nai

Project Progress

The concentration of free oxygen in Earth’s surface environment represents a balance between the accumulation of O₂, due to long-term burial of organic carbon in sediments, and the consumption of O₂ by weathering processes and the oxidation of reduced gases. The stability of modern O₂ levels is typically attributed to a negative feedback that emerges when the production and consumption fluxes are expressed as a function of O₂ concentration. Empirical studies of modern burial of organic carbon suggest that the production of O₂ is a logarithmically decreasing function of the duration of time—-the “oxygen exposure time (OET)”—over which sedimentary organic carbon is exposed to O₂. The OET hypothesis implies that a fraction of organic matter is physically protected from anaerobic decay by its association with clay-sized mineral surface area, but susceptible to aerobic decay, either oxidatively or via free extracellular hydrolytic enzymes. By assuming that the long-term aerobic degradation is diffusion-limited, we have constructed a mathematical model that predicts the observed logarithmic decrease of the OET curve.

The rise of O₂ in the Proterozoic may be associated with the loss of stability of the intial, lower O₂ levels. To explain this loss of stability, we propose a positive feedback whereby exposure to O₂ enhances not only degradation but also physical protection, due to the precipitation of iron oxides and clay minerals. When the rate of transformation from the unprotected state to the protected state exceeds a small fraction of the average oxidative degradation rate, our theoretical OET curve develops a maximum at small O₂ exposure times. In this case, the equilibrium O₂ concentration can lose its stability. These observations may help explain major fluctuations in Earth’s carbon cycle and the rise of O₂ during the Proterozoic (2000—542 Ma).