CO2 Concentrations in Vertisols: Seasonal Variability and Shrink–Swell
Daniel O. Breecker, Junyeon Yoon, Lauren A. Michel, Takele M. Dinka, Steven G. Driese, Jason S. Mintz, Lee C. Nordt, Katherine D. Romanak, Cristine L.S. Morgan, 2013. "CO2 Concentrations in Vertisols: Seasonal Variability and Shrink–Swell", New Frontiers in Paleopedology and Terrestrial Paleoclimatology: Paleosols and Soil Surface Analog Systems, Steven G. Driese, Lee C. Nordt
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The paleosol–carbonate CO2 barometer is widely accepted to be the most reliable method for reconstructing Earth’s atmospheric CO2 concentrations in deep time. Currently, the largest source of error in atmospheric CO2 calculated using the paleosol barometer originates from uncertainty in soil CO2 concentrations during soil carbonate formation. Many of the paleosols used for CO2 reconstruction were formed in clay-rich alluvium and have vertic properties, which may influence soil CO2.We hypothesized that the cracking during drying of shrink–swell clays results in rapid CO2 escape and low soil CO2 concentrations. We tested our hypothesis by monitoring soil cracking and the concentration of CO2 in the pore spaces of surface Vertisols (the Houston Black and Heiden series fine, smectitic, thermic Udic Haplusterts). Crack porosity of soils was estimated by measuring soil subsidence, and CO2 was measured in syringe samples collected from soil gas wells. During the period of study, pore-space CO2 concentrations at ~1-m soil depth varied by two orders of magnitude, from 10% during water-saturated conditions to <0.1% during hot, dry episodes. These large seasonal variations likely result in significant calcite dissolution and reprecipitation during pedogenesis. Soil CO2 concentrations decreased as soil-crack porosity increased. Moreover a compilation of published records of soil CO2 variability indicates that the variability of CO2 concentrations in Vertisols is significantly larger than the variability in other soil orders, regardless of climate or vegetation. These results suggest that soil cracking is a primary control on soil CO2 in Vertisols and that a soil-order-specific calibration of the paleosol barometer should be developed for application to clay-rich paleosols with soil cracks and/or pedogenic slickensides.
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New Frontiers in Paleopedology and Terrestrial Paleoclimatology: Paleosols and Soil Surface Analog Systems
After initial breakthroughs in the discovery of fossil soils, or paleosols in the 1970s and early 1980s, the last several decades of intensified research have revealed the much greater role that these deposits can play in reconstructing ancient Earth surface systems. Research currently focuses on terrestrial paleoclimatology, in which climates of the past are reconstructed at temporal scales ranging from hundreds to millions of years, using paleosols as archives of that information. Such research requires interdisciplinary study of soils conducted in both modern and ancient environments. These issues and many others were discussed at the joint SEPM-NSF Workshop “Paleosols and Soil Surface Analog Systems”, held at Petrified Forest National Park.