  The radial-dependent positions of snowlines of abundant oxygen- and
carbon-bearing molecules in protoplanetary discs will result in systematic
radial variations in the C/O ratios in the gas and ice. This variation is
proposed as a tracer of the formation location of gas-giant planets. However,
disc chemistry can affect the C/O ratios in the gas and ice, thus potentially
erasing the chemical fingerprint of snowlines in gas-giant atmospheres. We
calculate the molecular composition of hot Jupiter atmospheres using elemental
abundances extracted from a chemical kinetics model of a disc midplane where we
have varied the initial abundances and ionization rates. The models predict a
wider diversity of possible atmospheres than those predicted using elemental
ratios from snowlines only. As found in previous work, as the C/O ratio exceeds
the solar value, the mixing ratio of CH$_{4}$ increases in the lower
atmosphere, and those of C$_{2}$H$_{2}$ and HCN increase mainly in the upper
atmosphere. The mixing ratio of H$_{2}$O correspondingly decreases. We find
that hot Jupiters with C/O$>1$ can only form between the CO$_{2}$ and CH$_{4}$
snowlines. Moreover, they can only form in a disc which has fully inherited
interstellar abundances, and where negligible chemistry has occurred. Hence,
carbon-rich planets are likely rare, unless efficient transport of
hydrocarbon-rich ices via pebble drift to within the CH$_{4}$ snowline is a
common phenomenon. We predict combinations of C/O ratios and elemental
abundances that can constrain gas-giant planet formation locations relative to
snowline positions, and that can provide insight into the disc chemical
history.
