Coccolithophores and other haptophyte algae acquire the carbon required for metabolic processes from the water in which they live. Whether carbon is actively moved across the cell membrane via a carbon concentrating mechanism, or passively through diffusion, is important for haptophyte biochemistry. The possible utilization of carbon concentrating mechanisms also has the potential to over-print one proxy method by which ancient atmospheric CO₂ concentration is reconstructed using alkenone isotopes. Here I show that carbon concentrating mechanisms are likely used when aqueous carbon dioxide concentrations are below 7 µmol L⁻¹. I compile published alkenone-based CO₂ reconstructions from multiple sites over the Pleistocene and recalculate them using a common methodology, which allows comparison to be made with ice core CO₂ records. Interrogating these records reveals that the relationship between proxy CO₂ and ice core CO₂ breaks down when local aqueous CO₂ concentration falls below 7 µmol L⁻¹. The recognition of this threshold explains why many alkenone-based CO₂ records fail to accurately replicate ice core CO₂ records, and it suggests the alkenone proxy is likely robust for much of the Cenozoic when this threshold was unlikely to be reached in much of the global ocean.

中文翻译：

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当含水二氧化碳浓度降至7 µmol L ^-1以下时，烯酮同位素显示出在球墨镜中活性炭富集机制的证据

球花珊瑚和其他触藻藻类从它们生活的水中获取代谢过程所需的碳。碳是通过碳浓缩机制在细胞膜上主动移动，还是通过扩散被动地移动，对于触藻类生物化学而言很重要。碳浓缩机制的可能利用也有可能套印一种替代方法，通过该方法可以使用烯酮同位素重建古代大气中的CO ₂浓度。在此我表明，当含水二氧化碳浓度低于7 µ mol L ^-1时，可能会使用碳浓缩机制 。我编译发布的基于烯酮的CO ₂从更新世的多个地点进行重建，并使用通用方法重新计算它们，从而可以与冰芯CO ₂记录进行比较。对这些记录的查询表明，当局部含水CO ₂浓度低于7  µ mol L ^-1时，代理CO ₂与冰芯CO ₂之间的关系破裂。此阈值的识别说明了为什么许多基于​​烯酮的CO ₂记录无法准确复制冰芯CO _2的原因 记录，这表明当在全球大部分海洋中不太可能达到该阈值时，烯醛酮替代物可能对许多新生代都具有稳健性。