  Recent measurements of Si IV/C IV ratios in the high-redshift Ly-alpha forest
(Songaila & Cowie, AJ, 112, 335 [1996a]; Savaglio et al., A&A, in press [1997])
have opened a new window on chemical enrichment and the first generations of
stars. However, the derivation of accurate Si/C abundances requires reliable
ionization corrections, which are strongly dependent on the spectral shape of
the metagalactic ionizing background and on the ``local effects'' of hot stars
in nearby galaxies. Recent models have assumed power-law quasar ionizing
backgrounds plus a decrement at 4 Ryd to account for He II attenuation in
intervening clouds. However, we show that realistic ionizing backgrounds based
on cosmological radiative transfer models produce more complex ionizing spectra
between 1-5 Ryd that are critical to interpreting ions of Si and C. We also
make a preliminary investigation of the effects of He II ionization front
non-overlap. Because the attenuation and re-emission by intervening clouds
enhance Si IV relative to C IV, the observed high Si IV/C IV ratios do not
require an unrealistic Si overproduction [Si/C $\geq 3 (Si/C)_{\odot}$]. If the
ionizing spectrum is dominated by ``local effects'' from massive stars, even
larger Si IV/C IV ratios are possible. However, unless stellar radiation
dominates quasars by more than a factor of 10, we confirm the evidence for some
Si overproduction by massive stars; values Si/C $\approx 2 (Si/C)_{\odot}$ fit
the measurements better than solar abundances. Ultimately, an adequate
interpretation of the ratios of C IV, Si IV, and C II may require hot,
collisionally ionized gas in a multiphase medium.
