Relating Weathering Fronts for Acid Neutralization and Oxidation to pCO2 and pO2

As mineral assemblages formed at depth reequilibrate at Earth's surface, reactions with O₂ and CO₂ are recorded as depletion profiles in regolith. We use models to explore reaction fronts in regolith developed on two lithologies exposed on ridgetops in the Virginia Piedmont (United States). The ratios, R⁰, of capacities to consume O₂:CO₂ differ between the Virginia diabase (R⁰=0.04) and granite (0.02) protoliths. However, the ratios, R, of actual consumption of O₂ (by FeO oxidation) versus CO₂ (by silicate weathering) recorded by regolith are identical (~0.02). Although soil gases were not measured, we propose that R=~0.02 because the ratio of soil pO₂ to pCO₂ (=R') equals ~0.02 for both sites. For Fe-rich diabase, however, R'<R⁰ and CO₂ is depleted deeper than O₂, allowing ferrous iron to be lost from regolith. In contrast, R'>R⁰ for Fe-poor granite and O₂ is consumed deeper than CO₂, causing iron to be retained. In the granite, the volume constraints during oxidation promote fracturing. In turn, fracturing promotes advection and development of thick regolith. Indeed, modern regolith is generally thicker on granites than diabase, as expected if the condition, 0.02<R'<0.04, characterizes many modern soils. Perhaps for ancient atmospheres with pO₂/pCO₂<~0.02, thick regolith was also not likely.