Algal biomarkers as a proxy for pCO₂: Constraints from late quaternary sapropels in the eastern Mediterranean

Records of carbon dioxide concentrations (partial pressure expressed as pCO₂) over Earth's history provide trends that are critical to understand our changing world. To better constrain pCO₂ estimations, here we test organic pCO₂ proxies against the direct measurements of pCO₂ recorded in ice cores. Based on the concept of stable carbon isotopic fractionation due to photosynthetic CO₂ fixation (Ɛ_p), we use the stable carbon isotopic composition (δ¹³C) of the recently proposed biomarker phytol (from all photoautotrophs), as well as the conventionally used alkenone biomarkers (from specific species) for comparison, to reconstruct pCO₂ over several Quaternary sapropel formation periods (S1, S3, S4, and S5) in the eastern Mediterranean Sea. The reconstructed pCO₂ values are within error of the ice core values but consistently exceed the ice core values by ca. 100 µatm. This offset corresponds with atmospheric disequilibrium of present day CO_2[aq] concentrations in the Mediterranean Sea from global pCO₂, equivalent to ca. 100 µatm, although pCO₂ estimates derived from individual horizons within each sapropel do not covary with the ice core values. This may possibly be due to greater variability in local CO_2[aq] concentration changes in the Mediterranean, as compared with the global average pCO₂, or possibly due to biases in the proxy, such as variable growth rate or carbon-concentrating mechanisms. Thus, the offset is likely a combination of physiological or environmental factors. Nevertheless, our results demonstrate that alkenone- and phytol-based pCO₂ proxies yield statistically similar estimations (P-value = 0.02, Pearson's r-value = 0.56), and yield reasonable absolute estimations although with relatively large uncertainties (±100 µatm).