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

Dana L. Royer, Kylen M. Moynihan, Melissa L. McKee, Liliana Londoño, Peter J. Franks
2018 Climate of the Past Discussions  
<p><strong>Abstract.</strong> Leaf gas-exchange models show considerable promise as paleo-CO<sub>2</sub> proxies. They are largely mechanistic in nature, provide well-constrained estimates even when CO<sub>2</sub> is high, and can be applied to most subaerial, stomata-bearing leaves from C<sub>3</sub> taxa, regardless of age or taxonomy. Here we place additional observational and theoretical constraints on one of these models, the <q>Franks</q> model. In order to gauge the model's general
more » ... cy in a way that is appropriate for fossil studies, we estimated CO<sub>2</sub> from 40 species of extant angiosperms, conifers, and ferns based only on measurements that can be made directly from fossils (leaf δ<sup>13</sup>C and stomatal density and size) and a limited sample size (1&amp;ndash;3 leaves per species). The mean error rate is 28 %, which is similar to or better than the accuracy of other leading paleo-CO<sub>2</sub> proxies. We find that leaf temperature and photorespiration do not strongly affect estimated CO<sub>2</sub>, although more work is warranted on the possible influence of O<sub>2</sub> concentration on photorespiration. Leaves from the lowermost 1&amp;ndash;2 m of closed-canopy forests should not be used because the local air δ<sup>13</sup>C 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.</p>
doi:10.5194/cp-2018-156 fatcat:7nb2f4xuwjapnjgmjtgplpqv4u