A phytol εp-based core-top calibration to reconstruct past changes in atmospheric CO2

Proxy-based reconstructions of past changes in atmospheric carbon dioxide concentrations (pCO2) are essential for understanding climate dynamics. A common method for reconstructing past pCO2 is based on the carbon isotopic fractionation during photosynthesis by Rubisco (εp). This proxy method is based upon the difference (εp) between the stable carbon isotopic composition (δ13C) of dissolved CO2 and the δ13C of marine photoautotroph biomass, which depends on the concentration of dissolved CO2, related to pCO2 through Henry’s Law. This method has been applied to the general phytoplankton biomarker chlorophyll (preserved as isoprenoids like phytol, phytane, and pristane in the sedimentary record) to reconstruct photoautotroph biomass δ13C. The long-term stability of these chlorophyll-derived biomarkers in the sedimentary record has currently allowed the reconstruction of pCO2 across the Phanerozoic (∼450 million years). However, the chlorophyll-derived biomarker proxy currently lacks a robust validation within modern settings. Here we investigate the relationship between the δ13C of chlorophyll (as phytol) and the concentration of dissolved CO2 in the modern ocean using a globally distributed set of 30 marine core top sediments and 75 suspended particulate matter samples. Our results demonstrate a positive relationship between the extent of fractionation (higher phytol εp) and dissolved CO2 concentration. This marks the first empirical calibration between phytol εp and the concentration of dissolved CO2 in natural settings. We find that terrestrial input negatively affects this observed relationship, and the exclusion of coastal samples from our dataset improves the correlation. When applied to previously published Pleistocene proxy data, our new calibration provides an improved pCO2 reconstruction with estimates that are statistically like direct pCO2 measurements from the Antarctic ice cores. When applied to published data from the entire Phanerozoic, our calibration provides estimates in line with those of other proxy methods, emphasizing the potential of chlorophyll for reconstructions of pCO2 across geological time.