Atmospheric oxygen over Phanerozoic time

Abstract

It is quite possible that the level of atmospheric oxygen has varied (roughly between 15 and 30% O2) over the past 550 million years. This variation is suggested by modeling of the carbon and sulfur cycles, by the excessive sediment burial of organic matter that accompanied the advent of large vascular land plants, and by recent physiological studies that relate to biological evolution.

Figure 1

The long-term global cycles of carbon and sulfur (pyrite, FeS₂). Because little carbon can be stored in the ocean, inequalities between the weathering and burial of organic matter must result in the reciprocal formation or loss of CaCO₃ to conserve carbon. The same is true for pyrite (FeS₂), calcium sulfate, and sulfur. Also, any prolonged imbalance in the net flux between oxidized and reduced reservoirs of carbon must be balanced approximately by opposite fluxes between reduced and oxidized reservoirs of sulfur to avoid fluctuations in atmospheric O₂ that would be too large for the maintenance of higher forms of life over geologic time. (Degassing caused by the deep thermal decomposition of organic matter and pyrite is here lumped under the heading of “weathering,” because the overall process of degassing, followed by air oxidation, results in an overall reaction chemically equivalent to oxidative weathering on the continents.)

Figure 2

Plot of atmospheric oxygen level vs. time for the Phanerozoic (past 550 million years) calculated via a sediment abundance model with rapid recycling (15).