Abstract:
Concentrations of the Fe(CO3)OH component in goethites from Phanerozoic oolitic ironstones appear to record information on the partial pressures of soil CO2 in ancient subaerial weathering environments. Application of a simple steady-state, one-dimensional, Fickian diffusion model to ancient goethite-bearing soils suggests that it may be possible to calculate lower limits for the partial pressure of oxygen in the Earth's atmosphere by using both the inferred soil CO2 partial pressure and estimates of the partial pressure of atmospheric CO2. Extant data from oolitic goethites indicate that the atmosphericPO2 value was no lower than about 13% of the present atmospheric level (PAL) in the Late Ordovician. This value affirms existing evidence for abundant molecular oxygen in the Earth's atmosphere in the Early Paleozoic, i.e., before the widespread advent of vascular plants. Extensive colonization of the continents by vascular plants in the Devonian was associated with calculated atmospheric oxygen partial pressures that were no lower than about 39% of PAL at 360 Ma BP. For Early Jurassic to Late Cretaceous samples, the calculated lower limit of atmospheric oxygen ranged from about 20 to 25% of the present value. It remains to be established whether or not there is a systematic relationship between calculated minimum PO2 values (or soil respiration rates) and the actual partial pressure of atmospheric oxygen.