We present the results of the largest survey to date for intergalactic metals at redshifts z > 5 , using near-IR spectra of nine QSOs with emission redshifts z _ { em } > 5.7 .
We detect three strong C iv doublets at z _ { abs } = 5.7 –5.8 , two low ionisation systems at z _ { abs } > 5 , and numerous Mg ii absorbers at z _ { abs } = 2.5 –2.8 .
We find , for the first time , a change in the comoving mass density of C iv ions as we look back to redshifts z > 5 .
At a mean \langle z \rangle = 5.76 , we deduce \Omega _ { C \textsc { iv } } = ( 4.4 \pm 2.6 ) \times 10 ^ { -9 } which implies a drop by a factor of \sim 3.5 compared to the value at z < 4.7 , after accounting for the differing sensitivities of different surveys .
The observed number of C iv doublets is also lower by a similar factor , compared to expectations for a non-evolving column density distribution of absorbers .

These results point to a rapid build-up of intergalactic C iv over a period of only \sim 300 Myr ; such a build-up could reflect the accumulation of metals associated with the rising levels of star formation activity from z \sim 9 indicated by galaxy counts , and/or an increasing degree of ionisation of the intergalactic medium ( IGM ) , following the overlap of ionisation fronts from star-forming regions .
If the value of \Omega _ { C \textsc { iv } } we derive is typical of the IGM at large , it would imply a metallicity Z _ { IGM } \lower 2.15 pt \hbox { $ \buildrel > \over { \sim } $ } 10 ^ { -4 } Z _ { \odot } .
The early-type stars responsible for synthesising these metals would have emitted only about one Lyman continuum photon per baryon prior to z = 5.8 ; such a background is insufficient to keep the IGM ionised and we speculate on possible factors which could make up the required shortfall .