Elevated weathering rates in the Rocky Mountains during the Early Eocene Climatic Optimum

Abstract

During the chemical weathering of silicate minerals, atmospheric carbon dioxide is incorporated into carbonate minerals and buried. As the rate of silicate weathering is thought to increase in response to increasing atmospheric CO2 concentrations, this represents an important negative feedback mechanism1. Quaternary records of weathering reflect a narrow range of pCO2 (180–300 p.p.m.v.)2; therefore, the extent of this feedback has been difficult to predict for increasing concentrations of atmospheric CO2. However, high CO2 levels of up to 1,125 p.p.m.v. have been suggested for the Early Eocene Climatic Optimum (52 to 50 million years ago)3,4,5. Here, we combine 40Ar/39Ar ages6 and the measured volumes of river-derived sediments and sodium-bearing evaporites to determine rates of physical erosion and chemical weathering in the Green River Basin, western United State of America, during the Early Eocene Climatic Optimum7. We find physical erosion rates of 420±79 t km2 yr−1 and chemical weathering rates of 62.5±21.9 t km2 yr−1. The calculated denudation rates of 175±30 m Myr−1 rival the highest documented non-glacial Quaternary rates for crystalline bedrock8. We suggest that elevated atmospheric CO2 levels during the Early Eocene epoch led to enhanced silicate dissolution rates9, and thus to increased production of loose rock material and higher rates of physical weathering and denudation.

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Figure 1: Map of the central Rocky Mountain lake province showing the Early Eocene palaeocatchment of the GGRB6.
Figure 2: Early Eocene strata in the GGRB.
Figure 3: Comparison of Eocene chemical weathering, physical erosion and overall denudation rates to Quaternary rates for unglaciated granitic catchments.

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Acknowledgements

We thank B. S. Singer, J. R. Dyni, J. C. Knox, D. C. Kelly, B. Tikoff and J. P. Smoot for assistance or discussions, and appreciate the thoughtful review by R.S. Anderson. Financial support was provided by National Science Foundation grants EAR-0230123, EAR-0114055 and EAR-0516760, and the Bailey Distinguished Graduate Fellowship at the University of Wisconsin.

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Correspondence to M. Elliot Smith.

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Supplementary tables S1-S3 and figure S1 (PDF 243 kb)

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Elliot Smith, M., Carroll, A. & Mueller, E. Elevated weathering rates in the Rocky Mountains during the Early Eocene Climatic Optimum. Nature Geosci 1, 370–374 (2008). https://doi.org/10.1038/ngeo205

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