  (Modified) We study the abundance of silicon in the intergalactic medium by
analyzing the statistics of SiIV, CIV, and HI pixel optical depths in a sample
of 19 high-quality quasar absorption spectra spanning redshifts z ~ 2 - 4,
which we compare to realistic spectra drawn from a hydrodynamical simulation.
We find that silicon is highly overabundant relative to carbon and that the
scatter in Si/C is much smaller than that in C/H, implying a common origin for
Si and C. The inferred [Si/C] depends upon the shape of the UV background (UVB)
(harder backgrounds leading to higher [Si/C]), ranging from [Si/C] ~ 1.5 for a
quasar-only UVB, to [Si/C] ~ 0.25 for a UVB including both galaxies and an
artificial softening. For our fiducial UVB, which includes both quasars and
galaxies, we find [Si/C]=0.77 +/- 0.05, with a possible systematic error of ~
0.1 dex. There is no evidence for evolution in [Si/C] and the data are
inconsistent with previous claims of a sharp change in the SiIV/CIV ratio (or
the UVB) at z ~ 3. Comparisons with low-metallicity halo stars and
nucleosynthetic yields suggest that either our fiducial UVB is too hard or that
supermassive Pop III stars might have to be included. The inferred [Si/C]
corresponds to a contribution to the cosmic Si abundance of [Si/H] = -2.0, a
significant fraction of all Si production expected by z ~ 3.
