Ratios of C¹⁸O abundance obtained with a depleted level of carbon and with an ISM-like C abundance, rescaled by the level of carbon depletion. Where the ratio is higher than unity (colors from light blue to red), the C¹⁸O abundance changes more slowly than the elemental carbon abundance. Upper panels show a 10^-5 M_⊙ T Tauri disk model, bottom panels show a 10^-1 M_⊙ T Tauri disk model. Other disk parameters are: R_c = 60 au, γ = 0.8, h_c = 0.1, and ψ = 0.1. The white solid line describes the A_V = 1 surface, where photodissociating photons are mainly absorbed by the dust.