Chemical controls on iron distributions across the subsurface South Pacific Ocean

Iron and nitrogen are the primary nutrients that limit productivity in the ocean. While nitrogen cycling is largely controlled by biology, iron cycling is strongly determined by chemistry because iron losses are driven by abiotic formation of authigenic iron hydroxides (authFeOH). Here, we apply a mechanistic approach to examine how organic matter across the dissolved-particulate size spectrum controls authFeOH formation in subsurface waters (>250 m) of the South Pacific Ocean. We find that accounting for the chemical heterogeneity of organic matter is essential for predicting widespread authFeOH formation. Predicted dissolved and particulate iron concentrations matched observations in the ocean interior, while discrepancies were linked to kinetic control of authFeOH formation or inputs of particles from the seafloor. Our results highlight the need to represent complexity in abiotic interactions to better resolve the interplay of chemical and biological controls on ocean iron cycling.