Chemical Abundances in High-Redshift Neutral Clouds

Neutral hydrogen clouds with high column density detected towards distant quasars are unique probes of elemental nucleosynthesis and chemical evolution in the low metallicity regime. They provide measurements for several elements at very early times which are unfeasible in other astrophysical environments. Comparison between refractory and non-refractory elements provides evidence for the presence of dust, and the recently measured Ar probes photoionization. A prominent characteristic is the dominance of a solar abundance pattern, which is somewhat unexpected at low metallicities. It is argued that this property and Nitrogen observations can be used to constrain the age of the Damped Ly_alpha systems and the epoch of star formation.