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ALMA observations of $z>6$ galaxies have revealed abnormally high[OIII]$_{\rm 88\mu m}$/[CII]$_{\rm 158\mu m}$ ratios and [CII]$_{\rm 158\mu m}$deficits compared to local galaxies. Numerous solutions have been proposedincluding differences in C and O abundance ratios, observational bias, anddifferences in ISM properties. In order to elucidate the underlying physicsthat drives this high-redshift phenomenon, we employ SPHINX$^{20}$, astate-of-the-art, cosmological radiation-hydrodynamics simulation, thatresolves detailed ISM properties of thousands of galaxies in the epoch ofreionization. We find that the observed $z>6$ [OIII]-SFR and [CII]-SFRrelations can only be reproduced when the C/O abundance ratio is $\sim8\times$lower than Solar and the total metal production is $\sim4\times$ higher thanthat of a Kroupa IMF. This implies that high-redshift galaxies are potentiallyprimarily enriched by low-metallicity core-collapse supernovae with a moretop-heavy IMF. As AGB stars and type-Ia supernova begin to contribute to thegalaxy metallicity, both the [CII]-SFR and [CII] luminosity functions arepredicted to converge to observed values at $z\sim4.5$. While we demonstratethat ionisation parameter, LyC escape fraction, ISM gas density, and CMBattenuation all drive galaxies towards higher [OIII]/[CII], observed values at$z>6$ can only be reproduced with substantially lower C/O abundances comparedto Solar. The combination of [CII] and [OIII] can be used to predict the valuesof ionisation parameter, ISM gas density, and LyC escape fraction and weprovide estimates of these quantities for nine observed $z>6$ galaxies.Finally, we demonstrate that [OI]$_{\rm 63\mu m}$ can be used as a replacementfor [CII] in high-redshift galaxies where [CII] is unobserved and argue thatmore observation time should be used to target [OI] at $z>6$. (Abridged)

monthly notices of the royal astronomical society

The Nature of High ${\rm [OIII]}_{\rm 88\mu m}$/${\rm [CII]}_{\rm 158\mu  m}$ Galaxies in the Epoch of Reionization: Low Carbon Abundance and a  Top-Heavy IMF?