John S. Carr, Joan R. Najita

Inner disk chemistry offers a valuable window onto disk evolution and planet formation. Key planet formation processes, including pebble drift, planetesimal and planet formation, dust traps, and snow lines, determine the delivery rates of oxygen and carbon to the inner disk. As a result, measurements of molecules sensitive to the gas-phase C/O ratio, in disks over a range of ages, can constrain the relative importance and time evolution of these processes. Here we report JWST/MIRI observations of T Tauri disks in the young cluster IC 348, extending to older ages previous studies of disks. Like younger disks, IC 348 sources show a broad range of molecular ratios, indicating diverse planet formation histories. However, the similar HCN/H$_2$O and C$_2$H$_2$/H$_2$O flux ratio distributions of IC 348 (2--5 Myr old) and Taurus (1--2 Myr old) sources imply little to no evolution in the inner disk C/O ratio over this time interval, even as disk masses decline by a factor of 5 on average. This result contradicts the general prediction of increasing C/O ratio with time in models of disk chemical evolution, indicating the need to better understand the interplay between (and efficiencies of) planet formation and disk evolution processes. We also find that the cold-to-warm water flux ratios show no evolution and do not correlate with the hydrocarbon-to-water ratios. These results suggest that most inner T Tauri disks are not dominated by rapid icy pebble drift and avoid the ''meter-size barrier problem'' once thought to be an obstacle to planet formation.