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We present measurements of carbon, oxygen, silicon, and iron in quasarabsorption systems existing when the universe was roughly one billion yearsold. We measure column densities in nine low-ionization systems at 4.7 < z <6.3 using Keck, Magellan, and VLT optical and near-infrared spectra withmoderate to high resolution. The column density ratios among C II, O I, Si II,and Fe II are nearly identical to sub-DLAs and metal-poor ([M/H] < -1) DLAs atlower redshifts, with no significant evolution over 2 < z < 6. The estimatedintrinsic scatter in the ratio of any two elements is also small, with atypical r.m.s. deviation of <0.1 dex. These facts suggest that dust depletionand ionization effects are minimal in our z > 4.7 systems, as in thelower-redshift DLAs, and that the column density ratios are close to theintrinsic relative element abundances. The abundances in our z > 4.7 systemsare therefore likely to represent the typical integrated yields from stellarpopulations within the first gigayear of cosmic history. Due to the time limitimposed by the age of the universe at these redshifts, our measurements thusplace direct constraints on the metal production of massive stars, includingiron yields of prompt supernovae. The lack of redshift evolution furthersuggests that the metal inventories of most metal-poor absorption systems at z> 2 are also dominated by massive stars, with minimal contributions fromdelayed Type Ia supernovae or AGB winds. The relative abundances in our systemsbroadly agree with those in very metal-poor, non-carbon-enhanced Galactic halostars. This is consistent with the picture in which present-day metal-poorstars were potentially formed as early as one billion years after the Big Bang.

Iron and alpha-element Production in the First One Billion Years after the Big Bang