  We present the first detection of 13CCH in a protoplanetary disk (TW Hya).
Using observations of C2H we measure CCH/13CCH = 65 +/- 20 in gas with a CO
isotopic ratio of 12CO/13CO = 21 +/- 5 (Yoshida et al. 2022a). The TW Hya disk
exhibits a gas phase C/O that exceeds unity and C2H is the tracer of this
excess carbon. We confirm that the TW Hya gaseous disk exhibits two separate
carbon isotopic reservoirs as noted previously (Yoshida et al. 2022a). We
explore two theoretical solutions for the development of this dichotomy. One
model represents TW Hya today with a protoplanetary disk exposed to a cosmic
ray ionization rate that is below interstellar as consistent with current
estimates. We find that this model does not have sufficient ionization in cold
(T < 40 K) layers to activate carbon isotopic fractionation. The second model
investigates a younger TW Hya protostellar disk exposed to an interstellar
cosmic ray ionization rate. We find that the younger model has sources of
ionization deeper in a colder disk that generates two independent isotopic
reservoirs. One reservoir is 12C-enriched carried by methane/hydrocarbon ices
and the other is 13C-enriched carried by gaseous CO. The former potentially
provides a source of methane/hydrocarbon ices to power the chemistry that
generates the anomalously strong C$_2$H emission in this (and other) disk
systems in later stages. The latter provides a source of gaseous 13C-rich
material to generate isotopic enrichments in forming giant planets as recently
detected in the super-Jupiter TYC 8998-760-1 b by Zhang et al. (2021).
