Light-element abundances from z=0 to z=5

Plausible ratios of deuterium to hydrogen D/H as a function of metallicity, time, and redshift are investigated. Guided by the heavy-element abundance patterns observed locally in Galactic dwarf stars and at large redshift in quasi-stellar object (QSO) absorption-line systems, empirical evolution of the relative abundance ratios Li/D, B/D, N/D, O/D, and F/D for QSO absorption line systems are given for the possible evolutionary patterns in D/H. Limits imposed from the few observations of deuterium and hydrogen in QSO absorption-line systems (QSOALSs) at z similar to 3 are consistent with the expected amount of oxygen produced in stars for the amount of deuterium burned in stars. The atomic properties suggest that the O/D and N/D ratios will be free of substantial ionization, continuum placement, or optical depth errors. The unique advantages of using the O/D and N/D ratio, instead of the D/H ratio, in determining the primordial abundance and subsequent evolution of deuterium is as an important and recurring concept of this paper. Transitions of O I and N I that may be useful in determining precise O/D and N/D ratios in QSOALSs, along with some QSOALS spectra of systems that may be of use in subsequent follow-up searches, are given in the appendices. Definitive detection of lithium, boron, and fluorine in QSOALSs at an appropriate redshift may be able to assist in identifying their chief nucleosynthetic origin sites(s), although the practicality of making positive detections is much lower than for O/D and N/D. Positive detection of any fluorine at a sufficiently large redshift (z greater than or similar to 1.5) would suggest strongly a positive detection of the neutrino process operating in massive stars. Observation of these light elements at several redshifts could also provide a useful diagnostic on the conditions of the early universe.