MODELING THE LONG-TERM EVOLUTION OF ATMOSPHERIC OXYGEN AND CARBON DIOXIDE

Holland (1978) initially suggested that the atmospheric O₂ level had been regulated by two major negative feedback mechanisms: (1) the negative response of the marine burial flux of organic carbon to pO₂; and (2) the positive response of the O₂ consumption flux by soil to pO₂. This hypothesis has subsequently been discarded by Holland and other researchers for the lack of supportive evidence. Based on the following types of analysis, however, we have come to conclude that the atmospheric pO₂ level has indeed been regulated by the above two negative feedback mechanisms: (a) a three-dimensional statistical analysis of the data reported in recent literature on the organic contents, DO and sedimentation rates of recent marine sediments; (b) a theoretical analysis of the kinetics of decomposition of organic matter by sulfur-reducing bacteria; and (c) an analysis of the available experimental data on the oxidation kinetics of organic carbon within the dynamics model of soil weathering.

We have also developed a series of rate equations linking the production and consumption fluxes of O₂ to various parameters, such as atmospheric pO₂ and pCO₂, the global soil thickness, the total land area, and the volcanic fluxes of CO₂, CH₄ + CO, and H₂. Using these equations, we have simulated the long-term (up to 4 Ga) changes in the pO₂ and pCO₂ of the atmosphere, the contents and carbon isotopic compositions of organic C and carbonate in new sediments, and the masses of organic C and carbonate reservoirs under a variety of scenarios. The results support the Dimroth-Ohmoto model for atmospheric evolution that suggests the development of a stable oxic atmosphere in < 50 Ma since the appearance of cyanobacteria > 3.5 Ga and a gradual decrease in pCO₂ from ~1000 PAL to 1 PAL in ~4 Ga.