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USING PALEOSOLS TO UNDERSTAND PALEO-CARBON BURIAL
NATHAN D. SHELDON
Department of Earth and Environmental Sciences, 1100 N. University Avenue, University of
Michigan, Ann Arbor, Michigan 48109, USA
e-mail: nsheldon@umich.edu
AND
NEIL J. TABOR
Huffington Department of Earth Sciences, P O Box 750395, Southern Methodist University,
Dallas, Texas 75275, USA
ABSTRACT: It has long been understood that the primary control on atmospheric carbon dioxide levels
over geologic time (10 6 –10 7 years) is silicate weathering. Schematically, this relationship is given by the
“Urey Equation,” such as CaSiO 3 + CO 2 = CaCO 3 + SiO 2 , 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 CO 2 from all sources (diffusion, rainfall, in situ productivity) to
weather 1 mole of silicate Ca 2+ . Thus, by characterizing chemical losses during pedogenesis, it is possible
to determine the total CO 2 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 CO 2 consumption is not
simply a function of soil formation time and that it is instead controlled by the atmospheric CO 2 level.
Benthic foraminiferal δ 13 C values also indicate a carbon burial event at this time that is consistent with the
paleosol carbon sequestration estimates. As atmospheric CO 2 declined toward the end of the middle
Miocene climatic optimum by a factor of three, CO 2 consumption by silicate weathering dropped by at
least 50%, indicating a strong relationship between the two, even on relatively short timescales (10 4 –10 5
years).