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Climate of the Past An interactive open-access journal of the European Geosciences Union
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Discussion papers
https://doi.org/10.5194/cp-2018-156
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/cp-2018-156
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 21 Nov 2018

Research article | 21 Nov 2018

Review status
This discussion paper is a preprint. It is a manuscript under review for the journal Climate of the Past (CP).

Sensitivity of a leaf gas-exchange model for estimating paleoatmospheric CO2 concentration

Dana L. Royer1, Kylen M. Moynihan1, Melissa L. McKee1, Liliana Londoño2, and Peter J. Franks3 Dana L. Royer et al.
  • 1Department of Earth and Environmental Sciences, Wesleyan University, Middletown, Connecticut, USA
  • 2Smithsonian Tropical Research Institute, Balboa, Ancón, Republic of Panamá
  • 3Faculty of Agriculture and Environment, University of Sydney, Sydney, New South Wales, Australia

Abstract. Leaf gas-exchange models show considerable promise as paleo-CO2 proxies. They are largely mechanistic in nature, provide well-constrained estimates even when CO2 is high, and can be applied to most subaerial, stomata-bearing leaves from C3 taxa, regardless of age or taxonomy. Here we place additional observational and theoretical constraints on one of these models, the Franks model. In order to gauge the model's general accuracy in a way that is appropriate for fossil studies, we estimated CO2 from 40 species of extant angiosperms, conifers, and ferns based only on measurements that can be made directly from fossils (leaf δ13C and stomatal density and size) and a limited sample size (1–3 leaves per species). The mean error rate is 28%, which is similar to or better than the accuracy of other leading paleo-CO2 proxies. We find that leaf temperature and photorespiration do not strongly affect estimated CO2, although more work is warranted on the possible influence of O2 concentration on photorespiration. Leaves from the lowermost 1–2m of closed-canopy forests should not be used because the local air δ13C value is lower than the global well-mixed value. Such leaves are not common in the fossil record, but can be identified by morphological and isotopic means.

Dana L. Royer et al.
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Short summary
Plant-based proxies for estimating atmospheric CO2 in the geologic past are becoming more popular. Here we test in a wide range of living plants the reliability of a method based on leaf gas-exchange principles. Overall, the average error rate (~ 28 %) is broadly similar to other paleo-CO2 proxies. Our results should increase confidence in using this recently developed method.
Plant-based proxies for estimating atmospheric CO2 in the geologic past are becoming more...
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