Stable isotopes as a guide to biogeochemical processes

Isotopic distribution is a function of physical and biological processes. In general, equilibrium-controlled reactions occur at high temperatures among solid phases, whereas low-temperature reactions, especially those mediated by organisms, are kinetically controlled. The solar and stellar abundance of isotopes is poorly known. Knowledge of extra-terrestrial distributions of the isotopes of light elements presently comes from lunar and meteorite measurements. On Earth, the elements associated with biological synthesis which have been studied most intensively, are hydrogen, carbon, nitrogen, oxygen and sulphur. In general, reduced products of metabolism are enriched in light isotopes. Thus, $^{1}$H and $^{12}$C are enriched in hydrogen gas and methane, when produced by fermentation or CO$_{2}$ reduction. Nitrogen gas is enriched in $^{14}$N when produced by denitrification, and H$_{2}$S is highly enriched in $^{32}$S when it results from sulphate reduction. A pattern of biological enrichment factors has been recognized on Earth, but until now, not in lunar or meteorite samples.