An experimental evaluation of the use of Δ13C as a proxy for palaeoatmospheric CO2

Barry H Lomax,Phillip E Jardine,Janice A Lake,Melanie J Leng

doi:10.1016/j.gca.2018.12.026

Abstract

Abstract Understanding changes in atmospheric CO2 over geological time via the development of well constrained and tested proxies is of increasing importance within the Earth sciences. Recently a new proxy (identified as the C3 proxy) has been proposed that is based on the relationship between CO2 and carbon isotope discrimination (Δ13C) of plant leaf tissue. Initial work suggests that this proxy has the capacity to deliver accurate and potentially precise palaeo-CO2 reconstructions through geological time since the origins of vascular plants (∼450 Mya). However, the proposed model has yet to be fully validated through independent experiments. Using the model plant Arabidopsis thaliana exposed to different watering regimes and grown over a wide range of CO2 concentrations (380, 400, 760, 1000, 1200, 1500, 2000 and 3000 ppm) relevant to plant evolution we provide an experimental framework that allows for such validation. Our experiments show that a wide variation in Δ13C as a function of water availability is independent of CO2 treatment. Validation of the C3 proxy was undertaken by comparing growth CO2 to estimates of CO2 derived from Δ13C. Our results show significant differences between predicted and observed CO2 across all CO2 treatments and water availabilities, with a strong under prediction of CO2 in experiments designed to simulate Cenozoic and Mesozoic atmospheric conditions (≥1500 ppm). Further assessment of Δ13C to predict CO2 was undertaken using Monte Carlo error propagation. This suite of analysis revealed a lack of convergence between predicted and observed CO2. Collectively these findings suggest that the relationship between Δ13C and CO2 is poorly constrained. Consequently the use of Δ13C as a proxy to reconstruct palaeoatmospheric CO2 is of limited use as the estimates of CO2 are not accurate when compared to known growth conditions.

Highlights

Understanding both the long term carbon cycle and rapid perturbations in atmospheric CO2 observed through the geological record has become an increasingly important area of scientific enquiry
Six separate CO2 experiments were conducted, with CO2 held at concentrations of 380, 760, 1000, 1500, 2000 and 3000 ppm with the d13Ca signature becoming more negative as CO2 increases
Our data demonstrates considerable spread in D13C within each CO2 treatment as a function of watering regime (Fig. 2) suggesting that other factors not previously investigated in the context of the C3 plant proxy (Schubert and Jahren, 2012) have the potential to influence estimates of CO2 based on D13C

Summary

Introduction

Understanding both the long term carbon cycle and rapid perturbations in atmospheric CO2 observed through the geological record has become an increasingly important area of scientific enquiry. A major limiting step in understanding the climate system sensitivity to changes in atmospheric CO2 over geological time has been the variability in modelled solutions of palaeo-CO2 concentration which vary considerably both between (GEOCARB vs COPSE (Berner and Kothavala, 2001; Bergman et al, 2004)) and within model families (GEOCARB III vs GEOCARBSULF (Berner and Kothavala, 2001; Berner, 2006)). B.H. Lomax et al / Geochimica et Cosmochimica Acta 247 (2019) 162–174 suggests modelled values ranging from $3400 ppm in the Early Triassic to $500 ppm in the Late Triassic. Lomax et al / Geochimica et Cosmochimica Acta 247 (2019) 162–174 suggests modelled values ranging from $3400 ppm in the Early Triassic to $500 ppm in the Late Triassic To constrain these models and evaluate refinements made through model development requires the development of mechanistically based CO2 proxies that have been independently tested and fully validated (Lomax and Fraser, 2015)

Methods

Results

Discussion

Conclusion