Discovery of excess OI absorption toward the z=6.42 QSO SDSS J1148+5251

We present a search for O I in the spectra of nine 4.9 <= z(QSO) <= 6.4 QSOs taken with Keck HIRES. We detect six systems with N-O (I) > 10(13.7) cm(-2) in the redshift intervals where O I lambda 1302 falls redward of the Ly alpha forest. Four of these lie toward SDSS J1148+5251 ( zQSO 6: 42). This imbalance is unlikely to arise from variations in sensitivity among our data or from a statistical fluctuation. The excess O I occurs over a redshift interval that also contains transmission in Ly alpha and Ly beta. Therefore, if these O I systems represent pockets of neutral gas, then they must occur within or near regions of the IGM that are highly ionized. In contrast, no O I is detected toward SDSS J1030+0524 ( zQSO 6: 30), whose spectrum shows complete absorption in Ly alpha and Ly beta over Delta z similar to 0.2. Assuming no ionization corrections, we measure mean abundance ratios <[O/Si]> = 0.04 +/- 0.06, <[C/O]> = -0.31 +/- 0.09, and <[C/Si]> = -0.34 +/- 0.07 (2 sigma), which are consistent with enrichment dominated by Type II supernovae. The O/Si ratio limits the fraction of silicon in these systems contributed by metal-free very massive stars to less than or similar to 30%, a result that is insensitive to ionization corrections. The ionic comoving mass densities along the zQSO > 6: 2 sight lines, including only the detected systems, are Omega(O) (I) (7.0 +/- 0.6)x 10(-8), Omega(Si) (II) (9.6 +/- 0.9)x 10(-9), and Omega(C) (II) (1.5 +/- 0.2)x 10(-8).