National, Eel, Luquillo, Shale Hills, INVESTIGATOR, COLLABORATOR
The pCO2 concentrations of early Earth’s atmosphere are a key component for resolving the faint young Sun paradox and have been intensely debated for some time. Regolith models that use mass-balance geochemical arguments, based on the consumption of CO2 through the neutralization of soil, saprolite and rock, prove to be one of the more reliable methods for estimating atmospheric pCO2 [1, 2, 3]. Yet, many of the assumptions used to estimate the early atmosphere are difficult to test in the modern biotic Earth. This study serves as a review of modern regolith gas research and explores the potential use of deep weathering profiles for refining paleoatmosphere reconstructions. We compiled 248 soil CO2 and O2 assays from 29 regolith studies spanning a range of environments that includes deep (>3 m) profiles. Soil CO2 values (n=174) range from 0.01 to 15.5% by vol., whereas soil O2 values (n=74) range from 0.2 to 22.85% by vol. The compilation of regolith gas data shows that only 30% of the assays were collected from a depth >3 m; and few of these studies include both gas and bulk geochemical data. The data that do extend to depths >3 m show O2:CO2 ratios that exponentially decline with increasing depth. This deep portion of the modern sub-surface Critical Zone has lower O2:CO2 ratios and presumed lower biomass concentrations than the modern surface. These observations suggest that the highly acidic, low-oxidizing soil atmosphere deep in modern regolith may be a suitable analogue for Precambrian abiotic near-surface weathering. Contemporary deep (> 3 m) regolith studies that include both bulk geochemical and gaseous phase data are not only lacking but could provide a much-needed empirical dataset for refining Precambrian atmosphere estimates using paleosols.  Sheldon, 2006, Precambrian Res 147, 148-155.  Driese et al., 2011, Precambrian Res 189, 1-17.  Brantley et al., in press, In Treatise of Geochemistry, The Atmosphere – History.
G. E. Stinchcomb and S. L. Brantley (2013): A review of CO2 and O2 gas dynamics within the sub-surface Critical Zone and implications for early-atmosphere studies using paleosols. Goldschmidt 2013.
This Paper/Book acknowledges NSF CZO grant support.