Carbon Isotope Fractionation in Noelaerhabdaceae Algae in Culture and a Critical Evaluation of the Alkenone Paleobarometer

Abstract

Abstract The carbon isotope fractionation in algal organic matter (ε p ), including the long‐chain alkenones produced by the coccolithophorid family Noelaerhabdaceae, is used to reconstruct past atmospheric CO 2 levels. The conventional proxy linearly relates ε p to changes in cellular carbon demand relative to diffusive CO 2 supply, with larger ε p values occurring at lower carbon demand relative to supply (i.e., abundant CO 2 ). However, the response of Gephyrocapsa oceanica , one of the dominant alkenone producers of the last few million years, has not been studied closely. Here, we subject G. oceanica to various CO 2 levels by increasing pCO 2 in the culture headspace, as opposed to increasing dissolved inorganic carbon (DIC) and alkalinity concentrations at constant pH. We note no substantial change in physiology, but observe an increase in ε p as carbon demand relative to supply decreases, consistent with DIC manipulations. We compile existing Noelaerhabdaceae ε p data and show that the diffusive model poorly describes the data. A meta‐analysis of individual treatments (unique combinations of lab, strain, and light conditions) shows that the slope of the ε p response depends on the light conditions and range of carbon demand relative to CO 2 supply in the treatment, which is incompatible with the diffusive model. We model ε p as a multilinear function of key physiological and environmental variables and find that both photoperiod duration and light intensity are critical parameters, in addition to CO 2 and cell size. While alkenone carbon isotope ratios indeed record CO 2 information, irradiance and other factors are also necessary to properly describe alkenone ε p .