Ratio of C II/C IV (left panels) and Si II/Si IV (right panels) column densities as a function of redshift (upper panels) and velocity separation from the systemic redshift of the QSO (lower panels) for our sample of O I systems. The dotted vertical lines in the upper panels are drawn at z = 5.7, the limit beyond which the low ionization phase is expected to dominate in the gas (Cooper et al. 2019). The vertical dotted lines in the lower panels mark a velocity separation of 5000 km s⁻¹, which separates PDLAs from intervening DLAs. In each panel, the three systems with black contours (diamond, square and star) are the same highlighted in Figs. 7 and 8.