A phytol ε_p-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 (ε_p). This proxy method is based upon the difference (ε_p) 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 phytol, phytane, and pristane in the sedimentary record) to reconstruct photoautotroph biomass δ¹³C. The stability of these chlorophyll-derived biomarkers in the sedimentary record has currently allowed the reconstruction of pCO₂ across the Phanerozoic (~540 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 ε_p) and dissolved CO₂ concentration. This marks the first empirical calibration between phytol ε_p 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.