Evolution and radial distribution of the midplane C/O in the gas and solids in one of the disk models used for this work. Specifically, we show the chemistry of the disk in the reset scenario of refractory erosion, which governs the high C/O early on in the disk’s life (A). We similarly note the region of the disk with high and low water vapour abundances (B and D, respectively), the carbon-poor region due to refractory erosion (E), and a region where gaseous CO is converted to frozen CO₂ (C). The more carbon-rich region (F) exists at larger radii than the carbon erosion front. Orange denotes carbon-rich regions (C/O > 1) while blue denotes carbon-poor regions (C/O < 1). The nominalvolatile C/O = 0.4 and the refractory C/O = 2.