DEMONSTRATION OF THE C3 PLANT PROXY: CONTROLLED GROWTH CHAMBERS TO DEEP-TIME APPLICATIONS

Controlled growth experiments across a wide range of atmospheric carbon dioxide (CO₂) levels (up to 4200 ppm) showed that CO₂ concentration affects the carbon isotope (δ¹³C) value of C₃ plant tissue. This work produced a unifying hyperbolic relationship between CO₂ and net carbon isotope discrimination and reconciled a wide range of field and chamber studies conducted across smaller changes in CO₂. The applicability of this relationship as a proxy for CO₂ was demonstrated through comparison with CO₂ data from ice core records across the Pleistocene-Holocene transition, and provided the framework for quantifying CO₂ in deep-time records from C₃ plant remains. However, some studies have misapplied this application, while others have confirmed use of the C₃ plant proxy. These disparate evaluations have added to the general confusion as to best practices for interpreting δ¹³C change in recent and deep-time terrestrial settings. Here we dissect each issue and demonstrate the proper application of our work across a wide range of previously published datasets. Quantitative reanalysis bolsters our proposed relationship between CO₂ and carbon isotope value, and therefore, the efficacy of the C₃ plant proxy. Our integrated evaluation of existing datasets allows us to describe the common flaws encountered when applying the C₃ plant proxy, while further demonstrating the value of its correct application across Geologic Time.