Significance Phytoplankton assimilate carbon dioxide, produce oxygen, and nourish food webs on a scale that impacts planetary processes but are limited by iron deficiency over much of the global surface ocean. Therefore, we must understand processes that regulate the ocean’s iron inventory to accurately simulate and predict the ocean’s response to change. This study reveals that the widespread production of nanosized iron colloids from the weathering of lithogenic material drives sedimentary iron supply throughout the deep ocean. The discovery accounts for the unexplained occurrence of colloids and patterns of iron isotope variation previously observed in other parts of the deep ocean and suggests how long-standing assumptions we have used to simulate iron supply in ocean models must be revised.