  Recent observations of high redshift quasars at z~6 have finally revealed
complete Gunn-Peterson absorption. However, this at best constrains the
volume-weighted and mass-weighted hydrogen neutral fractions to be greater than
10^-3 and 10^-2 respectively; stronger constraints are not possible due to the
high optical depth for hydrogen Lyman transitions. Here I suggest certain metal
lines as tracers of the hydrogen neutral fraction. These lines should cause
unsaturated absorption when the IGM is almost fully neutral, if it is polluted
to metallicities Z ~ 10^-3.5--10^-2.5 Z_solar. Such a minimal level of metal
pollution is inevitable in the middle to late stages of reionization unless
quasars rather than stars are the dominant source of ionizing photons. The OI
line at 1302 Angstroms is particularly promising: the OI and H ionization
potentials are almost identical, and OI should be in very tight charge exchange
equilibrium with H. The SiII 1260 Angstrom transition might also be observable.
At high redshift, overdense regions are the first to be polluted to high
metallicity but the last to remain permanently ionized, due to the short
recombination times. Such regions should produce a fluctuating OI and SiII
forest which, if observed, would indicate large quantities of neutral hydrogen.
The OI forest may already be detectable in the SDSS z=6.28 quasar. If seen in
future high-redshift quasars, the OI and SiII forests will probe the topology
of reionization and metal pollution in the early universe. If in additional the
HI optical depth can be measured from the damping wing of a high-redshift
gamma-ray burst, they will yield a very robust measure of the metallicity of
the high-redshift universe.
