Abstract 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 (Ɛp), 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 CO2[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 CO2[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).

中文翻译：

---

藻类生物标志物作为 pCO2 的代表：地中海东部第四纪晚期腐泥的限制

摘要 地球历史上二氧化碳浓度（分压表示为 pCO2）的记录提供了对了解我们不断变化的世界至关重要的趋势。为了更好地限制 pCO2 估计，我们在这里测试了有机 pCO2 代理与冰芯中记录的 pCO2 的直接测量值。基于由于光合 CO2 固定 (Ɛp) 引起的稳定碳同位素分馏的概念，我们使用最近提出的生物标志物植醇（来自所有光合自养生物）的稳定碳同位素组成（δ13C），以及常规使用的烯酮生物标志物（来自特定物种）进行比较，以重建地中海东部几个第四纪腐泥形成时期（S1、S3、S4 和 S5）的 pCO2。重建的 pCO2 值在冰芯值的误差范围内，但始终超过冰芯值约。100 微米。该偏移量与全球 pCO2 中地中海当前 CO2[aq] 浓度的大气不平衡相对应，相当于约 100 µatm，尽管从每个腐烂物内的单个层位得出的 pCO2 估计值与冰芯值不共变。这可能是由于与全球平均 pCO2 相比，地中海局部 CO2[aq] 浓度变化的可变性更大，或者可能是由于代理中的偏差，例如可变增长率或碳浓缩机制。因此，偏移很可能是生理或环境因素的组合。尽管如此，