Quantitative critique of leaf‐based paleo‐CO₂ proxies: Consequences for their reliability and applicability

Abstract

A variety of proxies have been developed to reconstruct paleo-CO 2 from fossil leaves. These proxies rely on some combination of stomatal morphology, leaf δ 13 C, and leaf gas exchange. A common conceptual framework for evaluating these proxies is lacking, which has hampered efforts for inter-comparison. Here we develop such a framework, based on the underlying physics and biochemistry. From this conceptual framework, we find that the more extensively parameterised proxies, such as the optimisation model, are likely to be the most robust. The simpler proxies, such as the stomatal ratio model, tend to under-predict CO 2 , especially in warm (>15 C) and moist (>50% humidity) environments. This identification of a structural underprediction may help to explain the common observation that the simpler proxies often produce estimates of paleo-CO 2 that are lower than those from the more complex proxies and other, non-leaf-based CO 2 proxies. The use of extensively parameterised models is not always possible, depending on the preservation state of the fossils and the state of knowledge about the fossil's nearest living relative. With this caveat in mind, our analysis highlights the value of using the most complex leafbased model as possible.

The Miocene: The Future of the Past

“…All rights reserved. exchange modelling (Franks et al, 2014;Konrad et al, 2017Konrad et al, , 2020. The stomatal ratio and transfer function methods are taxon dependent, whereas the gas-exchange modelling approach is taxonindependent and incorporates additional factors apart from stomatal densities, such as stomatal conductance, and δ 13 C of the fossil leaves and atmospheres.…”

“…All rights reserved. Schubert, 2016), has been questioned because it does not account for water stress and can produce results that are unrealistically low (e.g., <0 ppm) or high (infinite; Kohn, 2016;Konrad et al, 2020;Lomax et al, 2019;Porter et al, 2019). The C3 plant δ 13 C proxy can produce similar results as other proxies, but the lack of an explicit dependence on precipitation or moisture availability is a weakness.…”

“…Stomatal density is defined either as the number of stomata per unit area (SD = stomata/mm 2 ) or as the percentage of stomata relative to all epidermal cells, the stomatal index (SI [%]). Three methods of stomatal proxy paleo‐ p CO 2 reconstructions are currently in use: the stomatal ratio method (McElwain & Chaloner, 1996), transfer functions (e.g., Barclay & Wing, 2016) and gas‐exchange modeling (Franks et al., 2014; Konrad et al., 2017, 2020). The stomatal ratio and transfer function methods are taxon dependent, whereas the gas‐exchange modeling approach is taxon‐independent and incorporates additional factors apart from stomatal densities, such as stomatal conductance, and δ 13 C of the fossil leaves and atmospheres.…”

Past climates inform our future

“…1). leading to misinterpretations of the signal (10). Proxy methodologies and assumptions continue to be refined, and there is some indication that CO 2 at the end of the Cretaceous may have been higher than shown in Fig.…”

Introduction to NECLIME Special Issue: Biodiversity and floral patterns in the course of Cenozoic climate change

“…Konrad, Royer, Franks, and Roth‐Nebelsick (2021) develop a conceptual framework based on physics and biochemistry laws for evaluating the proxies developed to reconstruct palaeo‐CO 2 from fossil leaves on a common basis.…”