Causes and consequences of the rise of atmospheric oxygen - David Catling

Causes and consequences of the rise of atmospheric oxygen
- David Catling
University of Washington
Oxygenation of the atmosphere and oceans can be understood in terms of changing source and sink fluxes of O2. I will discuss how the relative magnitude of such fluxes set stages of Earth history with different mean atmospheric O2 levels, e.g., anoxia before the Great Oxidation Event (GOE), oxygenated air in Proterozoic, and highly oxygenated air in the Phanerozoic. Change in atmospheric composition at the GOE (e.g., growth of the ozone layer and the end of mass-independent sulfur isotope fractionation) depends on time-dependent photochemistry. Atmospheres with ground-level O2 mixing ratios ~10^-8 to ~10^-4 are found to be unstable to perturbations. Hence, the latter sets a lower limit for the much debated O2 level in the mid-Proterozoic atmosphere. Finally, I will discuss the ultimate drivers of Earth’s one-way oxidation. Escape of hydrogen (a strong reducing agent) to space is widely accepted as the ultimate cause of Venus’ and Mars’ oxidized atmospheres and the red color of Mars. On Earth, excess oxidation (in Fe(3+), S(6+), and O2) exceeds reduced carbon in the continental crust by ~2. Reduced carbon requires photosynthesis but photosynthesis alone cannot explain the big redox imbalance. Gradual atmospheric xenon isotope fractionation throughout the Archean, ceasing around the GOE, implies that hydrogen escape (which dragged out xenon ions) was important for Earth’s net oxidation, as proposed two decades ago (Catling et al. 2001 Science) before such compelling evidence was known.