jats:titleAbstract</jats:title> jats:pPhosphorus (P) availability exerts a strong influence on primary productivity in global oceans. However, its abundance and role as a limiting nutrient prior to the start of the Great Oxygenation Event (GOE) 2.45–2.32 Ga is unclear. Low concentrations of seawater P have been proposed to explain the apparent delay between the early appearance of oxygen-producing Cyanobacteria and the onset of atmospheric oxygenation. We report evidence for seawater precipitation of Ca-phosphate nanoparticles in 2.46–2.40 Ga iron formations deposited on a marine shelf, including shallow-water facies, immediately prior to the onset of the GOE. Our modeling shows that the co-precipitation of Ca-phosphate and ferrous silicate (greenalite) required ferruginous seawater with dissolved P concentrations many orders of magnitude higher than in today’s photic zone. If correct, it follows that P availability is unlikely to have suppressed the expansion of Cyanobacteria prior to the GOE. A reservoir of P-rich surface water shortly before 2.40 Ga could ultimately have triggered a rapid rise in atmospheric oxygen by fueling a sharp increase in primary productivity and organic-carbon burial. We speculate that the enigmatic Lomagundi positive carbon-isotope excursion, recorded in 2.32–2.06 Ga shallow-water carbonates, may mark a key step in the transition toward a modern biosphere of high biological productivity controlled by nutrient availability.</jats:p>