oceans respond to climate change. However, scientists are still unsure about the relative importance of different sources of iron to the ocean, making it difficult to predict how the iron supply and biological activity in the ocean to store carbon will change with climate. Over the past 20 years, researchers have used iron isotopes to track these sources, but overlapping and variable isotope signatures create uncertainty. This study used a box model to explore how uncertainties in iron and iron isotopes from sediments affect seawater iron isotopes, and to make a first‐order estimate of the global sedimentary input of iron. The simple box model matches observed iron levels and isotope compositions in the main ocean regions with a sediment input of iron of 7.5–15 Gmol per year, offering insights for more complex global models. Key Points An ocean box model has been developed to describe the marine cycle of iron and its stable isotopes Model‐data comparisons reveal the necessity to consider spatially distinct isotopic signals and fractionation processes in seawater The global sedimentary source is constrained between 7.5−15 Gmol Fe which will be further explored in full ocean biogeochemistry models