Alkenone isotopes show evidence of active carbon concentrating mechanisms in coccolithophores as aqueous carbon dioxide concentrations fall below 7 µmol L−1

• Published in: Biogeosciences
• Main Author: Badger, Marcus P. S.
• Format: Text
• Language: English
• Published: 2021
• Subjects: ice core
• Online Access: https://doi.org/10.5194/bg-18-1149-2021; https://bg.copernicus.org/articles/18/1149/2021/

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 CO 2 concentration is reconstructed using alkenone isotopes. Here I show that carbon concentrating mechanisms are likely used when aqueous carbon dioxide concentrations are below 7 µ mol L −1 . I compile published alkenone-based CO 2 reconstructions from multiple sites over the Pleistocene and recalculate them using a common methodology, which allows comparison to be made with ice core CO 2 records. Interrogating these records reveals that the relationship between proxy CO 2 and ice core CO 2 breaks down when local aqueous CO 2 concentration falls below 7 µ mol L −1 . The recognition of this threshold explains why many alkenone-based CO 2 records fail to accurately replicate ice core CO 2 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.