Iron (Fe) is an essential micronutrient for marine productivity. In recent decades, our understanding of oceanic Fe supply has broadened from dust-dominated sources to include shelf sediments, hydrothermal vents, sea ice, rivers, and mesoscale eddies. Yet, quantitative constraints on coastal dissolved Fe (dFe) fluxes, speciation dynamics, and coupled biogeochemical processes remain unclear. Coastal zones are poorly represented in the only quality-controlled global database for dFe in seawater (the GEOTRACES Intermediate Data Product), yet extensive dFe data is available from regional studies for many shelf regions worldwide. Here, we compile a new quality-controlled global dataset comprising ~9,000 dFe measurements from ca. 250 published sources and individual contributions. We apply a hybrid mechanistic approach of Fe solubility modelling and particle tracking experiments, to explore how coastal dFe distributions relate to salinity and Fe chemistry globally. Salinity emerges as the dominant control, with consistently negative correlations that vary regionally from conservative mixing in river plumes (r²= 0.65) to significant decoupling in upwelling systems (r²= 0.18). By combining data synthesis with a statistical analysis of the factors that control dFe in coastal zones, and new model analysis incorporating coastal dFe data in ECCO-Darwin, we aim to provide a comprehensive assessment of coastal Fe cycling. This is critical to understand the influence of the shelf on Fe supply to the global ocean.

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