  The abundance of carbon relative to oxygen (C/O) is a promising probe of star
formation history in the early universe, as the ratio changes with time due to
production of these elements by different nucleosynthesis pathways. We present
a measurement of $\log{\mathrm{(C/O)}} = -1.01\pm0.12$ (stat) $\pm0.15$ (sys)
in a $z=6.23$ galaxy observed as part of the GLASS-JWST Early Release Science
Program. Notably, we achieve good precision thanks to the detection of the
rest-frame ultraviolet O III], C III], and C IV emission lines delivered by
JWST/NIRSpec. The C/O abundance is $\sim$0.8 dex lower than the solar value and
is consistent with the expected yield from core-collapse supernovae, indicating
that longer-lived intermediate mass stars have not fully contributed to carbon
enrichment. This in turn implies rapid buildup of a young stellar population
with age $\lesssim100$ Myr in a galaxy seen $\sim$900 million years after the
Big Bang. Our chemical abundance analysis is consistent with spectral energy
distribution modeling of JWST/NIRCam photometric data, which indicates a
current stellar mass $\log\,\mathrm{M}_* / \mathrm{M_{sun}} =
8.4^{+0.4}_{-0.2}$ and specific star formation rate sSFR $\simeq 20$
Gyr$^{-1}$. These results showcase the value of chemical abundances and C/O in
particular to study the earliest stages of galaxy assembly.
