Iron availability regulates marine productivity, nitrogen fixation and carbon export, yet the deep-sea sediment–water exchanges and internal cycling processes that sustain and redistribute dissolved Fe remain difficult to quantify. Here we combine machine-learning reconstructions of global dissolved Fe and δ⁵⁶Fe with source partitioning and circulation diagnostics to evaluate how sediment-derived inputs, hydrothermal supply, atmospheric deposition and particle-reactive internal cycling shape the marine Fe cycle. We show that Fe concentrations and isotopes encode distinct facets of the marine iron cycle. Dissolved Fe is spatially heterogeneous and strongly overprinted by local inputs, scavenging and regeneration, whereas δ⁵⁶Fe follows basin-scale water-mass structure. Empirical endmember analysis indicates that apparent δ⁵⁶Fe source signatures vary among ocean basins. Among the resolved external inputs, reductive sedimentary Fe exceeds non-reductive sedimentary and hydrothermal Fe, with particularly large contributions in the Arctic and North Pacific, highlighting the importance of redox-sensitive sediment–water exchange. Along the North Atlantic Deep Water pathway, δ⁵⁶Fe evolution cannot be explained by heavy atmospheric inputs alone, but instead records source signatures transformed by mixing and internal Fe cycling. These results suggest that seawater δ⁵⁶Fe integrates benthic Fe supply, particle-reactive recycling and large-scale ocean circulation, providing a basin-scale tracer of sediment-influenced biogeochemical coupling in the ocean interior.