Deep carbon cycling drives the splitting of the 520-km mantle discontinuity

Carbon cycling into Earth’s deep interior regulates melting, redox state, and volatile transport, yet its imprint on mantle seismic structure remains unclear. A potential seismic expressure of deep carbon cycling is the sporadic splitting of the 520-km discontinuity, expressed as a secondary reflector near 560 km beneath slabs and mantle upwellings, has been attributed to davemaoite (CaSiO3) exsolution, although its shear-wave properties cannot produce the required 2–4% impedance contrast. We conduct high pressure-temperature experiments (~20 GPa, 1200–1600 °C) to determine how carbonate-altered oceanic crust reorganizes mantle layering. Here we show that carbonate addition drives Ca-Mg exchange with silicates, increases Ca incorporation into silicates under Fe-rich conditions, and enhances davemaoite exsolution. This process yields 12–33(2) vol.% davemaoite near ~ 560 km, sufficient to match impedance contrasts observed. Davemaoite-rich crust could be recycled by mantle plumes, accounting for the 560-km reflector beneath hot regions. Subducted carbonates thus reshape mineral equilibria, reinforce chemical stratification, and generate persistent seismic signatures of deep carbon cycling. This study shows that carbon recycled into mantle promotes davemaoite formation at the mantle transition zone, providing a mineralogical explanation for the 560-km seismic reflector and revealing how deep carbon cycling shapes Earth’s interior.