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To date, at least three comets -- 2I/Borisov, C/2016 R2 (PanSTARRS), andC/2009 P1 (Garradd) -- have been observed to have unusually high COconcentrations compared to water. We attempt to explain these observations bymodeling the effect of drifting solid (ice and dust) material on the icecompositions in protoplanetary disks. We find that, independent of the exactdisk model parameters, we always obtain a region of enhanced ice-phase CO/H2Othat spreads out in radius over time. The inner edge of this feature coincideswith the CO snowline. Almost every model achieves at least CO/H2O of unity, andone model reaches a CO/H2O ratio > 10. After running our simulations for 1 Myr,an average of 40% of the disk ice mass contains more CO than H2O ice. In lightof this, a population of CO ice enhanced planetesimals are likely to generallyform in the outer regions of disks, and we speculate that the aforementionedCO-rich comets may be more common, both in our own Solar System and inextrasolar systems, than previously expected.

Ice-Coated Pebble Drift as a Possible Explanation for Peculiar Cometary CO/H2O Ratios