We present new constraints on gas-phase C, N, and O abundances in the molecular layer of the IM Lup protoplanetary disk. Building on previous physical and chemical modeling of this disk, we use new ALMA observations of C2H to constrain the C/O ratio in the molecular layer to be ∼0.8, i.e., higher than the solar value of ∼0.54. We use archival ALMA observations of HCN and H13CN to show that no depletion of N is required (assuming an interstellar abundance of 7.5 × 10‑5 per H). These results suggest that an appreciable fraction of O is sequestered in water ice in large grains settled to the disk midplane. Similarly, a fraction of the available C is locked up in less volatile molecules. By contrast, N remains largely unprocessed, likely as N2. This pattern of depletion suggests the presence of true abundance variations in this disk, and not a simple overall depletion of gas mass. If these results hold more generally, then combined CO, C2H, and HCN observations of disks may provide a promising path for constraining gas-phase C/O and N/O during planet-formation. Together, these tracers offer the opportunity to link the volatile compositions of disks to the atmospheres of planets formed from them.
