Two-dimensional posterior distribution of ¹⁶O/¹⁸O versus ﹩\mathrm{log}{N}_{{\rm{CO}}}﹩ from the MCMC analysis. The color scale indicates the number of samples per bin; overplotted contours mark the 68% and 95% credible regions. The elongated ridge illustrates the strong positive covariance between ¹⁶O/¹⁸O and N_CO: larger oxygen isotope ratios can be compensated by higher CO columns while preserving the C¹⁸O(1–0) flux.