MODELING THE RISE OF ATMOSPHERIC OXYGEN

A complete understanding of how and why free oxygen accumulated in Earth's atmosphere approximately 2.4 billion years ago remains elusive. Several lines of evidence suggest that oxygenic photosynthesis evolved significantly prior to the rise of oxygen [1, 2]. Hence, any model for a rise in oxygen levels must explain how the atmosphere remains largely anoxic after the advent of oxygenic photosynthesis, and also describe physical parameters that control the timing of the oxygen transition. Many qualitative descriptions for the rise in oxygen exist, while quantitative models are lacking. We developed a time-dependent box model which tracks redox fluxes of carbon, oxygen, and iron which we use to test various hypothess regarding the initial oxygenation of Earth.

Our model solves simple differential equations that determine the sources and sinks of biogenic oxygen and methane. The primary source of Earth's free oxygen derives from a photosynthesis, while sinks include photochemical destruction (with a large gross cycle consumption by biogenic methane,) reduced volcanic and metamorphic gases, continental weathering, and hydrogen escape. The model displays kinetic control of oxygen via the increased proportions of reduced gases from a more reduced mantle and crust, via volcanism and metamorphism, respectively. We show that a reduced atmosphere can persist long after the evolution of oxygenic photosynthesis, that an inhabited Earth remains in a Titan-like methane greenhouse in the absence of hydrogen escape, and that the oxic transition occurs when the flux of reduced species from volcanic and metamorphic gases drops below the flux of oxygen from organic carbon burial. The timing is most dramatically affected by the amount of reduced iron in the continental crust and/or mantle.

1. Canfield, D.E. Annual Review of Earth and Planetary Science, (33), 2005 2. Catling D.C and Claire, M.W., Earth and Planetary Science Letters, in review, 2005