The late Cambrian Steptoean positive carbon isotope excursion (SPICE) coincided with the end-Marjuman trilobite extinction and an episode of global ocean euxinia. However, the mechanism that sustained euxinia for similar to 2-3 m.y. remains unclear. To address this, a novel combination of carbonate-associated phosphorus (CAP) and iodine [I/(Ca + Mg)] ratios was analyzed at four sites representing a range of water depths across the central Missouri intrashelf basin. Our data demonstrate that CAP rose during the SPICE, in parallel with elevated delta 13C values and expanded euxinia, indicating increased phosphate availability in the surface ocean. In contrast, I/(Ca + Mg) ratios remained low, demonstrating persistent oxygen-poor conditions in shallow seas. These characteristics can be linked to enhanced phosphorus release and recycling in the ocean due to expanded seafloor euxinia. Based on a coupled oceanic carbon-phosphorus model, we infer that elevated phosphorus levels boosted primary productivity in the surface ocean, leading to increased oxygen generation and enhanced particulate export, sustaining oceanic euxinia until the feedback was broken by rising atmospheric O2 levels. Our study highlights phosphorus recycling as a key driver of late Cambrian ocean redox instability.