Introduction

Oxygen is the most abundant element in the earth's crust, it accounts for 89% of the mass of the ocean, and it is the second most abundant element in the earth's atmosphere. Much work on the oxygen cycle has focused on the question of the origin of atmospheric O and its variations over geologic time [see Kump et al., 1991, and references therein). This review focuses on several other aspects of the oxygen cycle including the short-term controls on the oxygen isotopic abundance of atmospheric CO and O, and the short-term variabilitiy in the O/N ratio.

These aspects of the oxygen cycle depend mainly on material exchanges between the atmosphere and living organisms at the earth's surface or in the ocean. Like several other atmospheric variables which have received much attention recently, e.g., the abundances of CO, CH, and NO, the oxygen isotopic content of CO and O and the O/N ratio have atmospheric lifetimes that are long relative to the time scale of atmospheric mixing and thus reflect an integration of material exchanges over the globe. Recently, our knowledge of these variables has expanded through laboratory experiments exploring the exchange pathways, and through measurements on contemporary air samples or in ancient air samples extracted from polar ice cores. This review summarizes recent literature on these subjects, and also emphasizes how these aspects of the global oxygen cycle can provide new information on the material exchanges between the atmosphere and biota integrated over large areas.