Stable Iron Isotopes Constrain the Sedimentary Input of Dissolved Iron to the Ocean

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 ${\text{yr}}^{-1}$, 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.