Testing the accuracy of new paleoatmospheric CO2 proxies based on plant stable carbon isotopic composition and stomatal traits in a range of simulated paleoatmospheric O2:CO2 ratios

Abstract

A drive to improve long-term estimates of atmospheric CO 2 change through Earths history has led to the development of novel paleoproxy CO 2 methods applicable to fossil plants. This paper compares two of these paleoproxy CO 2 methods (1) the empirical model of Schubert and Jahren (2012, 2015) termed the C3 plant proxy, which was developed using the hyperbolic relationship observed between plant carbon isotope discrimination (Δ 13 C) and p CO 2 and (2) the mechanistic model of Franks et al. (2014), which utilizes stomatal anatomy measurements and plant carbon isotopic composition and is based on established equations for leaf gas exchange and photosynthesis. To date both models lack detailed experimental testing of the robustness and accuracy of their p CO 2 predictions in relation to phylogenetic differences in Δ 13 C between C3 plant groups and/or species and atmospheric O 2 concentration, which has co-fluctuated with CO 2 in the geological past. Here, we investigate if these novel paleoproxy CO 2 approaches can produce phylogenetically independent estimates of p CO 2 that are not influenced by variations in atmospheric oxygen. To address this, model estimates of p CO 2 were compared with measured CO 2 values for ten plant species representing four major vascular plant groups (lycophytes, monilophytes, gymnosperms and angiosperms) grown for 6 months in walk-in plant growth chambers under varying O 2 :CO 2 ratios. Results from the mechanistic model reveal that species-specific plant responses to atmospheric CO 2 accounted for the large variability in CO 2 predictions between species and overestimations of p CO 2 by ∼+232 ppm to 940 ppm. Adjustments to the model that involved: (1) corrections to the photorespiratory compensation point (to account for fluctuating oxygen) and (2) removal of the phylogenetic effect on Δ 13 C, reduced between-species variability (by 50%) and led to better p CO 2 estimates within 58–229 ppm of measured values. The C3 plant proxy (empirical approach) produced accurate CO 2 estimates within +37 to +71 ppm of measured values, however it was affected by species-specific differences in Δ 13 C and for some species resulted in negative estimates of p CO 2 . Sub-ambient O 2 (16%) resulted in erroneously high CO 2 estimates (∼100 ppm higher than the control) for a number of species, as plant responses to decreasing O 2 mimicked those of increasing CO 2 . We conclude that both models (with phylogenetic corrections to Franks et al. (2014)) can produce accurate estimates of paleo-CO 2 when a mix of three to four species, preferably containing representatives from both pteridophytes and spermatophytes, is used to obtain a consensus p CO 2 estimate. We advocate a fossil assemblage rather than a single-species approach to paleo-CO 2 estimation in future application of either method.