This paper presents ionization-corrected measurements of the carbon abundance in intergalactic gas at 4.0 < z < 4.5 , using spectra of three bright quasars obtained with the MIKE spectrograph on Magellan .
By measuring the C iv strength in a sample of 131 discrete H i -selected quasar absorbers with \rho / \bar { \rho } \geq 1.6 , we derive a median carbon abundance of [ C/H ] = -3.55 , with lognormal scatter of approximately \sigma \approx 0.8 dex .
This median value is a factor of two to three lower than similar measurements made at z \sim 2.4 using C iv and O vi .
The strength of evolution is modestly dependent on the choice of UV background spectrum used to make ionization corrections , although our detection of an abundance evolution is generally robust with respect to this model uncertainty .
We present a framework for analyzing the effects of spatial fluctuations in the UV ionizing background at frequencies relevant for C iv production .
We also explore the effects of reduced flux between 3-4 Rydbergs ( as from He ii Lyman series absorption ) on our abundance estimates .
At He ii line absorption levels similar to published estimates the effects are very small , although a larger optical depth could reduce the strength of the abundance evolution .
Our results imply that \sim 50 \% of the heavy elements seen in the IGM at z \sim 2.4 were deposited in the 1.3 Gyr between z \sim 4.3 and z \sim 2.4 .
The total implied mass flux of carbon into the Ly- \alpha forest would constitute \sim 30 \% of the IMF-weighted carbon yield from known star forming populations over this period .