A mechanistic algal 13C-fractionation model for constraining pCO2 and light availability

The CO2-dependency of stable carbon isotope fractionation (εp) during marine algal carbon acquisition is widely used as a proxy for past atmospheric pCO2. However, a robust mechanistic model linking εp and pCO2 suitable for reconstructions is still lacking, hampering proper evaluation of uncertainties and confounding factors such as light availability. Here, we present a multi-compartment model of an algal cell that simulates environmental control on passive CO2 diffusion, active HCO3- uptake, and 13C fractionation during C-fixation. The model is fitted to dinoflagellate and Gephyrocapsa huxleyi culture and sedimentary data. This shows robust performance across a wide range of parameter values, implying an appropriate representation of the mechanistic control of the system in the model. Subsequently, given ice-core records of pCO2, we apply the model inversely to late Pleistocene G. huxleyi-derived alkenone εp records are used to reconstruct light availability in the Caribbean Sea. In addition, we use εp records of Spiniferites ramosus dinoflagellate cysts and alkenones to produce a pCO2 reconstruction across the Miocene Climatic Optimum (~19 – 15 Ma). Our results support the use of fossil dinoflagellate cyst εp for pCO2 reconstructions and show the potential of this proxy for constraining past light availability using alkenone εp, when pCO2 is known.