Abstract Haptophyte algal biomarkers called alkenones are widely used to reconstruct atmospheric CO2 in Earth’s Cenozoic history. This method is based on the notion that the algal carbon isotope fractionation during photosynthesis, as represented by ep37:2, is a function of seawater CO2 concentration and algal physiology. Constraining the algal physiological parameter, known as the ‘b’ term, is the key for successful applications of the alkenone-CO2 method. Using sensitivity analyses, we show that the growth rate (μ), cell size (r), and membrane permeability (P) are the most important variables to determine b. For all life on Earth, body size is a key factor that regulates metabolic rates. Exploiting the interdependence between phytoplankton cell size and growth rate, and specifically, the r – μ relationship for coccolithophores, we show that the length of fossil coccoliths (Lcoccolith) produced by ancient alkenone-synthesizers can be used to estimate r and therefore μ. Combining our new Lcoccolith data and published ep37:2 from the South China Sea Site MD01-2392, existing results from ODP Site 925, and ice core CO2, we determined the cell membrane permeability (P = 5.09 × 10−5 m s−1) for the Pleistocene community employing a bootstrap resampling technique. These new methods of constraining r, μ and P, combined with proxy-derived temperature (T), led us to rebuild b as a variable for each sample individually, which is subsequently used for alkenone-CO2 calculations. Application of this approach established pCO2 of the last 3 glacial-interglacial cycles, which turns out to be comparable with the ice core data in both the amplitude of changes and absolute values. It also reconciles the published Eocene – Oligocene alkenone-CO2 data which showed large geographical differences, with the new estimates much more consistent among different sites, and in line with other proxy-based results and ice sheet model predictions.

中文翻译：

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精炼烯酮-pCO2 方法二：解决生理参数“b”

摘要 称为链烯酮的触藻藻类生物标志物被广泛用于重建地球新生代历史中的大气 CO2。该方法基于这样一种观点，即光合作用过程中藻类碳同位素分馏（如 ep37:2）是海水 CO2 浓度和藻类生理学的函数。限制藻类生理参数，称为“b”项，是烯酮-CO2 方法成功应用的关键。使用敏感性分析，我们表明生长速率 (μ)、细胞大小 (r) 和膜渗透性 (P) 是确定 b 的最重要变量。对于地球上的所有生命，体型是调节代谢率的关键因素。利用浮游植物细胞大小和生长速率之间的相互依赖性，特别是球石藻的 r - μ 关系，我们表明，古代烯酮合成器产生的化石颗石 (Lcoccolith) 的长度可用于估计 r，从而估计 μ。结合我们新的 Lcoccolith 数据和来自南海站点 MD01-2392 的 ep37:2、来自 ODP 站点 925 的现有结果和冰芯 CO2，我们确定了细胞膜通透性 (P = 5.09 × 10−5 ms−1)对于采用自举重采样技术的更新世社区。这些约束 r、μ 和 P 的新方法，结合代理衍生的温度 (T)，使我们能够将 b 分别重建为每个样本的变量，随后将其用于烯酮-CO2 计算。这种方法的应用确定了最后 3 个冰期-间冰期循环的 pCO2，结果证明它在变化幅度和绝对值方面与冰芯数据相当。