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.
|Original language||English (US)|
|Title of host publication||The Atmosphere - History|
|Number of pages||26|
|State||Published - Nov 2013|
All Science Journal Classification (ASJC) codes
- Earth and Planetary Sciences(all)
- Environmental Science(all)