In the history of the earth, the cycles of carbon and atmospheric molecular oxygen are closely coupled to the development of life because of the fundamental biochemical reactions occurring during photosynthesis and respiration. However, the role of the two cycles in the earth system, al least on lime scales up to 106 years, is distinctly different. The major atmospheric branch of the global carbon cycle, i.e., carbon dioxide, constitutes a potent greenhouse gas with the potential to control the climate of the earth. On the other hand, changes in the abundant atmospheric oxygen, at least on time scales less than 106 years, are too small to significantly impact the radiative balance of the atmosphere and have been proven very difficult to measure directly. Therefore, up to now the cycle of atmospheric oxygen has not received much attention in global change science. However, because of the tight coupling of the carbon and oxygen cycles, variations in atmospheric oxygen reflect also important processes in the carbon cycle. Now, with recently developed analytical techniques to accurately measure the variations in atmospheric 02, the global cycle of oxygen as a diagnostic tool has drawn much interest.

Oxygen has three stable oxygen isotopes: l60, ' O, and lsO. Most biological, chemical, and physical processes in which oxygen is involved are affected by the different masses of the oxygen atoms, leading to fractionation processes that induce varying isotopic ratios of the different oxygen-containing molecules in the earth system. Most of these fractionation processes are relatively well understood and/or empirically measured, which makes observations of the oxygen isotope ratios an additional important diagnostic tool.

Here I briefly review some recent applications of using oxygen and its isotopes as diagnostic tracers of the global carbon cycle. The focus is on atmospheric 02 and CO, and the oxygen isotope ratios in each of these molecules. Because of the vigorous mixing in the atmosphere, spatiotemporal variations of these species in atmospheric air reflect large-scale surface processes. The unraveling and quantification of this information necessitates a combination of a model of atmospheric transport and a model of the surface processes. This review outlines some components of the latter, though it does not intend to be comprehensive. A more extensive review on some of the topics addressed here, albeit with a different point of view, maybe found in Keeling (1995).

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