CARBON PROCESSING AND CYCLING: THE RISE AND STABILIZATION OF ATMOSPHERIC OXYGEN

A three reservoir model for surface carbon can yield a temporal model consistent with carbon isotope data from carbonate rocks and other constraints associated with the transformation of the oxidation state of the atmosphere. Reasonable estimates of the time variation of the amount of oxidized versus reduced carbon species yield appropriate timing of free oxygen in the atmosphere, while maintaining the observed nearly constant carbon fractionation in carbonates with time. Although any reasonable model has a great deal of flexibility because of limited constraints on many of the factors important in carbon speciation, it is gratifying that it is not necessary to adopt any extreme parameters.

An important additional factor that must be considered in any realistic model is the amount of reducing power at the earth's surface and its time variation. There are some values of reducing power that will prevent evolution of the atmosphere to a free oxygen state. That free oxygen has been present for somewhat more than the last 2Ga constrains the size and evolution of the reducing reservoir.

Ultimately there must be processes controlling the overall average oxidation state of surface carbon if the level of free oxygen in the surface environment is to stabilize. Several mechanisms, including some that have been previously proposed, can help regulate carbon cycling and speciation. The details of the relatively recent (geologically speaking) stabilization of free oxygen in the atmosphere (assuming that it is, in fact approximately stabilized), may well involve as complicated a model as the earlier transition from reducing to oxidizing.