EXPERIMENTAL CONSTRAINTS ON A NEW LEAF GAS-EXCHANGE METHOD FOR PALEO-CO2 RECONSTRUCTION

Franks et al. (2014) presented a new stomatal-based proxy for paleo-CO₂ reconstruction based on basic plant-gas exchange principles. In contrast to other stomatal-based methods, this new approach produces well-bounded CO­₂ estimates, even at high CO₂, and does not require that the fossil species are still living today. The method therefore opens up, for the first time, much of the paleobotanical record for quantitative paleo-CO₂ inference. Despite its promise, only a handful of extant species have been tested, the majority of which are in clades with recent radiations; moreover, only one species has been tested at elevated CO₂. It therefore is not clear how applicable the method is to ancient lineages, especially at high CO₂. Here we place important experimental constraints on the Franks et al. (2014) method by testing two ferns (Stenochlaena palustris and Osmunda cinnomamea), one angiosperm (Euptelea pleiosperma), and one gymnosperm (Cedrus deodara). Plant species were grown in growth chambers at elevated (1000 ppm) and near-ambient CO₂ (500 ppm) levels before measuring the key leaf inputs needed in the model: photosynthetic rate, stomatal conductance, stomatal size and density, and stable carbon isotopic composition. We also compare CO₂ estimates from the Franks et al. (2014) method to previous stomatal index-based estimates (Beerling et al., 2002) for the fossil fern aff. Stenochlaena dating immediately above the K/P boundary. Overall, our study establishes important constraints for thinking quantitatively about an exciting, nascent paleo-CO₂ method.

Beerling, D. J., Lomax, B. H., Royer, D. L., Upchurch, G. R., and Kump, L. R., 2002, An atmospheric pCO2 reconstruction across the Cretaceous-Tertiary boundary from leaf megafossils: Proceedings of the National Academy of Sciences, v. 99, no. 12, p. 7836-7840.

Franks, P. J., Royer, D. L., Beerling, D. J., Van de Water, P. K., Cantrill, D. J., Barbour, M. M., and Berry, J. A., 2014, New constraints on atmospheric CO₂ concentration for the Phanerozoic: Geophysical Research Letters, v. 41, p. 4685-4694.