In this Letter we report the CO abundance relative to H2 derived toward the circumstellar disk of the T-Tauri star TW Hya from the HD (1-0) and C18O (2-1) emission lines. The HD (1-0) line was observed by the Herschel Space Observatory Photodetector Array Camera and Spectrometer whereas C18O (2-1) observations were carried out with the Submillimeter Array at a spatial resolution of 2.8 x 1.9 (corresponding to 142 x 97 AU). In the disks warm molecular layer (T>20 K) we measure a disk-averaged gas-phase CO abundance relative to H2 of $chi{rm(CO)}=(0.1-3)x10^{-5}$, substantially lower than the canonical value of $chi{rm(CO)}=10^{-4}$. We infer that the best explanation of this low $chi$(CO) is the chemical destruction of CO followed by rapid formation of carbon chains, or perhaps CO2, that can subsequently freeze-out, resulting in the bulk mass of carbon locked up in ice grain mantles and oxygen in water. As a consequence of this likely time-dependent carbon sink mechanism, CO may be an unreliable tracer of H2 gas mass.
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