CO₂ proxies based on leaf gas-exchange principles help in part to address the limitations of the stomatal frequency proxies. A well-vetted model (see figure) for carbon assimilation (photosynthesis) describes the rate of carbon assimilation (A_n) to be equal to the product between the physical capacity of the leaf to exchange gases with the atmosphere (i.e. total leaf conductance to CO₂, g_c(tot)) and the gradient between atmospheric (c_a) and intercellular (c_i) CO₂ (c_a – c_i). That is, a higher rate of photosynthesis is associated with a higher leaf conductance and/or a strong drawdown of intercellular CO₂ relative to the atmospheric concentration. Rearranging for atmospheric CO₂, the equation becomes:

In fossils, total leaf conductance can be estimated from direct measurements of stomatal size and density, along with other parameters that are in part calibrated using present-day relatives. The c_i/c_a ratio is determined from measurements of fossil leaf δ¹³C along with an estimate of paleoatmospheric δ¹³C (for details on this, please see FAQ How exactly do paleo-CO₂ proxies work?, in particular the general background information). The assimilation rate (A_n) cannot be measured directly on fossils and is normally inferred from a nearest living relative, along with a mechanistic model that describes how the assimilation rate scales with atmospheric CO₂.

The leaf gas-exchange proxies are largely mechanistic in nature, can be applied to most stomatal-bearing fossil leaves, and produce estimates of CO₂ that remain well-bounded (i.e. their upper uncertainties do not go to infinity, as is the case with the stomatal density and stomatal index proxies), even at high CO₂.

The figures above show a simple flow chart of the different parameters required to calculate CO₂ from this proxy. A template for entering new paleo-CO₂ estimates from the leaf gas exchange proxies can be found here: Franks method, Konrad ROM method, and Konrad FOM method. For adding new data, please fill out the template and submit it to the paleo-CO₂ database at the NOAA National Centers for Environmental Information (formerly known as the National Climatic Data Center, NCDC), using email address paleo@noaa.gov.

References cited

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 41, 4685-4694.