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.