Currently, there is only one paleo‐CO2 record from plant macrofossils that has sufficient stratigraphic resolution to potentially capture a transient spike related to rapid carbon release at the Cretaceous‐Paleogene (K‐Pg) boundary. Unfortunately, the associated measurements of stomatal index are off‐calibration, leading to a qualitative interpretation of >2,300‐ppm CO2. Here we reevaluate this record with a paleo‐CO2 proxy based on leaf gas exchange principles. We also test the proxy with three living species grown at 500‐ and 1,000‐ppm CO2, including the nearest living relative of the K‐Pg fern, and find a mean error rate of ~22%, which is comparable to other leading paleo‐CO2 proxies. Our fossils record a ~250‐ppm increase in CO2 across the K‐Pg boundary from ~625 to ~875 ppm. A small CO2 spike associated with the end‐Cretaceous mass extinction is consistent with many temperature records and with carbon cycle modeling of Deccan volcanism and the meteorite impact.

In many woody dicot plant species, colder temperatures correlate with a greater degree of leaf dissection and with larger and more abundant leaf teeth (the serrated edges along margins). The measurement of site-mean characteristics of leaf size and shape (physiognomy), including leaf dissection and tooth morphology, has been an important paleoclimate tool for over a century. These physiognomic-based climate proxies require that all woody dicot plants at a site, regardless of species, change their leaf shape rapidly and predictably in response to temperature. Here we experimentally test these assumptions by growing five woody species in growth cabinets under two temperatures (17 and 25°C). In keeping with global site-based patterns, plants tend to develop more dissected leaves with more abundant and larger leaf teeth in the cool treatment. Overall, this upholds the assumption that leaf shape responds in a particular direction to temperature change. The assumption that leaf shape variables respond to temperature in the same way regardless of species did not hold because the responses varied by species. Leaf physiognomic models for inferring paleoclimate should take into account these species-specific responses.

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 fossil 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 on a limited sample size (one to three 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 1–2 m 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.

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 1–2 m 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.

The physiognomy (size and shape) of fossilized leaves has been analyzed to reconstruct the mean annual temperature (MAT) of ancient environments. Colder temperatures often select for larger and more abundant leaf teeth-serrated edges on leaf margins-as well as a greater degree of leaf dissection. Accurate paleotemperature estimates play important roles in testing climate models used for predicting future patterns of climate change. Paleotemperature estimates can be compared with atmospheric CO2 concentrations from the same period to help constrain climate sensitivity. However, to be able to accurately predict paleotemperature from the morphology of fossilized leaves, leaves must be able to react quickly and in a predictable manner to changes in temperature. By growing Acer rubrum plants in growth cabinets under different temperatures, Royer (2012) found that this species develops more highly dissected leaves (larger, more frequent teeth, and a higher perimeter/area ratio) in cooler temperatures than it does in warmer temperatures. To expand this work, my research examines the extent to which temperature affects leaf morphology in four additional tree species: Caprinus caroliniana, Acer negundo, Ilex opaca, and Ostrya virginiana. Saplings of these species were grown in two growth cabinets under different temperatures. Carpinus caroliniana leaves had a significantly lower total number of leaf teeth as well as a lower ratio of total number of leaf teeth to internal perimeter, and Acer negundo leaves had a significantly lower feret diameter ratio (measure of leaf dissection) in the warm temperature treatment compared to the cool treatment. In addition, a two-way ANOVA was used to test the influence of temperature and species on leaf