| Summary: | Records of carbon dioxide concentrations (partial pressure expressed as p CO 2 ) over Earth’s history provide trends that arecritical to understand our changing world. To better constrain p CO 2 estimations, here we test organic p CO 2 proxies against the direct measurements of p CO 2 recorded in ice cores. Based on the concept of stable carbon isotopic fractionation due to photosynthetic CO 2 fixation (Ɛ p ), we use the stable carbon isotopic composition (δ 13 C) of the recently proposed biomarker phytol (from all photoautotrophs), as well as the conventionally used alkenone biomarkers (from specific species) for comparison, to reconstruct p CO 2 over several Quaternary sapropel formation periods (S1, S3, S4, and S5) in the eastern Mediterranean Sea. The reconstructed p CO 2 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 p CO 2 , equivalent to ca. 100 µatm, although p CO 2 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 p CO 2 , 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 p CO 2 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).
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