From Soils to Paleosols
It has long been understood that the primary control on atmospheric carbon dioxide levels over geologic time (106−107 years) is silicate weathering. Schematically, this relationship is given by the “Urey Equation,” such as CaSiO3 + CO2 = CaCO3 + SiO2, where the equation represents weathering going from left to right and metamorphism going from right to left. The logic of the Urey Equation can be inverted to look instead at the consumption (and therefore burial) of carbon due to weathering because, for example, it requires 2 moles of CO2 from all sources (diffusion, rainfall, in situ productivity) to weather 1 mole of silicate Ca2+. Thus, by characterizing chemical losses during pedogenesis, it is possible to determine the total CO2 that was consumed during pedogenesis. With reasonable estimates of formation time, the gross consumption can be reconfigured as a rate of carbon consumption. This theoretical framework is applied to basalt-parented paleosols from the Picture Gorge Subgroup (Oregon) that span the middle Miocene climatic optimum. The calculations indicate that CO2 consumption is not simply a function of soil formation time and that it is instead controlled by the atmospheric CO2 level. Benthic foraminiferal δ13C values also indicate a carbon burial event at this time that is consistent with the paleosol carbon sequestration estimates. As atmospheric CO2 declined toward the end of the middle Miocene climatic optimum by a factor of three, CO2 consumption by silicate weathering dropped by at least 50%, indicating a strong relationship between the two, even on relatively short timescales (104–105 years).
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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.