THE GREAT OXIDATION EVENT, MINERAL DIVERSIFICATION, AND MINERAL CORRELATIONS WITH ATMOSPHERIC COMPOSITION THROUGH GEOLOGIC TIME

Before the Great Oxidation Event (GOE) the mineralogical record of the near surface continental environment indicates a low partial pressure of oxygen during weathering that restricted many elements to a low oxidation state, thus limiting the number of possible minerals of these elements. Calculations indicate that local pulses in the production of O₂ by photosynthesis (or pulses of some other oxidant) could mobilize metals such as Mo and Re in near surface waters without mobilizing other metals such as U. However, this O₂ would be completely consumed and would not accumulate in the atmosphere. After the GOE, when O₂ levels in the atmosphere were high enough to participate in chemical reactions between minerals and natural waters, many chemical elements such as U, V, Mn, Cu, Se, Te, As, Sb, Bi, Ag and Hg could be present in one or more oxidized forms in minerals in the near surface environment. This development resulted in an explosive growth in the diversification of minerals in the near-surface environment. Among the most abundant are clay minerals, which are well suited for hosting a great variety of chemical elements in multiple oxidation states and whose formation during weathering reflect O₂ and CO₂ levels in the near surface environment. Dramatic global changes in the levels of atmospheric O₂ and CO₂ over multimillion year timescales during the Phanerozoic are well established based on quantitative model analyses of geological and biological processes and their carbon, sulfur and strontium isotopic records. We show that the relative abundances of clay minerals in ancient shales track the global atmospheric changes: kaolinite and chlorite show negative and positive correlations, respectively, with O₂ levels, whereas montmorillonite and illite show negative and positive correlations, respectively, with CO₂ levels. Within these overall correlations, changes in the patterns of the correlations suggest that relative clay mineral abundances are sensitive indicators of major biological events relevant to the weathering zone, including the rise of deep-rooted vascular plants and their associated mycorrhizal fungi. The evolution of the composition of Earth's atmosphere and the mineralogical record are thus intimately linked to the evolution of Earth’s biosphere throughout much of Earth history.