Molecular fossils from phytoplankton reveal secular PCO₂ trend over the phanerozoic

Past changes in the atmospheric concentration of carbon dioxide (PCO₂) have had a major impact on earth system dynamics; yet, reconstructing secular trends of past PCO₂ remains a prevalent challenge in paleoclimate studies. The current long-term PCO₂ reconstructions rely largely on the compilation of many different proxies, often with discrepancies among proxies, particularly for periods older than 100 million years (Ma). Here, we reconstructed Phanerozoic PCO₂ from a single proxy: the stable carbon isotopic fractionation associated with photosynthesis (Ɛ_p) that increases as PCO₂ increases. This concept has been widely applied to alkenones, but here, we expand this concept both spatially and temporally by applying it to all marine phytoplankton via a diagenetic product of chlorophyll, phytane. We obtained data from 306 marine sediments and oils, which showed that Ɛ_p ranges from 11 to 24, agreeing with the observed range of maximum fractionation of Rubisco (i.e., 25 to 28). The observed secular PCO₂ trend derived from phytane-based Ɛ_p mirrors the available compilations of PCO₂ over the past 420 Ma, except for two periods in which our higher estimates agree with the warm climate during those time periods. Our record currently provides the longest secular trend in PCO₂ based on a single marine proxy, covering the past 500 Ma of Earth history.