OI abs. search in Keck and VLT spectra of 199 QSOs

The number density over 4.9<z<5.7 is a factor of 1.7-4.1 lower (68% confidence) than that over 5.7<z<6.5, with a lower value at z<5.7 favored with 99% confidence. This decrease suggests that the fraction of metals in a low-ionization phase is larger at z~6 than at lower redshifts. Absorption from highly ionized metals traced by CIV is also weaker in higher-redshift OI systems, supporting this picture.

The evolution of OI absorbers implies that metal-enriched circumgalactic gas at z~6 is undergoing an ionization transition driven by a strengthening ultraviolet background. This in turn suggests that the reionization of the diffuse intergalactic medium may still be ongoing at or only recently ended by this epoch.