Molecular fossils from phytoplankton reveal secular Pco2 trend over the Phanerozoic

Past changes in the atmospheric concentration of carbon dioxide ( P co 2 ) have had a major impact on earth system dynamics; yet, reconstructing secular trends of past P co 2 remains a prevalent challenge in paleoclimate studies. The current long-term P co 2 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 P co 2 from a single proxy: the stable carbon isotopic fractionation associated with photosynthesis (Ɛ p ) that increases as P co 2 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 P co 2 trend derived from phytane-based Ɛ p mirrors the available compilations of P co 2 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 P co 2 based on a single marine proxy, covering the past 500 Ma of Earth history.