E-XQR-30: The evolution of Mg ii, C ii, and O i across 2 < z < 6

Intervening metal absorbers in quasar spectra at z &gt; 6 can be used as probes to study the chemical enrichment of the Universe during the Epoch of Reionization. This work presents the comoving line densities (dn/dX) of low-ionization absorbers, namely, Mg ii (2796 Å), C ii (1334 Å), and O i (1302 Å) across 2 &lt; z &lt; 6 using the E-XQR-30 metal absorber catalogue prepared from 42 XSHOOTER quasar spectra at 5.8 &lt; z &lt; 6.6. Here, we analyse 280 Mg ii (1.9 &lt; z &lt; 6.4), 22 C ii (5.2 &lt; z &lt; 6.4), and 10 O i (5.3 &lt; z &lt; 6.4) intervening absorbers, thereby building up on previous studies with improved sensitivity of 50 per cent completeness at an equivalent width of W &gt; 0.03 Å. For the first time, we present the comoving line densities of 131 weak (W &lt; 0.3 Å) intervening Mg ii absorbers at 1.9 &lt; z &lt; 6.4 which exhibit constant evolution with redshift similar to medium (0.3 &lt; W &lt; 1.0 Å) absorbers. However, the cosmic mass density of Mg ii-dominated by strong Mg ii systems-traces the evolution of global star formation history from redshift 1.9 to 5.5. E-XQR-30 also increases the absorption path-length by a factor of 50 per cent for C ii and O i whose line densities show a rising trend towards z &gt; 5, in agreement with previous works. In the context of a decline in the metal enrichment of the Universe at z &gt; 5, the overall evolution in the incidence rates of absorption systems can be explained by a weak-possibly soft fluctuating-ultraviolet background. Our results, thereby, provide evidence for a late reionization continuing to occur in metal-enriched and therefore, biased regions in the Universe.