Testing the general phytoplankton biomarker-based pCO2 proxy across the last 350,000 years

Understanding how atmospheric carbon dioxide concentrations ( p CO 2 ) changed through time is vital for assessing how carbon cycle perturbations will impact our near future. A widely used p CO 2 proxy is based on the photosynthetic carbon isotopic fractionation recorded by phytoplankton biomarkers, such as derivatives of chlorophyll-a (phytol/phytane) or more typically haptophyte-derived alkenones. However, the phytol-based proxy has limited Pleistocene data available, hindering validation against ice core p CO 2 records. The alkenone-based proxy has demonstrated mixed success in reconstructing Pleistocene ice core p CO 2 variations, presumably due to how the carbon demand parameter b has been constrained, using either empirical phosphate-growth rate relationships or cell size considerations. Here, we test the ability of both biomarker approaches to reconstruct p CO 2 across the last ~350,000 years. Employing a bulk organic matter-derived b for phytol and a phosphate-derived b for alkenones yields p CO 2 estimates which consistently exceed ice core values by ~50–300 ppm. By applying a modern ocean phytol-derived calibration for our phytol-based reconstruction and incorporating published coccolith size corrections into our alkenone-based reconstruction, we obtain lower p CO 2 estimates, with phytol-based p CO 2 values being more consistent with values from the ice core records. However, both lack the full amplitude of glacial-interglacial variations. We suggest this dampened variability is associated with two main issues: proxy insensitivity to small p CO 2 changes and the upregulation of carbon concentrating mechanisms. Our findings highlight the need for further research on understanding the mechanistic limitations of the phytoplankton biomarker-based p CO 2 proxies during low p CO 2 periods of Earth's history.