We have vastly increased the \ion C4 statistics at intermediate redshift by surveying the thousands of quasars in the Sloan Digital Sky Survey Data-Release 7 .
We visually verified over 16,000 \ion C4 systems with 1.46 < z < 4.55 —a sample size that renders Poisson error negligible .
Detailed Monte Carlo simulations show we are approximately 50 % complete down to rest equivalent widths W _ { r } \approx 0.6 \mbox { \AA } .
We analyzed the sample as a whole and in ten small redshift bins with approximately 1500 doublets each .
The equivalent width frequency distributions f ( W _ { r } ) were well modeled by an exponential , with little evolution in shape .
In contrast with previous studies that modeled the frequency distribution as a single power law , the fitted exponential gives a finite mass density for the \ion C4 ions .
The co-moving line density d \mathcal { N } _ { \mathrm { C IV } } / dX evolved smoothly with redshift , increasing by a factor of 2.37 \pm 0.09 from z = 4.55 to 1.96 , then plateauing at d \mathcal { N } _ { \mathrm { C IV } } / dX \approx 0.34 for z = 1.96 to 1.46 .
Comparing our SDSS sample with z < 1 ( ultraviolet ) and z > 5 ( infrared ) surveys , we see an approximately 10-fold increase in d \mathcal { N } _ { \mathrm { C IV } } / dX over z \approx 6 \rightarrow 0 , for W _ { r } \geq 0.6 \mbox { \AA } .
This suggests a monotonic and significant increase in the enrichment of gas outside galaxies over the 12 Gyr lifetime of the universe .