BRYOCARB: A process-based model of thallose liverwort carbon isotope fractionation in response to CO2, O2, light and temperature

Evidence from laboratory experiments indicates that fractionation against the heavy stable isotope of carbon (Δ13C) by bryophytes (liverworts and mosses) is strongly dependent on atmospheric CO2. This physiological response may therefore provide the basis for developing a new terrestrial CO2 proxy [Fletcher, B.J., Beerling, D.J., Brentnall, S.J., Royer, D.L., 2005. Fossil bryophytes as recorders of ancient CO2 levels: experimental evidence and a Cretaceous case study. Global Biogeochem. Cycles19, GB3012]. Here, we establish a theoretical basis for the proxy by developing an extended model of bryophyte carbon isotope fractionation (BRYOCARB) that integrates the biochemical theory of photosynthetic CO2 assimilation with controls on CO2 supply by diffusion from the atmosphere. The BRYOCARB model is evaluated against measurements of the response of liverwort photosynthesis and Δ13C to variations in atmospheric O2, temperature and irradiance at different CO2 concentrations. We show that the bryophyte proxy is at least as sensitive to variations in atmosphere CO2 as the two other leading carbon isotope-based approaches to estimating palaeo-CO2 levels (δ13C of phytoplankton and of paleosols). Mathematical inversion of BRYOCARB provides a mechanistic means of estimating atmospheric CO2 levels from fossil bryophyte carbon that can explicitly account for the effects of past differences in O2 and climate.